ECMAScript® 2019 Language Specification

ECMAScript® 2019 Language Specification
Pins
Table of Contents
Introduction
Scope
Conformance
Normative References
Overview
4.1
Web Scripting
4.2
ECMAScript Overview
4.2.1
Objects
4.2.2
The Strict Variant of ECMAScript
4.3
Terms and Definitions
4.3.1
type
4.3.2
primitive value
4.3.3
object
4.3.4
constructor
4.3.5
prototype
4.3.6
ordinary object
4.3.7
exotic object
4.3.8
standard object
4.3.9
built-in object
4.3.10
undefined value
4.3.11
Undefined type
4.3.12
null value
4.3.13
Null type
4.3.14
Boolean value
4.3.15
Boolean type
4.3.16
Boolean object
4.3.17
String value
4.3.18
String type
4.3.19
String object
4.3.20
Number value
4.3.21
Number type
4.3.22
Number object
4.3.23
Infinity
4.3.24
NaN
4.3.25
Symbol value
4.3.26
Symbol type
4.3.27
Symbol object
4.3.28
function
4.3.29
built-in function
4.3.30
property
4.3.31
method
4.3.32
built-in method
4.3.33
attribute
4.3.34
own property
4.3.35
inherited property
4.4
Organization of This Specification
Notational Conventions
5.1
Syntactic and Lexical Grammars
5.1.1
Context-Free Grammars
5.1.2
The Lexical and RegExp Grammars
5.1.3
The Numeric String Grammar
5.1.4
The Syntactic Grammar
5.1.5
Grammar Notation
5.2
Algorithm Conventions
5.2.1
Abstract Operations
5.2.2
Syntax-Directed Operations
5.2.3
Runtime Semantics
5.2.3.1
Implicit Completion Values
5.2.3.2
Throw an Exception
5.2.3.3
ReturnIfAbrupt
5.2.3.4
ReturnIfAbrupt Shorthands
5.2.4
Static Semantics
5.2.5
Mathematical Operations
ECMAScript Data Types and Values
6.1
ECMAScript Language Types
6.1.1
The Undefined Type
6.1.2
The Null Type
6.1.3
The Boolean Type
6.1.4
The String Type
6.1.5
The Symbol Type
6.1.5.1
Well-Known Symbols
6.1.6
The Number Type
6.1.7
The Object Type
6.1.7.1
Property Attributes
6.1.7.2
Object Internal Methods and Internal Slots
6.1.7.3
Invariants of the Essential Internal Methods
6.1.7.4
Well-Known Intrinsic Objects
6.2
ECMAScript Specification Types
6.2.1
The List and Record Specification Types
6.2.2
The Set and Relation Specification Types
6.2.3
The Completion Record Specification Type
6.2.3.1
Await
6.2.3.1.1
Await Fulfilled Functions
6.2.3.1.2
Await Rejected Functions
6.2.3.2
NormalCompletion
6.2.3.3
ThrowCompletion
6.2.3.4
UpdateEmpty (
completionRecord
value
6.2.4
The Reference Specification Type
6.2.4.1
GetBase (
6.2.4.2
GetReferencedName (
6.2.4.3
IsStrictReference (
6.2.4.4
HasPrimitiveBase (
6.2.4.5
IsPropertyReference (
6.2.4.6
IsUnresolvableReference (
6.2.4.7
IsSuperReference (
6.2.4.8
GetValue (
6.2.4.9
PutValue (
6.2.4.10
GetThisValue (
6.2.4.11
InitializeReferencedBinding (
6.2.5
The Property Descriptor Specification Type
6.2.5.1
IsAccessorDescriptor (
Desc
6.2.5.2
IsDataDescriptor (
Desc
6.2.5.3
IsGenericDescriptor (
Desc
6.2.5.4
FromPropertyDescriptor (
Desc
6.2.5.5
ToPropertyDescriptor (
Obj
6.2.5.6
CompletePropertyDescriptor (
Desc
6.2.6
The Lexical Environment and Environment Record Specification Types
6.2.7
Data Blocks
6.2.7.1
CreateByteDataBlock (
size
6.2.7.2
CreateSharedByteDataBlock (
size
6.2.7.3
CopyDataBlockBytes (
toBlock
toIndex
fromBlock
fromIndex
count
Abstract Operations
7.1
Type Conversion
7.1.1
ToPrimitive (
input
[ ,
PreferredType
] )
7.1.1.1
OrdinaryToPrimitive (
hint
7.1.2
ToBoolean (
argument
7.1.3
ToNumber (
argument
7.1.3.1
ToNumber Applied to the String Type
7.1.3.1.1
RS: MV
7.1.4
ToInteger (
argument
7.1.5
ToInt32 (
argument
7.1.6
ToUint32 (
argument
7.1.7
ToInt16 (
argument
7.1.8
ToUint16 (
argument
7.1.9
ToInt8 (
argument
7.1.10
ToUint8 (
argument
7.1.11
ToUint8Clamp (
argument
7.1.12
ToString (
argument
7.1.12.1
NumberToString (
7.1.13
ToObject (
argument
7.1.14
ToPropertyKey (
argument
7.1.15
ToLength (
argument
7.1.16
CanonicalNumericIndexString (
argument
7.1.17
ToIndex (
value
7.2
Testing and Comparison Operations
7.2.1
RequireObjectCoercible (
argument
7.2.2
IsArray (
argument
7.2.3
IsCallable (
argument
7.2.4
IsConstructor (
argument
7.2.5
IsExtensible (
7.2.6
IsInteger (
argument
7.2.7
IsPropertyKey (
argument
7.2.8
IsRegExp (
argument
7.2.9
IsStringPrefix (
7.2.10
SameValue (
7.2.11
SameValueZero (
7.2.12
SameValueNonNumber (
7.2.13
Abstract Relational Comparison
7.2.14
Abstract Equality Comparison
7.2.15
Strict Equality Comparison
7.3
Operations on Objects
7.3.1
Get (
7.3.2
GetV (
7.3.3
Set (
Throw
7.3.4
CreateDataProperty (
7.3.5
CreateMethodProperty (
7.3.6
CreateDataPropertyOrThrow (
7.3.7
DefinePropertyOrThrow (
desc
7.3.8
DeletePropertyOrThrow (
7.3.9
GetMethod (
7.3.10
HasProperty (
7.3.11
HasOwnProperty (
7.3.12
Call (
[ ,
argumentsList
] )
7.3.13
Construct (
[ ,
argumentsList
[ ,
newTarget
] ] )
7.3.14
SetIntegrityLevel (
level
7.3.15
TestIntegrityLevel (
level
7.3.16
CreateArrayFromList (
elements
7.3.17
CreateListFromArrayLike (
obj
[ ,
elementTypes
] )
7.3.18
Invoke (
[ ,
argumentsList
] )
7.3.19
OrdinaryHasInstance (
7.3.20
SpeciesConstructor (
defaultConstructor
7.3.21
EnumerableOwnPropertyNames (
kind
7.3.22
GetFunctionRealm (
obj
7.3.23
CopyDataProperties (
target
source
excludedItems
7.4
Operations on Iterator Objects
7.4.1
GetIterator (
obj
[ ,
hint
[ ,
method
] ] )
7.4.2
IteratorNext (
iteratorRecord
[ ,
value
] )
7.4.3
IteratorComplete (
iterResult
7.4.4
IteratorValue (
iterResult
7.4.5
IteratorStep (
iteratorRecord
7.4.6
IteratorClose (
iteratorRecord
completion
7.4.7
AsyncIteratorClose (
iteratorRecord
completion
7.4.8
CreateIterResultObject (
value
done
7.4.9
CreateListIteratorRecord (
list
7.4.9.1
ListIterator next ( )
Executable Code and Execution Contexts
8.1
Lexical Environments
8.1.1
Environment Records
8.1.1.1
Declarative Environment Records
8.1.1.1.1
HasBinding (
8.1.1.1.2
CreateMutableBinding (
8.1.1.1.3
CreateImmutableBinding (
8.1.1.1.4
InitializeBinding (
8.1.1.1.5
SetMutableBinding (
8.1.1.1.6
GetBindingValue (
8.1.1.1.7
DeleteBinding (
8.1.1.1.8
HasThisBinding ( )
8.1.1.1.9
HasSuperBinding ( )
8.1.1.1.10
WithBaseObject ( )
8.1.1.2
Object Environment Records
8.1.1.2.1
HasBinding (
8.1.1.2.2
CreateMutableBinding (
8.1.1.2.3
CreateImmutableBinding (
8.1.1.2.4
InitializeBinding (
8.1.1.2.5
SetMutableBinding (
8.1.1.2.6
GetBindingValue (
8.1.1.2.7
DeleteBinding (
8.1.1.2.8
HasThisBinding ( )
8.1.1.2.9
HasSuperBinding ( )
8.1.1.2.10
WithBaseObject ( )
8.1.1.3
Function Environment Records
8.1.1.3.1
BindThisValue (
8.1.1.3.2
HasThisBinding ( )
8.1.1.3.3
HasSuperBinding ( )
8.1.1.3.4
GetThisBinding ( )
8.1.1.3.5
GetSuperBase ( )
8.1.1.4
Global Environment Records
8.1.1.4.1
HasBinding (
8.1.1.4.2
CreateMutableBinding (
8.1.1.4.3
CreateImmutableBinding (
8.1.1.4.4
InitializeBinding (
8.1.1.4.5
SetMutableBinding (
8.1.1.4.6
GetBindingValue (
8.1.1.4.7
DeleteBinding (
8.1.1.4.8
HasThisBinding ( )
8.1.1.4.9
HasSuperBinding ( )
8.1.1.4.10
WithBaseObject ( )
8.1.1.4.11
GetThisBinding ( )
8.1.1.4.12
HasVarDeclaration (
8.1.1.4.13
HasLexicalDeclaration (
8.1.1.4.14
HasRestrictedGlobalProperty (
8.1.1.4.15
CanDeclareGlobalVar (
8.1.1.4.16
CanDeclareGlobalFunction (
8.1.1.4.17
CreateGlobalVarBinding (
8.1.1.4.18
CreateGlobalFunctionBinding (
8.1.1.5
Module Environment Records
8.1.1.5.1
GetBindingValue (
8.1.1.5.2
DeleteBinding (
8.1.1.5.3
HasThisBinding ( )
8.1.1.5.4
GetThisBinding ( )
8.1.1.5.5
CreateImportBinding (
N2
8.1.2
Lexical Environment Operations
8.1.2.1
GetIdentifierReference (
lex
name
strict
8.1.2.2
NewDeclarativeEnvironment (
8.1.2.3
NewObjectEnvironment (
8.1.2.4
NewFunctionEnvironment (
newTarget
8.1.2.5
NewGlobalEnvironment (
thisValue
8.1.2.6
NewModuleEnvironment (
8.2
Realms
8.2.1
CreateRealm ( )
8.2.2
CreateIntrinsics (
realmRec
8.2.3
SetRealmGlobalObject (
realmRec
globalObj
thisValue
8.2.4
SetDefaultGlobalBindings (
realmRec
8.3
Execution Contexts
8.3.1
GetActiveScriptOrModule ( )
8.3.2
ResolveBinding (
name
[ ,
env
] )
8.3.3
GetThisEnvironment ( )
8.3.4
ResolveThisBinding ( )
8.3.5
GetNewTarget ( )
8.3.6
GetGlobalObject ( )
8.4
Jobs and Job Queues
8.4.1
EnqueueJob (
queueName
job
arguments
8.5
InitializeHostDefinedRealm ( )
8.6
RunJobs ( )
8.7
Agents
8.7.1
AgentSignifier ( )
8.7.2
AgentCanSuspend ( )
8.8
Agent Clusters
8.9
Forward Progress
Ordinary and Exotic Objects Behaviours
9.1
Ordinary Object Internal Methods and Internal Slots
9.1.1
[[GetPrototypeOf]] ( )
9.1.1.1
OrdinaryGetPrototypeOf (
9.1.2
[[SetPrototypeOf]] (
9.1.2.1
OrdinarySetPrototypeOf (
9.1.3
[[IsExtensible]] ( )
9.1.3.1
OrdinaryIsExtensible (
9.1.4
[[PreventExtensions]] ( )
9.1.4.1
OrdinaryPreventExtensions (
9.1.5
[[GetOwnProperty]] (
9.1.5.1
OrdinaryGetOwnProperty (
9.1.6
[[DefineOwnProperty]] (
Desc
9.1.6.1
OrdinaryDefineOwnProperty (
Desc
9.1.6.2
IsCompatiblePropertyDescriptor (
Extensible
Desc
Current
9.1.6.3
ValidateAndApplyPropertyDescriptor (
extensible
Desc
current
9.1.7
[[HasProperty]] (
9.1.7.1
OrdinaryHasProperty (
9.1.8
[[Get]] (
Receiver
9.1.8.1
OrdinaryGet (
Receiver
9.1.9
[[Set]] (
Receiver
9.1.9.1
OrdinarySet (
Receiver
9.1.9.2
OrdinarySetWithOwnDescriptor (
Receiver
ownDesc
9.1.10
[[Delete]] (
9.1.10.1
OrdinaryDelete (
9.1.11
[[OwnPropertyKeys]] ( )
9.1.11.1
OrdinaryOwnPropertyKeys (
9.1.12
ObjectCreate (
proto
[ ,
internalSlotsList
] )
9.1.13
OrdinaryCreateFromConstructor (
constructor
intrinsicDefaultProto
[ ,
internalSlotsList
] )
9.1.14
GetPrototypeFromConstructor (
constructor
intrinsicDefaultProto
9.2
ECMAScript Function Objects
9.2.1
[[Call]] (
thisArgument
argumentsList
9.2.1.1
PrepareForOrdinaryCall (
newTarget
9.2.1.2
OrdinaryCallBindThis (
calleeContext
thisArgument
9.2.1.3
OrdinaryCallEvaluateBody (
argumentsList
9.2.2
[[Construct]] (
argumentsList
newTarget
9.2.3
FunctionAllocate (
functionPrototype
strict
functionKind
9.2.4
FunctionInitialize (
kind
ParameterList
Body
Scope
9.2.5
FunctionCreate (
kind
ParameterList
Body
Scope
Strict
[ ,
prototype
] )
9.2.6
GeneratorFunctionCreate (
kind
ParameterList
Body
Scope
Strict
9.2.7
AsyncGeneratorFunctionCreate (
kind
ParameterList
Body
Scope
Strict
9.2.8
AsyncFunctionCreate (
kind
parameters
body
Scope
Strict
9.2.9
AddRestrictedFunctionProperties (
realm
9.2.9.1
%ThrowTypeError% ( )
9.2.10
MakeConstructor (
[ ,
writablePrototype
[ ,
prototype
] ] )
9.2.11
MakeClassConstructor (
9.2.12
MakeMethod (
homeObject
9.2.13
SetFunctionName (
name
[ ,
prefix
] )
9.2.14
SetFunctionLength (
length
9.2.15
FunctionDeclarationInstantiation (
func
argumentsList
9.3
Built-in Function Objects
9.3.1
[[Call]] (
thisArgument
argumentsList
9.3.2
[[Construct]] (
argumentsList
newTarget
9.3.3
CreateBuiltinFunction (
steps
internalSlotsList
[ ,
realm
[ ,
prototype
] ] )
9.4
Built-in Exotic Object Internal Methods and Slots
9.4.1
Bound Function Exotic Objects
9.4.1.1
[[Call]] (
thisArgument
argumentsList
9.4.1.2
[[Construct]] (
argumentsList
newTarget
9.4.1.3
BoundFunctionCreate (
targetFunction
boundThis
boundArgs
9.4.2
Array Exotic Objects
9.4.2.1
[[DefineOwnProperty]] (
Desc
9.4.2.2
ArrayCreate (
length
[ ,
proto
] )
9.4.2.3
ArraySpeciesCreate (
originalArray
length
9.4.2.4
ArraySetLength (
Desc
9.4.3
String Exotic Objects
9.4.3.1
[[GetOwnProperty]] (
9.4.3.2
[[DefineOwnProperty]] (
Desc
9.4.3.3
[[OwnPropertyKeys]] ( )
9.4.3.4
StringCreate (
value
prototype
9.4.3.5
StringGetOwnProperty (
9.4.4
Arguments Exotic Objects
9.4.4.1
[[GetOwnProperty]] (
9.4.4.2
[[DefineOwnProperty]] (
Desc
9.4.4.3
[[Get]] (
Receiver
9.4.4.4
[[Set]] (
Receiver
9.4.4.5
[[Delete]] (
9.4.4.6
CreateUnmappedArgumentsObject (
argumentsList
9.4.4.7
CreateMappedArgumentsObject (
func
formals
argumentsList
env
9.4.4.7.1
MakeArgGetter (
name
env
9.4.4.7.2
MakeArgSetter (
name
env
9.4.5
Integer-Indexed Exotic Objects
9.4.5.1
[[GetOwnProperty]] (
9.4.5.2
[[HasProperty]] (
9.4.5.3
[[DefineOwnProperty]] (
Desc
9.4.5.4
[[Get]] (
Receiver
9.4.5.5
[[Set]] (
Receiver
9.4.5.6
[[OwnPropertyKeys]] ( )
9.4.5.7
IntegerIndexedObjectCreate (
prototype
internalSlotsList
9.4.5.8
IntegerIndexedElementGet (
index
9.4.5.9
IntegerIndexedElementSet (
index
value
9.4.6
Module Namespace Exotic Objects
9.4.6.1
[[SetPrototypeOf]] (
9.4.6.2
[[IsExtensible]] ( )
9.4.6.3
[[PreventExtensions]] ( )
9.4.6.4
[[GetOwnProperty]] (
9.4.6.5
[[DefineOwnProperty]] (
Desc
9.4.6.6
[[HasProperty]] (
9.4.6.7
[[Get]] (
Receiver
9.4.6.8
[[Set]] (
Receiver
9.4.6.9
[[Delete]] (
9.4.6.10
[[OwnPropertyKeys]] ( )
9.4.6.11
ModuleNamespaceCreate (
module
exports
9.4.7
Immutable Prototype Exotic Objects
9.4.7.1
[[SetPrototypeOf]] (
9.4.7.2
SetImmutablePrototype (
9.5
Proxy Object Internal Methods and Internal Slots
9.5.1
[[GetPrototypeOf]] ( )
9.5.2
[[SetPrototypeOf]] (
9.5.3
[[IsExtensible]] ( )
9.5.4
[[PreventExtensions]] ( )
9.5.5
[[GetOwnProperty]] (
9.5.6
[[DefineOwnProperty]] (
Desc
9.5.7
[[HasProperty]] (
9.5.8
[[Get]] (
Receiver
9.5.9
[[Set]] (
Receiver
9.5.10
[[Delete]] (
9.5.11
[[OwnPropertyKeys]] ( )
9.5.12
[[Call]] (
thisArgument
argumentsList
9.5.13
[[Construct]] (
argumentsList
newTarget
9.5.14
ProxyCreate (
target
handler
10
ECMAScript Language: Source Code
10.1
Source Text
10.1.1
SS: UTF16Encoding (
cp
10.1.2
SS: UTF16Decode (
lead
trail
10.2
Types of Source Code
10.2.1
Strict Mode Code
10.2.2
Non-ECMAScript Functions
11
ECMAScript Language: Lexical Grammar
11.1
Unicode Format-Control Characters
11.2
White Space
11.3
Line Terminators
11.4
Comments
11.5
Tokens
11.6
Names and Keywords
11.6.1
Identifier Names
11.6.1.1
SS: Early Errors
11.6.1.2
SS: StringValue
11.6.2
Reserved Words
11.6.2.1
Keywords
11.6.2.2
Future Reserved Words
11.7
Punctuators
11.8
Literals
11.8.1
Null Literals
11.8.2
Boolean Literals
11.8.3
Numeric Literals
11.8.3.1
SS: MV
11.8.4
String Literals
11.8.4.1
SS: StringValue
11.8.4.2
SS: SV
11.8.5
Regular Expression Literals
11.8.5.1
SS: Early Errors
11.8.5.2
SS: BodyText
11.8.5.3
SS: FlagText
11.8.6
Template Literal Lexical Components
11.8.6.1
SS: TV and TRV
11.9
Automatic Semicolon Insertion
11.9.1
Rules of Automatic Semicolon Insertion
11.9.2
Examples of Automatic Semicolon Insertion
12
ECMAScript Language: Expressions
12.1
Identifiers
12.1.1
SS: Early Errors
12.1.2
SS: BoundNames
12.1.3
SS: AssignmentTargetType
12.1.4
SS: StringValue
12.1.5
RS: BindingInitialization
12.1.5.1
RS: InitializeBoundName (
name
value
environment
12.1.6
RS: Evaluation
12.2
Primary Expression
12.2.1
Semantics
12.2.1.1
SS: CoveredParenthesizedExpression
12.2.1.2
SS: HasName
12.2.1.3
SS: IsFunctionDefinition
12.2.1.4
SS: IsIdentifierRef
12.2.1.5
SS: AssignmentTargetType
12.2.2
The
this
Keyword
12.2.2.1
RS: Evaluation
12.2.3
Identifier Reference
12.2.4
Literals
12.2.4.1
RS: Evaluation
12.2.5
Array Initializer
12.2.5.1
SS: ElisionWidth
12.2.5.2
RS: ArrayAccumulation
12.2.5.3
RS: Evaluation
12.2.6
Object Initializer
12.2.6.1
SS: Early Errors
12.2.6.2
SS: ComputedPropertyContains
12.2.6.3
SS: Contains
12.2.6.4
SS: IsComputedPropertyKey
12.2.6.5
SS: PropName
12.2.6.6
SS: PropertyNameList
12.2.6.7
RS: Evaluation
12.2.6.8
RS: PropertyDefinitionEvaluation
12.2.7
Function Defining Expressions
12.2.8
Regular Expression Literals
12.2.8.1
SS: Early Errors
12.2.8.2
RS: Evaluation
12.2.9
Template Literals
12.2.9.1
SS: Early Errors
12.2.9.2
SS: TemplateStrings
12.2.9.3
RS: ArgumentListEvaluation
12.2.9.4
RS: GetTemplateObject (
templateLiteral
12.2.9.5
RS: SubstitutionEvaluation
12.2.9.6
RS: Evaluation
12.2.10
The Grouping Operator
12.2.10.1
SS: Early Errors
12.2.10.2
SS: IsFunctionDefinition
12.2.10.3
SS: AssignmentTargetType
12.2.10.4
RS: NamedEvaluation
12.2.10.5
RS: Evaluation
12.3
Left-Hand-Side Expressions
12.3.1
Static Semantics
12.3.1.1
SS: CoveredCallExpression
12.3.1.2
SS: Contains
12.3.1.3
SS: IsFunctionDefinition
12.3.1.4
SS: IsDestructuring
12.3.1.5
SS: IsIdentifierRef
12.3.1.6
SS: AssignmentTargetType
12.3.2
Property Accessors
12.3.2.1
RS: Evaluation
12.3.3
The
new
Operator
12.3.3.1
RS: Evaluation
12.3.3.1.1
RS: EvaluateNew (
constructExpr
arguments
12.3.4
Function Calls
12.3.4.1
RS: Evaluation
12.3.4.2
RS: EvaluateCall (
func
ref
arguments
tailPosition
12.3.5
The
super
Keyword
12.3.5.1
RS: Evaluation
12.3.5.2
RS: GetSuperConstructor ( )
12.3.5.3
RS: MakeSuperPropertyReference (
actualThis
propertyKey
strict
12.3.6
Argument Lists
12.3.6.1
RS: ArgumentListEvaluation
12.3.7
Tagged Templates
12.3.7.1
RS: Evaluation
12.3.8
Meta Properties
12.3.8.1
RS: Evaluation
12.4
Update Expressions
12.4.1
SS: Early Errors
12.4.2
SS: IsFunctionDefinition
12.4.3
SS: AssignmentTargetType
12.4.4
Postfix Increment Operator
12.4.4.1
RS: Evaluation
12.4.5
Postfix Decrement Operator
12.4.5.1
RS: Evaluation
12.4.6
Prefix Increment Operator
12.4.6.1
RS: Evaluation
12.4.7
Prefix Decrement Operator
12.4.7.1
RS: Evaluation
12.5
Unary Operators
12.5.1
SS: IsFunctionDefinition
12.5.2
SS: AssignmentTargetType
12.5.3
The
delete
Operator
12.5.3.1
SS: Early Errors
12.5.3.2
RS: Evaluation
12.5.4
The
void
Operator
12.5.4.1
RS: Evaluation
12.5.5
The
typeof
Operator
12.5.5.1
RS: Evaluation
12.5.6
Unary
Operator
12.5.6.1
RS: Evaluation
12.5.7
Unary
Operator
12.5.7.1
RS: Evaluation
12.5.8
Bitwise NOT Operator (
12.5.8.1
RS: Evaluation
12.5.9
Logical NOT Operator (
12.5.9.1
RS: Evaluation
12.6
Exponentiation Operator
12.6.1
SS: IsFunctionDefinition
12.6.2
SS: AssignmentTargetType
12.6.3
RS: Evaluation
12.6.4
Applying the
**
Operator
12.7
Multiplicative Operators
12.7.1
SS: IsFunctionDefinition
12.7.2
SS: AssignmentTargetType
12.7.3
RS: Evaluation
12.7.3.1
Applying the
Operator
12.7.3.2
Applying the
Operator
12.7.3.3
Applying the
Operator
12.8
Additive Operators
12.8.1
SS: IsFunctionDefinition
12.8.2
SS: AssignmentTargetType
12.8.3
The Addition Operator (
12.8.3.1
RS: Evaluation
12.8.4
The Subtraction Operator (
12.8.4.1
RS: Evaluation
12.8.5
Applying the Additive Operators to Numbers
12.9
Bitwise Shift Operators
12.9.1
SS: IsFunctionDefinition
12.9.2
SS: AssignmentTargetType
12.9.3
The Left Shift Operator (
<<
12.9.3.1
RS: Evaluation
12.9.4
The Signed Right Shift Operator (
>>
12.9.4.1
RS: Evaluation
12.9.5
The Unsigned Right Shift Operator (
>>>
12.9.5.1
RS: Evaluation
12.10
Relational Operators
12.10.1
SS: IsFunctionDefinition
12.10.2
SS: AssignmentTargetType
12.10.3
RS: Evaluation
12.10.4
RS: InstanceofOperator (
target
12.11
Equality Operators
12.11.1
SS: IsFunctionDefinition
12.11.2
SS: AssignmentTargetType
12.11.3
RS: Evaluation
12.12
Binary Bitwise Operators
12.12.1
SS: IsFunctionDefinition
12.12.2
SS: AssignmentTargetType
12.12.3
RS: Evaluation
12.13
Binary Logical Operators
12.13.1
SS: IsFunctionDefinition
12.13.2
SS: AssignmentTargetType
12.13.3
RS: Evaluation
12.14
Conditional Operator (
? :
12.14.1
SS: IsFunctionDefinition
12.14.2
SS: AssignmentTargetType
12.14.3
RS: Evaluation
12.15
Assignment Operators
12.15.1
SS: Early Errors
12.15.2
SS: IsFunctionDefinition
12.15.3
SS: AssignmentTargetType
12.15.4
RS: Evaluation
12.15.5
Destructuring Assignment
12.15.5.1
SS: Early Errors
12.15.5.2
RS: DestructuringAssignmentEvaluation
12.15.5.3
RS: PropertyDestructuringAssignmentEvaluation
12.15.5.4
RS: RestDestructuringAssignmentEvaluation
12.15.5.5
RS: IteratorDestructuringAssignmentEvaluation
12.15.5.6
RS: KeyedDestructuringAssignmentEvaluation
12.16
Comma Operator (
12.16.1
SS: IsFunctionDefinition
12.16.2
SS: AssignmentTargetType
12.16.3
RS: Evaluation
13
ECMAScript Language: Statements and Declarations
13.1
Statement Semantics
13.1.1
SS: ContainsDuplicateLabels
13.1.2
SS: ContainsUndefinedBreakTarget
13.1.3
SS: ContainsUndefinedContinueTarget
13.1.4
SS: DeclarationPart
13.1.5
SS: VarDeclaredNames
13.1.6
SS: VarScopedDeclarations
13.1.7
RS: LabelledEvaluation
13.1.8
RS: Evaluation
13.2
Block
13.2.1
SS: Early Errors
13.2.2
SS: ContainsDuplicateLabels
13.2.3
SS: ContainsUndefinedBreakTarget
13.2.4
SS: ContainsUndefinedContinueTarget
13.2.5
SS: LexicallyDeclaredNames
13.2.6
SS: LexicallyScopedDeclarations
13.2.7
SS: TopLevelLexicallyDeclaredNames
13.2.8
SS: TopLevelLexicallyScopedDeclarations
13.2.9
SS: TopLevelVarDeclaredNames
13.2.10
SS: TopLevelVarScopedDeclarations
13.2.11
SS: VarDeclaredNames
13.2.12
SS: VarScopedDeclarations
13.2.13
RS: Evaluation
13.2.14
RS: BlockDeclarationInstantiation (
code
env
13.3
Declarations and the Variable Statement
13.3.1
Let and Const Declarations
13.3.1.1
SS: Early Errors
13.3.1.2
SS: BoundNames
13.3.1.3
SS: IsConstantDeclaration
13.3.1.4
RS: Evaluation
13.3.2
Variable Statement
13.3.2.1
SS: BoundNames
13.3.2.2
SS: VarDeclaredNames
13.3.2.3
SS: VarScopedDeclarations
13.3.2.4
RS: Evaluation
13.3.3
Destructuring Binding Patterns
13.3.3.1
SS: BoundNames
13.3.3.2
SS: ContainsExpression
13.3.3.3
SS: HasInitializer
13.3.3.4
SS: IsSimpleParameterList
13.3.3.5
RS: BindingInitialization
13.3.3.6
RS: PropertyBindingInitialization
13.3.3.7
RS: RestBindingInitialization
13.3.3.8
RS: IteratorBindingInitialization
13.3.3.9
RS: KeyedBindingInitialization
13.4
Empty Statement
13.4.1
RS: Evaluation
13.5
Expression Statement
13.5.1
RS: Evaluation
13.6
The
if
Statement
13.6.1
SS: Early Errors
13.6.2
SS: ContainsDuplicateLabels
13.6.3
SS: ContainsUndefinedBreakTarget
13.6.4
SS: ContainsUndefinedContinueTarget
13.6.5
SS: VarDeclaredNames
13.6.6
SS: VarScopedDeclarations
13.6.7
RS: Evaluation
13.7
Iteration Statements
13.7.1
Semantics
13.7.1.1
SS: Early Errors
13.7.1.2
RS: LoopContinues (
completion
labelSet
13.7.2
The
do
while
Statement
13.7.2.1
SS: ContainsDuplicateLabels
13.7.2.2
SS: ContainsUndefinedBreakTarget
13.7.2.3
SS: ContainsUndefinedContinueTarget
13.7.2.4
SS: VarDeclaredNames
13.7.2.5
SS: VarScopedDeclarations
13.7.2.6
RS: LabelledEvaluation
13.7.3
The
while
Statement
13.7.3.1
SS: ContainsDuplicateLabels
13.7.3.2
SS: ContainsUndefinedBreakTarget
13.7.3.3
SS: ContainsUndefinedContinueTarget
13.7.3.4
SS: VarDeclaredNames
13.7.3.5
SS: VarScopedDeclarations
13.7.3.6
RS: LabelledEvaluation
13.7.4
The
for
Statement
13.7.4.1
SS: Early Errors
13.7.4.2
SS: ContainsDuplicateLabels
13.7.4.3
SS: ContainsUndefinedBreakTarget
13.7.4.4
SS: ContainsUndefinedContinueTarget
13.7.4.5
SS: VarDeclaredNames
13.7.4.6
SS: VarScopedDeclarations
13.7.4.7
RS: LabelledEvaluation
13.7.4.8
RS: ForBodyEvaluation (
test
increment
stmt
perIterationBindings
labelSet
13.7.4.9
RS: CreatePerIterationEnvironment (
perIterationBindings
13.7.5
The
for
in
for
of
, and
for
await
of
Statements
13.7.5.1
SS: Early Errors
13.7.5.2
SS: BoundNames
13.7.5.3
SS: ContainsDuplicateLabels
13.7.5.4
SS: ContainsUndefinedBreakTarget
13.7.5.5
SS: ContainsUndefinedContinueTarget
13.7.5.6
SS: IsDestructuring
13.7.5.7
SS: VarDeclaredNames
13.7.5.8
SS: VarScopedDeclarations
13.7.5.9
RS: BindingInitialization
13.7.5.10
RS: BindingInstantiation
13.7.5.11
RS: LabelledEvaluation
13.7.5.12
RS: ForIn/OfHeadEvaluation (
TDZnames
expr
iterationKind
13.7.5.13
RS: ForIn/OfBodyEvaluation (
lhs
stmt
iteratorRecord
iterationKind
lhsKind
labelSet
[ ,
iteratorKind
] )
13.7.5.14
RS: Evaluation
13.7.5.15
EnumerateObjectProperties (
13.8
The
continue
Statement
13.8.1
SS: Early Errors
13.8.2
SS: ContainsUndefinedContinueTarget
13.8.3
RS: Evaluation
13.9
The
break
Statement
13.9.1
SS: Early Errors
13.9.2
SS: ContainsUndefinedBreakTarget
13.9.3
RS: Evaluation
13.10
The
return
Statement
13.10.1
RS: Evaluation
13.11
The
with
Statement
13.11.1
SS: Early Errors
13.11.2
SS: ContainsDuplicateLabels
13.11.3
SS: ContainsUndefinedBreakTarget
13.11.4
SS: ContainsUndefinedContinueTarget
13.11.5
SS: VarDeclaredNames
13.11.6
SS: VarScopedDeclarations
13.11.7
RS: Evaluation
13.12
The
switch
Statement
13.12.1
SS: Early Errors
13.12.2
SS: ContainsDuplicateLabels
13.12.3
SS: ContainsUndefinedBreakTarget
13.12.4
SS: ContainsUndefinedContinueTarget
13.12.5
SS: LexicallyDeclaredNames
13.12.6
SS: LexicallyScopedDeclarations
13.12.7
SS: VarDeclaredNames
13.12.8
SS: VarScopedDeclarations
13.12.9
RS: CaseBlockEvaluation
13.12.10
RS: CaseClauseIsSelected (
input
13.12.11
RS: Evaluation
13.13
Labelled Statements
13.13.1
SS: Early Errors
13.13.2
SS: ContainsDuplicateLabels
13.13.3
SS: ContainsUndefinedBreakTarget
13.13.4
SS: ContainsUndefinedContinueTarget
13.13.5
SS: IsLabelledFunction (
stmt
13.13.6
SS: LexicallyDeclaredNames
13.13.7
SS: LexicallyScopedDeclarations
13.13.8
SS: TopLevelLexicallyDeclaredNames
13.13.9
SS: TopLevelLexicallyScopedDeclarations
13.13.10
SS: TopLevelVarDeclaredNames
13.13.11
SS: TopLevelVarScopedDeclarations
13.13.12
SS: VarDeclaredNames
13.13.13
SS: VarScopedDeclarations
13.13.14
RS: LabelledEvaluation
13.13.15
RS: Evaluation
13.14
The
throw
Statement
13.14.1
RS: Evaluation
13.15
The
try
Statement
13.15.1
SS: Early Errors
13.15.2
SS: ContainsDuplicateLabels
13.15.3
SS: ContainsUndefinedBreakTarget
13.15.4
SS: ContainsUndefinedContinueTarget
13.15.5
SS: VarDeclaredNames
13.15.6
SS: VarScopedDeclarations
13.15.7
RS: CatchClauseEvaluation
13.15.8
RS: Evaluation
13.16
The
debugger
Statement
13.16.1
RS: Evaluation
14
ECMAScript Language: Functions and Classes
14.1
Function Definitions
14.1.1
Directive Prologues and the Use Strict Directive
14.1.2
SS: Early Errors
14.1.3
SS: BoundNames
14.1.4
SS: Contains
14.1.5
SS: ContainsExpression
14.1.6
SS: ContainsUseStrict
14.1.7
SS: ExpectedArgumentCount
14.1.8
SS: HasInitializer
14.1.9
SS: HasName
14.1.10
SS: IsAnonymousFunctionDefinition (
expr
14.1.11
SS: IsConstantDeclaration
14.1.12
SS: IsFunctionDefinition
14.1.13
SS: IsSimpleParameterList
14.1.14
SS: LexicallyDeclaredNames
14.1.15
SS: LexicallyScopedDeclarations
14.1.16
SS: VarDeclaredNames
14.1.17
SS: VarScopedDeclarations
14.1.18
RS: EvaluateBody
14.1.19
RS: IteratorBindingInitialization
14.1.20
RS: InstantiateFunctionObject
14.1.21
RS: NamedEvaluation
14.1.22
RS: Evaluation
14.2
Arrow Function Definitions
14.2.1
SS: Early Errors
14.2.2
SS: BoundNames
14.2.3
SS: Contains
14.2.4
SS: ContainsExpression
14.2.5
SS: ContainsUseStrict
14.2.6
SS: ExpectedArgumentCount
14.2.7
SS: HasName
14.2.8
SS: IsSimpleParameterList
14.2.9
SS: CoveredFormalsList
14.2.10
SS: LexicallyDeclaredNames
14.2.11
SS: LexicallyScopedDeclarations
14.2.12
SS: VarDeclaredNames
14.2.13
SS: VarScopedDeclarations
14.2.14
RS: IteratorBindingInitialization
14.2.15
RS: EvaluateBody
14.2.16
RS: NamedEvaluation
14.2.17
RS: Evaluation
14.3
Method Definitions
14.3.1
SS: Early Errors
14.3.2
SS: ComputedPropertyContains
14.3.3
SS: ExpectedArgumentCount
14.3.4
SS: HasDirectSuper
14.3.5
SS: PropName
14.3.6
SS: SpecialMethod
14.3.7
RS: DefineMethod
14.3.8
RS: PropertyDefinitionEvaluation
14.4
Generator Function Definitions
14.4.1
SS: Early Errors
14.4.2
SS: BoundNames
14.4.3
SS: ComputedPropertyContains
14.4.4
SS: Contains
14.4.5
SS: HasDirectSuper
14.4.6
SS: HasName
14.4.7
SS: IsConstantDeclaration
14.4.8
SS: IsFunctionDefinition
14.4.9
SS: PropName
14.4.10
RS: EvaluateBody
14.4.11
RS: InstantiateFunctionObject
14.4.12
RS: PropertyDefinitionEvaluation
14.4.13
RS: NamedEvaluation
14.4.14
RS: Evaluation
14.5
Async Generator Function Definitions
14.5.1
SS: Early Errors
14.5.2
SS: BoundNames
14.5.3
SS: ComputedPropertyContains
14.5.4
SS: Contains
14.5.5
SS: HasDirectSuper
14.5.6
SS: HasName
14.5.7
SS: IsConstantDeclaration
14.5.8
SS: IsFunctionDefinition
14.5.9
SS: PropName
14.5.10
RS: EvaluateBody
14.5.11
RS: InstantiateFunctionObject
14.5.12
RS: PropertyDefinitionEvaluation
14.5.13
RS: NamedEvaluation
14.5.14
RS: Evaluation
14.6
Class Definitions
14.6.1
SS: Early Errors
14.6.2
SS: BoundNames
14.6.3
SS: ConstructorMethod
14.6.4
SS: Contains
14.6.5
SS: ComputedPropertyContains
14.6.6
SS: HasName
14.6.7
SS: IsConstantDeclaration
14.6.8
SS: IsFunctionDefinition
14.6.9
SS: IsStatic
14.6.10
SS: NonConstructorMethodDefinitions
14.6.11
SS: PrototypePropertyNameList
14.6.12
SS: PropName
14.6.13
RS: ClassDefinitionEvaluation
14.6.14
RS: BindingClassDeclarationEvaluation
14.6.15
RS: NamedEvaluation
14.6.16
RS: Evaluation
14.7
Async Function Definitions
14.7.1
SS: Early Errors
14.7.2
SS: BoundNames
14.7.3
SS: ComputedPropertyContains
14.7.4
SS: Contains
14.7.5
SS: HasDirectSuper
14.7.6
SS: HasName
14.7.7
SS: IsConstantDeclaration
14.7.8
SS: IsFunctionDefinition
14.7.9
SS: PropName
14.7.10
RS: InstantiateFunctionObject
14.7.11
RS: EvaluateBody
14.7.12
RS: PropertyDefinitionEvaluation
14.7.13
RS: NamedEvaluation
14.7.14
RS: Evaluation
14.8
Async Arrow Function Definitions
14.8.1
SS: Early Errors
14.8.2
SS: CoveredAsyncArrowHead
14.8.3
SS: BoundNames
14.8.4
SS: Contains
14.8.5
SS: ContainsExpression
14.8.6
SS: ExpectedArgumentCount
14.8.7
SS: HasName
14.8.8
SS: IsSimpleParameterList
14.8.9
SS: LexicallyDeclaredNames
14.8.10
SS: LexicallyScopedDeclarations
14.8.11
SS: VarDeclaredNames
14.8.12
SS: VarScopedDeclarations
14.8.13
RS: IteratorBindingInitialization
14.8.14
RS: EvaluateBody
14.8.15
RS: NamedEvaluation
14.8.16
RS: Evaluation
14.9
Tail Position Calls
14.9.1
SS: IsInTailPosition (
call
14.9.2
SS: HasCallInTailPosition
14.9.2.1
Statement Rules
14.9.2.2
Expression Rules
14.9.3
RS: PrepareForTailCall ( )
15
ECMAScript Language: Scripts and Modules
15.1
Scripts
15.1.1
SS: Early Errors
15.1.2
SS: IsStrict
15.1.3
SS: LexicallyDeclaredNames
15.1.4
SS: LexicallyScopedDeclarations
15.1.5
SS: VarDeclaredNames
15.1.6
SS: VarScopedDeclarations
15.1.7
RS: Evaluation
15.1.8
Script Records
15.1.9
ParseScript (
sourceText
realm
hostDefined
15.1.10
ScriptEvaluation (
scriptRecord
15.1.11
RS: GlobalDeclarationInstantiation (
script
env
15.1.12
RS: ScriptEvaluationJob (
sourceText
hostDefined
15.2
Modules
15.2.1
Module Semantics
15.2.1.1
SS: Early Errors
15.2.1.2
SS: ContainsDuplicateLabels
15.2.1.3
SS: ContainsUndefinedBreakTarget
15.2.1.4
SS: ContainsUndefinedContinueTarget
15.2.1.5
SS: ExportedBindings
15.2.1.6
SS: ExportedNames
15.2.1.7
SS: ExportEntries
15.2.1.8
SS: ImportEntries
15.2.1.9
SS: ImportedLocalNames (
importEntries
15.2.1.10
SS: ModuleRequests
15.2.1.11
SS: LexicallyDeclaredNames
15.2.1.12
SS: LexicallyScopedDeclarations
15.2.1.13
SS: VarDeclaredNames
15.2.1.14
SS: VarScopedDeclarations
15.2.1.15
Abstract Module Records
15.2.1.16
Cyclic Module Records
15.2.1.16.1
Instantiate ( ) Concrete Method
15.2.1.16.1.1
InnerModuleInstantiation (
module
stack
index
15.2.1.16.2
Evaluate ( ) Concrete Method
15.2.1.16.2.1
InnerModuleEvaluation (
module
stack
index
15.2.1.16.3
Example Cyclic Module Record Graphs
15.2.1.17
Source Text Module Records
15.2.1.17.1
ParseModule (
sourceText
realm
hostDefined
15.2.1.17.2
GetExportedNames (
exportStarSet
) Concrete Method
15.2.1.17.3
ResolveExport (
exportName
resolveSet
) Concrete Method
15.2.1.17.4
InitializeEnvironment ( ) Concrete Method
15.2.1.17.5
ExecuteModule ( ) Concrete Method
15.2.1.18
RS: HostResolveImportedModule (
referencingModule
specifier
15.2.1.19
RS: GetModuleNamespace (
module
15.2.1.20
RS: TopLevelModuleEvaluationJob (
sourceText
hostDefined
15.2.1.21
RS: Evaluation
15.2.2
Imports
15.2.2.1
SS: Early Errors
15.2.2.2
SS: BoundNames
15.2.2.3
SS: ImportEntries
15.2.2.4
SS: ImportEntriesForModule
15.2.2.5
SS: ModuleRequests
15.2.3
Exports
15.2.3.1
SS: Early Errors
15.2.3.2
SS: BoundNames
15.2.3.3
SS: ExportedBindings
15.2.3.4
SS: ExportedNames
15.2.3.5
SS: ExportEntries
15.2.3.6
SS: ExportEntriesForModule
15.2.3.7
SS: IsConstantDeclaration
15.2.3.8
SS: LexicallyScopedDeclarations
15.2.3.9
SS: ModuleRequests
15.2.3.10
SS: ReferencedBindings
15.2.3.11
RS: Evaluation
16
Error Handling and Language Extensions
16.1
HostReportErrors (
errorList
16.2
Forbidden Extensions
17
ECMAScript Standard Built-in Objects
18
The Global Object
18.1
Value Properties of the Global Object
18.1.1
Infinity
18.1.2
NaN
18.1.3
undefined
18.2
Function Properties of the Global Object
18.2.1
eval (
18.2.1.1
RS: PerformEval (
evalRealm
strictCaller
direct
18.2.1.1.1
Additional Early Error Rules for Eval Outside Functions
18.2.1.1.2
Additional Early Error Rules for Eval Outside Methods
18.2.1.1.3
Additional Early Error Rules for Eval Outside Constructor Methods
18.2.1.2
HostEnsureCanCompileStrings (
callerRealm
calleeRealm
18.2.1.3
RS: EvalDeclarationInstantiation (
body
varEnv
lexEnv
strict
18.2.2
isFinite (
number
18.2.3
isNaN (
number
18.2.4
parseFloat (
string
18.2.5
parseInt (
string
radix
18.2.6
URI Handling Functions
18.2.6.1
URI Syntax and Semantics
18.2.6.1.1
RS: Encode (
string
unescapedSet
18.2.6.1.2
RS: Decode (
string
reservedSet
18.2.6.2
decodeURI (
encodedURI
18.2.6.3
decodeURIComponent (
encodedURIComponent
18.2.6.4
encodeURI (
uri
18.2.6.5
encodeURIComponent (
uriComponent
18.3
Constructor Properties of the Global Object
18.3.1
Array ( . . . )
18.3.2
ArrayBuffer ( . . . )
18.3.3
Boolean ( . . . )
18.3.4
DataView ( . . . )
18.3.5
Date ( . . . )
18.3.6
Error ( . . . )
18.3.7
EvalError ( . . . )
18.3.8
Float32Array ( . . . )
18.3.9
Float64Array ( . . . )
18.3.10
Function ( . . . )
18.3.11
Int8Array ( . . . )
18.3.12
Int16Array ( . . . )
18.3.13
Int32Array ( . . . )
18.3.14
Map ( . . . )
18.3.15
Number ( . . . )
18.3.16
Object ( . . . )
18.3.17
Promise ( . . . )
18.3.18
Proxy ( . . . )
18.3.19
RangeError ( . . . )
18.3.20
ReferenceError ( . . . )
18.3.21
RegExp ( . . . )
18.3.22
Set ( . . . )
18.3.23
SharedArrayBuffer ( . . . )
18.3.24
String ( . . . )
18.3.25
Symbol ( . . . )
18.3.26
SyntaxError ( . . . )
18.3.27
TypeError ( . . . )
18.3.28
Uint8Array ( . . . )
18.3.29
Uint8ClampedArray ( . . . )
18.3.30
Uint16Array ( . . . )
18.3.31
Uint32Array ( . . . )
18.3.32
URIError ( . . . )
18.3.33
WeakMap ( . . . )
18.3.34
WeakSet ( . . . )
18.4
Other Properties of the Global Object
18.4.1
Atomics
18.4.2
JSON
18.4.3
Math
18.4.4
Reflect
19
Fundamental Objects
19.1
Object Objects
19.1.1
The Object Constructor
19.1.1.1
Object ( [
value
] )
19.1.2
Properties of the Object Constructor
19.1.2.1
Object.assign (
target
, ...
sources
19.1.2.2
Object.create (
Properties
19.1.2.3
Object.defineProperties (
Properties
19.1.2.3.1
RS: ObjectDefineProperties (
Properties
19.1.2.4
Object.defineProperty (
Attributes
19.1.2.5
Object.entries (
19.1.2.6
Object.freeze (
19.1.2.7
Object.fromEntries (
iterable
19.1.2.7.1
CreateDataPropertyOnObject Functions
19.1.2.8
Object.getOwnPropertyDescriptor (
19.1.2.9
Object.getOwnPropertyDescriptors (
19.1.2.10
Object.getOwnPropertyNames (
19.1.2.11
Object.getOwnPropertySymbols (
19.1.2.11.1
RS: GetOwnPropertyKeys (
type
19.1.2.12
Object.getPrototypeOf (
19.1.2.13
Object.is (
value1
value2
19.1.2.14
Object.isExtensible (
19.1.2.15
Object.isFrozen (
19.1.2.16
Object.isSealed (
19.1.2.17
Object.keys (
19.1.2.18
Object.preventExtensions (
19.1.2.19
Object.prototype
19.1.2.20
Object.seal (
19.1.2.21
Object.setPrototypeOf (
proto
19.1.2.22
Object.values (
19.1.3
Properties of the Object Prototype Object
19.1.3.1
Object.prototype.constructor
19.1.3.2
Object.prototype.hasOwnProperty (
19.1.3.3
Object.prototype.isPrototypeOf (
19.1.3.4
Object.prototype.propertyIsEnumerable (
19.1.3.5
Object.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
19.1.3.6
Object.prototype.toString ( )
19.1.3.7
Object.prototype.valueOf ( )
19.1.4
Properties of Object Instances
19.2
Function Objects
19.2.1
The Function Constructor
19.2.1.1
Function (
p1
p2
, … ,
pn
body
19.2.1.1.1
RS: CreateDynamicFunction (
constructor
newTarget
kind
args
19.2.2
Properties of the Function Constructor
19.2.2.1
Function.length
19.2.2.2
Function.prototype
19.2.3
Properties of the Function Prototype Object
19.2.3.1
Function.prototype.apply (
thisArg
argArray
19.2.3.2
Function.prototype.bind (
thisArg
, ...
args
19.2.3.3
Function.prototype.call (
thisArg
, ...
args
19.2.3.4
Function.prototype.constructor
19.2.3.5
Function.prototype.toString ( )
19.2.3.6
Function.prototype [ @@hasInstance ] (
19.2.4
Function Instances
19.2.4.1
length
19.2.4.2
name
19.2.4.3
prototype
19.2.5
HostHasSourceTextAvailable (
func
19.3
Boolean Objects
19.3.1
The Boolean Constructor
19.3.1.1
Boolean (
value
19.3.2
Properties of the Boolean Constructor
19.3.2.1
Boolean.prototype
19.3.3
Properties of the Boolean Prototype Object
19.3.3.1
Boolean.prototype.constructor
19.3.3.2
Boolean.prototype.toString ( )
19.3.3.3
Boolean.prototype.valueOf ( )
19.3.4
Properties of Boolean Instances
19.4
Symbol Objects
19.4.1
The Symbol Constructor
19.4.1.1
Symbol ( [
description
] )
19.4.2
Properties of the Symbol Constructor
19.4.2.1
Symbol.asyncIterator
19.4.2.2
Symbol.for (
key
19.4.2.3
Symbol.hasInstance
19.4.2.4
Symbol.isConcatSpreadable
19.4.2.5
Symbol.iterator
19.4.2.6
Symbol.keyFor (
sym
19.4.2.7
Symbol.match
19.4.2.8
Symbol.prototype
19.4.2.9
Symbol.replace
19.4.2.10
Symbol.search
19.4.2.11
Symbol.species
19.4.2.12
Symbol.split
19.4.2.13
Symbol.toPrimitive
19.4.2.14
Symbol.toStringTag
19.4.2.15
Symbol.unscopables
19.4.3
Properties of the Symbol Prototype Object
19.4.3.1
Symbol.prototype.constructor
19.4.3.2
get Symbol.prototype.description
19.4.3.3
Symbol.prototype.toString ( )
19.4.3.3.1
RS: SymbolDescriptiveString (
sym
19.4.3.4
Symbol.prototype.valueOf ( )
19.4.3.5
Symbol.prototype [ @@toPrimitive ] (
hint
19.4.3.6
Symbol.prototype [ @@toStringTag ]
19.4.4
Properties of Symbol Instances
19.5
Error Objects
19.5.1
The Error Constructor
19.5.1.1
Error (
message
19.5.2
Properties of the Error Constructor
19.5.2.1
Error.prototype
19.5.3
Properties of the Error Prototype Object
19.5.3.1
Error.prototype.constructor
19.5.3.2
Error.prototype.message
19.5.3.3
Error.prototype.name
19.5.3.4
Error.prototype.toString ( )
19.5.4
Properties of Error Instances
19.5.5
Native Error Types Used in This Standard
19.5.5.1
EvalError
19.5.5.2
RangeError
19.5.5.3
ReferenceError
19.5.5.4
SyntaxError
19.5.5.5
TypeError
19.5.5.6
URIError
19.5.6
NativeError
Object Structure
19.5.6.1
The
NativeError
Constructors
19.5.6.1.1
NativeError (
message
19.5.6.2
Properties of the
NativeError
Constructors
19.5.6.2.1
NativeError.prototype
19.5.6.3
Properties of the
NativeError
Prototype Objects
19.5.6.3.1
NativeError
.prototype.constructor
19.5.6.3.2
NativeError
.prototype.message
19.5.6.3.3
NativeError
.prototype.name
19.5.6.4
Properties of
NativeError
Instances
20
Numbers and Dates
20.1
Number Objects
20.1.1
The Number Constructor
20.1.1.1
Number (
value
20.1.2
Properties of the Number Constructor
20.1.2.1
Number.EPSILON
20.1.2.2
Number.isFinite (
number
20.1.2.3
Number.isInteger (
number
20.1.2.4
Number.isNaN (
number
20.1.2.5
Number.isSafeInteger (
number
20.1.2.6
Number.MAX_SAFE_INTEGER
20.1.2.7
Number.MAX_VALUE
20.1.2.8
Number.MIN_SAFE_INTEGER
20.1.2.9
Number.MIN_VALUE
20.1.2.10
Number.NaN
20.1.2.11
Number.NEGATIVE_INFINITY
20.1.2.12
Number.parseFloat (
string
20.1.2.13
Number.parseInt (
string
radix
20.1.2.14
Number.POSITIVE_INFINITY
20.1.2.15
Number.prototype
20.1.3
Properties of the Number Prototype Object
20.1.3.1
Number.prototype.constructor
20.1.3.2
Number.prototype.toExponential (
fractionDigits
20.1.3.3
Number.prototype.toFixed (
fractionDigits
20.1.3.4
Number.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
20.1.3.5
Number.prototype.toPrecision (
precision
20.1.3.6
Number.prototype.toString ( [
radix
] )
20.1.3.7
Number.prototype.valueOf ( )
20.1.4
Properties of Number Instances
20.2
The Math Object
20.2.1
Value Properties of the Math Object
20.2.1.1
Math.E
20.2.1.2
Math.LN10
20.2.1.3
Math.LN2
20.2.1.4
Math.LOG10E
20.2.1.5
Math.LOG2E
20.2.1.6
Math.PI
20.2.1.7
Math.SQRT1_2
20.2.1.8
Math.SQRT2
20.2.1.9
Math [ @@toStringTag ]
20.2.2
Function Properties of the Math Object
20.2.2.1
Math.abs (
20.2.2.2
Math.acos (
20.2.2.3
Math.acosh (
20.2.2.4
Math.asin (
20.2.2.5
Math.asinh (
20.2.2.6
Math.atan (
20.2.2.7
Math.atanh (
20.2.2.8
Math.atan2 (
20.2.2.9
Math.cbrt (
20.2.2.10
Math.ceil (
20.2.2.11
Math.clz32 (
20.2.2.12
Math.cos (
20.2.2.13
Math.cosh (
20.2.2.14
Math.exp (
20.2.2.15
Math.expm1 (
20.2.2.16
Math.floor (
20.2.2.17
Math.fround (
20.2.2.18
Math.hypot (
value1
value2
, ...
values
20.2.2.19
Math.imul (
20.2.2.20
Math.log (
20.2.2.21
Math.log1p (
20.2.2.22
Math.log10 (
20.2.2.23
Math.log2 (
20.2.2.24
Math.max (
value1
value2
, ...
values
20.2.2.25
Math.min (
value1
value2
, ...
values
20.2.2.26
Math.pow (
base
exponent
20.2.2.27
Math.random ( )
20.2.2.28
Math.round (
20.2.2.29
Math.sign (
20.2.2.30
Math.sin (
20.2.2.31
Math.sinh (
20.2.2.32
Math.sqrt (
20.2.2.33
Math.tan (
20.2.2.34
Math.tanh (
20.2.2.35
Math.trunc (
20.3
Date Objects
20.3.1
Overview of Date Objects and Definitions of Abstract Operations
20.3.1.1
Time Values and Time Range
20.3.1.2
Day Number and Time within Day
20.3.1.3
Year Number
20.3.1.4
Month Number
20.3.1.5
Date Number
20.3.1.6
Week Day
20.3.1.7
LocalTZA (
isUTC
20.3.1.8
LocalTime (
20.3.1.9
UTC (
20.3.1.10
Hours, Minutes, Second, and Milliseconds
20.3.1.11
MakeTime (
hour
min
sec
ms
20.3.1.12
MakeDay (
year
month
date
20.3.1.13
MakeDate (
day
time
20.3.1.14
TimeClip (
time
20.3.1.15
Date Time String Format
20.3.1.15.1
Expanded Years
20.3.2
The Date Constructor
20.3.2.1
Date (
year
month
[ ,
date
[ ,
hours
[ ,
minutes
[ ,
seconds
[ ,
ms
] ] ] ] ] )
20.3.2.2
Date (
value
20.3.2.3
Date ( )
20.3.3
Properties of the Date Constructor
20.3.3.1
Date.now ( )
20.3.3.2
Date.parse (
string
20.3.3.3
Date.prototype
20.3.3.4
Date.UTC (
year
[ ,
month
[ ,
date
[ ,
hours
[ ,
minutes
[ ,
seconds
[ ,
ms
] ] ] ] ] ] )
20.3.4
Properties of the Date Prototype Object
20.3.4.1
Date.prototype.constructor
20.3.4.2
Date.prototype.getDate ( )
20.3.4.3
Date.prototype.getDay ( )
20.3.4.4
Date.prototype.getFullYear ( )
20.3.4.5
Date.prototype.getHours ( )
20.3.4.6
Date.prototype.getMilliseconds ( )
20.3.4.7
Date.prototype.getMinutes ( )
20.3.4.8
Date.prototype.getMonth ( )
20.3.4.9
Date.prototype.getSeconds ( )
20.3.4.10
Date.prototype.getTime ( )
20.3.4.11
Date.prototype.getTimezoneOffset ( )
20.3.4.12
Date.prototype.getUTCDate ( )
20.3.4.13
Date.prototype.getUTCDay ( )
20.3.4.14
Date.prototype.getUTCFullYear ( )
20.3.4.15
Date.prototype.getUTCHours ( )
20.3.4.16
Date.prototype.getUTCMilliseconds ( )
20.3.4.17
Date.prototype.getUTCMinutes ( )
20.3.4.18
Date.prototype.getUTCMonth ( )
20.3.4.19
Date.prototype.getUTCSeconds ( )
20.3.4.20
Date.prototype.setDate (
date
20.3.4.21
Date.prototype.setFullYear (
year
[ ,
month
[ ,
date
] ] )
20.3.4.22
Date.prototype.setHours (
hour
[ ,
min
[ ,
sec
[ ,
ms
] ] ] )
20.3.4.23
Date.prototype.setMilliseconds (
ms
20.3.4.24
Date.prototype.setMinutes (
min
[ ,
sec
[ ,
ms
] ] )
20.3.4.25
Date.prototype.setMonth (
month
[ ,
date
] )
20.3.4.26
Date.prototype.setSeconds (
sec
[ ,
ms
] )
20.3.4.27
Date.prototype.setTime (
time
20.3.4.28
Date.prototype.setUTCDate (
date
20.3.4.29
Date.prototype.setUTCFullYear (
year
[ ,
month
[ ,
date
] ] )
20.3.4.30
Date.prototype.setUTCHours (
hour
[ ,
min
[ ,
sec
[ ,
ms
] ] ] )
20.3.4.31
Date.prototype.setUTCMilliseconds (
ms
20.3.4.32
Date.prototype.setUTCMinutes (
min
[ ,
sec
[ ,
ms
] ] )
20.3.4.33
Date.prototype.setUTCMonth (
month
[ ,
date
] )
20.3.4.34
Date.prototype.setUTCSeconds (
sec
[ ,
ms
] )
20.3.4.35
Date.prototype.toDateString ( )
20.3.4.36
Date.prototype.toISOString ( )
20.3.4.37
Date.prototype.toJSON (
key
20.3.4.38
Date.prototype.toLocaleDateString ( [
reserved1
[ ,
reserved2
] ] )
20.3.4.39
Date.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
20.3.4.40
Date.prototype.toLocaleTimeString ( [
reserved1
[ ,
reserved2
] ] )
20.3.4.41
Date.prototype.toString ( )
20.3.4.41.1
RS: TimeString (
tv
20.3.4.41.2
RS: DateString (
tv
20.3.4.41.3
RS: TimeZoneString (
tv
20.3.4.41.4
RS: ToDateString (
tv
20.3.4.42
Date.prototype.toTimeString ( )
20.3.4.43
Date.prototype.toUTCString ( )
20.3.4.44
Date.prototype.valueOf ( )
20.3.4.45
Date.prototype [ @@toPrimitive ] (
hint
20.3.5
Properties of Date Instances
21
Text Processing
21.1
String Objects
21.1.1
The String Constructor
21.1.1.1
String (
value
21.1.2
Properties of the String Constructor
21.1.2.1
String.fromCharCode ( ...
codeUnits
21.1.2.2
String.fromCodePoint ( ...
codePoints
21.1.2.3
String.prototype
21.1.2.4
String.raw (
template
, ...
substitutions
21.1.3
Properties of the String Prototype Object
21.1.3.1
String.prototype.charAt (
pos
21.1.3.2
String.prototype.charCodeAt (
pos
21.1.3.3
String.prototype.codePointAt (
pos
21.1.3.4
String.prototype.concat ( ...
args
21.1.3.5
String.prototype.constructor
21.1.3.6
String.prototype.endsWith (
searchString
[ ,
endPosition
] )
21.1.3.7
String.prototype.includes (
searchString
[ ,
position
] )
21.1.3.8
String.prototype.indexOf (
searchString
[ ,
position
] )
21.1.3.9
String.prototype.lastIndexOf (
searchString
[ ,
position
] )
21.1.3.10
String.prototype.localeCompare (
that
[ ,
reserved1
[ ,
reserved2
] ] )
21.1.3.11
String.prototype.match (
regexp
21.1.3.12
String.prototype.normalize ( [
form
] )
21.1.3.13
String.prototype.padEnd (
maxLength
[ ,
fillString
] )
21.1.3.14
String.prototype.padStart (
maxLength
[ ,
fillString
] )
21.1.3.15
String.prototype.repeat (
count
21.1.3.16
String.prototype.replace (
searchValue
replaceValue
21.1.3.16.1
RS: GetSubstitution (
matched
str
position
captures
namedCaptures
replacement
21.1.3.17
String.prototype.search (
regexp
21.1.3.18
String.prototype.slice (
start
end
21.1.3.19
String.prototype.split (
separator
limit
21.1.3.19.1
RS: SplitMatch (
21.1.3.20
String.prototype.startsWith (
searchString
[ ,
position
] )
21.1.3.21
String.prototype.substring (
start
end
21.1.3.22
String.prototype.toLocaleLowerCase ( [
reserved1
[ ,
reserved2
] ] )
21.1.3.23
String.prototype.toLocaleUpperCase ( [
reserved1
[ ,
reserved2
] ] )
21.1.3.24
String.prototype.toLowerCase ( )
21.1.3.25
String.prototype.toString ( )
21.1.3.26
String.prototype.toUpperCase ( )
21.1.3.27
String.prototype.trim ( )
21.1.3.27.1
RS: TrimString (
string
where
21.1.3.28
String.prototype.trimEnd ( )
21.1.3.29
String.prototype.trimStart ( )
21.1.3.30
String.prototype.valueOf ( )
21.1.3.31
String.prototype [ @@iterator ] ( )
21.1.4
Properties of String Instances
21.1.4.1
length
21.1.5
String Iterator Objects
21.1.5.1
CreateStringIterator (
string
21.1.5.2
The %StringIteratorPrototype% Object
21.1.5.2.1
%StringIteratorPrototype%.next ( )
21.1.5.2.2
%StringIteratorPrototype% [ @@toStringTag ]
21.1.5.3
Properties of String Iterator Instances
21.2
RegExp (Regular Expression) Objects
21.2.1
Patterns
21.2.1.1
SS: Early Errors
21.2.1.2
SS: CapturingGroupNumber
21.2.1.3
SS: IsCharacterClass
21.2.1.4
SS: CharacterValue
21.2.1.5
SS: SourceText
21.2.1.6
SS: StringValue
21.2.2
Pattern Semantics
21.2.2.1
Notation
21.2.2.2
Pattern
21.2.2.3
Disjunction
21.2.2.4
Alternative
21.2.2.5
Term
21.2.2.5.1
RS: RepeatMatcher (
min
max
greedy
parenIndex
parenCount
21.2.2.6
Assertion
21.2.2.6.1
RS: WordCharacters ( )
21.2.2.6.2
RS: IsWordChar (
21.2.2.7
Quantifier
21.2.2.8
Atom
21.2.2.8.1
RS: CharacterSetMatcher (
invert
direction
21.2.2.8.2
RS: Canonicalize (
ch
21.2.2.8.3
RS: UnicodeMatchProperty (
21.2.2.8.4
RS: UnicodeMatchPropertyValue (
21.2.2.9
AtomEscape
21.2.2.9.1
RS: BackreferenceMatcher (
direction
21.2.2.10
CharacterEscape
21.2.2.11
DecimalEscape
21.2.2.12
CharacterClassEscape
21.2.2.13
CharacterClass
21.2.2.14
ClassRanges
21.2.2.15
NonemptyClassRanges
21.2.2.15.1
RS: CharacterRange (
21.2.2.16
NonemptyClassRangesNoDash
21.2.2.17
ClassAtom
21.2.2.18
ClassAtomNoDash
21.2.2.19
ClassEscape
21.2.3
The RegExp Constructor
21.2.3.1
RegExp (
pattern
flags
21.2.3.2
Abstract Operations for the RegExp Constructor
21.2.3.2.1
RS: RegExpAlloc (
newTarget
21.2.3.2.2
RS: RegExpInitialize (
obj
pattern
flags
21.2.3.2.3
RS: RegExpCreate (
21.2.3.2.4
RS: EscapeRegExpPattern (
21.2.4
Properties of the RegExp Constructor
21.2.4.1
RegExp.prototype
21.2.4.2
get RegExp [ @@species ]
21.2.5
Properties of the RegExp Prototype Object
21.2.5.1
RegExp.prototype.constructor
21.2.5.2
RegExp.prototype.exec (
string
21.2.5.2.1
RS: RegExpExec (
21.2.5.2.2
RS: RegExpBuiltinExec (
21.2.5.2.3
AdvanceStringIndex (
index
unicode
21.2.5.3
get RegExp.prototype.dotAll
21.2.5.4
get RegExp.prototype.flags
21.2.5.5
get RegExp.prototype.global
21.2.5.6
get RegExp.prototype.ignoreCase
21.2.5.7
RegExp.prototype [ @@match ] (
string
21.2.5.8
get RegExp.prototype.multiline
21.2.5.9
RegExp.prototype [ @@replace ] (
string
replaceValue
21.2.5.10
RegExp.prototype [ @@search ] (
string
21.2.5.11
get RegExp.prototype.source
21.2.5.12
RegExp.prototype [ @@split ] (
string
limit
21.2.5.13
get RegExp.prototype.sticky
21.2.5.14
RegExp.prototype.test (
21.2.5.15
RegExp.prototype.toString ( )
21.2.5.16
get RegExp.prototype.unicode
21.2.6
Properties of RegExp Instances
21.2.6.1
lastIndex
22
Indexed Collections
22.1
Array Objects
22.1.1
The Array Constructor
22.1.1.1
Array ( )
22.1.1.2
Array (
len
22.1.1.3
Array ( ...
items
22.1.2
Properties of the Array Constructor
22.1.2.1
Array.from (
items
[ ,
mapfn
[ ,
thisArg
] ] )
22.1.2.2
Array.isArray (
arg
22.1.2.3
Array.of ( ...
items
22.1.2.4
Array.prototype
22.1.2.5
get Array [ @@species ]
22.1.3
Properties of the Array Prototype Object
22.1.3.1
Array.prototype.concat ( ...
arguments
22.1.3.1.1
RS: IsConcatSpreadable (
22.1.3.2
Array.prototype.constructor
22.1.3.3
Array.prototype.copyWithin (
target
start
[ ,
end
] )
22.1.3.4
Array.prototype.entries ( )
22.1.3.5
Array.prototype.every (
callbackfn
[ ,
thisArg
] )
22.1.3.6
Array.prototype.fill (
value
[ ,
start
[ ,
end
] ] )
22.1.3.7
Array.prototype.filter (
callbackfn
[ ,
thisArg
] )
22.1.3.8
Array.prototype.find (
predicate
[ ,
thisArg
] )
22.1.3.9
Array.prototype.findIndex (
predicate
[ ,
thisArg
] )
22.1.3.10
Array.prototype.flat( [
depth
] )
22.1.3.10.1
FlattenIntoArray(
target
source
sourceLen
start
depth
[ ,
mapperFunction
thisArg
])
22.1.3.11
Array.prototype.flatMap (
mapperFunction
[ ,
thisArg
] )
22.1.3.12
Array.prototype.forEach (
callbackfn
[ ,
thisArg
] )
22.1.3.13
Array.prototype.includes (
searchElement
[ ,
fromIndex
] )
22.1.3.14
Array.prototype.indexOf (
searchElement
[ ,
fromIndex
] )
22.1.3.15
Array.prototype.join (
separator
22.1.3.16
Array.prototype.keys ( )
22.1.3.17
Array.prototype.lastIndexOf (
searchElement
[ ,
fromIndex
] )
22.1.3.18
Array.prototype.map (
callbackfn
[ ,
thisArg
] )
22.1.3.19
Array.prototype.pop ( )
22.1.3.20
Array.prototype.push ( ...
items
22.1.3.21
Array.prototype.reduce (
callbackfn
[ ,
initialValue
] )
22.1.3.22
Array.prototype.reduceRight (
callbackfn
[ ,
initialValue
] )
22.1.3.23
Array.prototype.reverse ( )
22.1.3.24
Array.prototype.shift ( )
22.1.3.25
Array.prototype.slice (
start
end
22.1.3.26
Array.prototype.some (
callbackfn
[ ,
thisArg
] )
22.1.3.27
Array.prototype.sort (
comparefn
22.1.3.27.1
RS: SortCompare (
22.1.3.28
Array.prototype.splice (
start
deleteCount
, ...
items
22.1.3.29
Array.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
22.1.3.30
Array.prototype.toString ( )
22.1.3.31
Array.prototype.unshift ( ...
items
22.1.3.32
Array.prototype.values ( )
22.1.3.33
Array.prototype [ @@iterator ] ( )
22.1.3.34
Array.prototype [ @@unscopables ]
22.1.4
Properties of Array Instances
22.1.4.1
length
22.1.5
Array Iterator Objects
22.1.5.1
CreateArrayIterator (
array
kind
22.1.5.2
The %ArrayIteratorPrototype% Object
22.1.5.2.1
%ArrayIteratorPrototype%.next ( )
22.1.5.2.2
%ArrayIteratorPrototype% [ @@toStringTag ]
22.1.5.3
Properties of Array Iterator Instances
22.2
TypedArray Objects
22.2.1
The %TypedArray% Intrinsic Object
22.2.1.1
%TypedArray% ( )
22.2.2
Properties of the %TypedArray% Intrinsic Object
22.2.2.1
%TypedArray%.from (
source
[ ,
mapfn
[ ,
thisArg
] ] )
22.2.2.1.1
RS: IterableToList (
items
method
22.2.2.2
%TypedArray%.of ( ...
items
22.2.2.3
%TypedArray%.prototype
22.2.2.4
get %TypedArray% [ @@species ]
22.2.3
Properties of the %TypedArrayPrototype% Object
22.2.3.1
get %TypedArray%.prototype.buffer
22.2.3.2
get %TypedArray%.prototype.byteLength
22.2.3.3
get %TypedArray%.prototype.byteOffset
22.2.3.4
%TypedArray%.prototype.constructor
22.2.3.5
%TypedArray%.prototype.copyWithin (
target
start
[ ,
end
] )
22.2.3.5.1
RS: ValidateTypedArray (
22.2.3.6
%TypedArray%.prototype.entries ( )
22.2.3.7
%TypedArray%.prototype.every (
callbackfn
[ ,
thisArg
] )
22.2.3.8
%TypedArray%.prototype.fill (
value
[ ,
start
[ ,
end
] ] )
22.2.3.9
%TypedArray%.prototype.filter (
callbackfn
[ ,
thisArg
] )
22.2.3.10
%TypedArray%.prototype.find (
predicate
[ ,
thisArg
] )
22.2.3.11
%TypedArray%.prototype.findIndex (
predicate
[ ,
thisArg
] )
22.2.3.12
%TypedArray%.prototype.forEach (
callbackfn
[ ,
thisArg
] )
22.2.3.13
%TypedArray%.prototype.includes (
searchElement
[ ,
fromIndex
] )
22.2.3.14
%TypedArray%.prototype.indexOf (
searchElement
[ ,
fromIndex
] )
22.2.3.15
%TypedArray%.prototype.join (
separator
22.2.3.16
%TypedArray%.prototype.keys ( )
22.2.3.17
%TypedArray%.prototype.lastIndexOf (
searchElement
[ ,
fromIndex
] )
22.2.3.18
get %TypedArray%.prototype.length
22.2.3.19
%TypedArray%.prototype.map (
callbackfn
[ ,
thisArg
] )
22.2.3.20
%TypedArray%.prototype.reduce (
callbackfn
[ ,
initialValue
] )
22.2.3.21
%TypedArray%.prototype.reduceRight (
callbackfn
[ ,
initialValue
] )
22.2.3.22
%TypedArray%.prototype.reverse ( )
22.2.3.23
%TypedArray%.prototype.set (
overloaded
[ ,
offset
] )
22.2.3.23.1
%TypedArray%.prototype.set (
array
[ ,
offset
] )
22.2.3.23.2
%TypedArray%.prototype.set (
typedArray
[ ,
offset
] )
22.2.3.24
%TypedArray%.prototype.slice (
start
end
22.2.3.25
%TypedArray%.prototype.some (
callbackfn
[ ,
thisArg
] )
22.2.3.26
%TypedArray%.prototype.sort (
comparefn
22.2.3.27
%TypedArray%.prototype.subarray (
begin
end
22.2.3.28
%TypedArray%.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
22.2.3.29
%TypedArray%.prototype.toString ( )
22.2.3.30
%TypedArray%.prototype.values ( )
22.2.3.31
%TypedArray%.prototype [ @@iterator ] ( )
22.2.3.32
get %TypedArray%.prototype [ @@toStringTag ]
22.2.4
The
TypedArray
Constructors
22.2.4.1
TypedArray
( )
22.2.4.2
TypedArray
length
22.2.4.2.1
RS: AllocateTypedArray (
constructorName
newTarget
defaultProto
[ ,
length
] )
22.2.4.2.2
RS: AllocateTypedArrayBuffer (
length
22.2.4.3
TypedArray
typedArray
22.2.4.4
TypedArray
object
22.2.4.5
TypedArray
buffer
[ ,
byteOffset
[ ,
length
] ] )
22.2.4.6
TypedArrayCreate (
constructor
argumentList
22.2.4.7
TypedArraySpeciesCreate (
exemplar
argumentList
22.2.5
Properties of the
TypedArray
Constructors
22.2.5.1
TypedArray
.BYTES_PER_ELEMENT
22.2.5.2
TypedArray
.prototype
22.2.6
Properties of the
TypedArray
Prototype Objects
22.2.6.1
TypedArray
.prototype.BYTES_PER_ELEMENT
22.2.6.2
TypedArray
.prototype.constructor
22.2.7
Properties of
TypedArray
Instances
23
Keyed Collections
23.1
Map Objects
23.1.1
The Map Constructor
23.1.1.1
Map ( [
iterable
] )
23.1.1.2
AddEntriesFromIterable (
target
iterable
adder
23.1.2
Properties of the Map Constructor
23.1.2.1
Map.prototype
23.1.2.2
get Map [ @@species ]
23.1.3
Properties of the Map Prototype Object
23.1.3.1
Map.prototype.clear ( )
23.1.3.2
Map.prototype.constructor
23.1.3.3
Map.prototype.delete (
key
23.1.3.4
Map.prototype.entries ( )
23.1.3.5
Map.prototype.forEach (
callbackfn
[ ,
thisArg
] )
23.1.3.6
Map.prototype.get (
key
23.1.3.7
Map.prototype.has (
key
23.1.3.8
Map.prototype.keys ( )
23.1.3.9
Map.prototype.set (
key
value
23.1.3.10
get Map.prototype.size
23.1.3.11
Map.prototype.values ( )
23.1.3.12
Map.prototype [ @@iterator ] ( )
23.1.3.13
Map.prototype [ @@toStringTag ]
23.1.4
Properties of Map Instances
23.1.5
Map Iterator Objects
23.1.5.1
CreateMapIterator (
map
kind
23.1.5.2
The %MapIteratorPrototype% Object
23.1.5.2.1
%MapIteratorPrototype%.next ( )
23.1.5.2.2
%MapIteratorPrototype% [ @@toStringTag ]
23.1.5.3
Properties of Map Iterator Instances
23.2
Set Objects
23.2.1
The Set Constructor
23.2.1.1
Set ( [
iterable
] )
23.2.2
Properties of the Set Constructor
23.2.2.1
Set.prototype
23.2.2.2
get Set [ @@species ]
23.2.3
Properties of the Set Prototype Object
23.2.3.1
Set.prototype.add (
value
23.2.3.2
Set.prototype.clear ( )
23.2.3.3
Set.prototype.constructor
23.2.3.4
Set.prototype.delete (
value
23.2.3.5
Set.prototype.entries ( )
23.2.3.6
Set.prototype.forEach (
callbackfn
[ ,
thisArg
] )
23.2.3.7
Set.prototype.has (
value
23.2.3.8
Set.prototype.keys ( )
23.2.3.9
get Set.prototype.size
23.2.3.10
Set.prototype.values ( )
23.2.3.11
Set.prototype [ @@iterator ] ( )
23.2.3.12
Set.prototype [ @@toStringTag ]
23.2.4
Properties of Set Instances
23.2.5
Set Iterator Objects
23.2.5.1
CreateSetIterator (
set
kind
23.2.5.2
The %SetIteratorPrototype% Object
23.2.5.2.1
%SetIteratorPrototype%.next ( )
23.2.5.2.2
%SetIteratorPrototype% [ @@toStringTag ]
23.2.5.3
Properties of Set Iterator Instances
23.3
WeakMap Objects
23.3.1
The WeakMap Constructor
23.3.1.1
WeakMap ( [
iterable
] )
23.3.2
Properties of the WeakMap Constructor
23.3.2.1
WeakMap.prototype
23.3.3
Properties of the WeakMap Prototype Object
23.3.3.1
WeakMap.prototype.constructor
23.3.3.2
WeakMap.prototype.delete (
key
23.3.3.3
WeakMap.prototype.get (
key
23.3.3.4
WeakMap.prototype.has (
key
23.3.3.5
WeakMap.prototype.set (
key
value
23.3.3.6
WeakMap.prototype [ @@toStringTag ]
23.3.4
Properties of WeakMap Instances
23.4
WeakSet Objects
23.4.1
The WeakSet Constructor
23.4.1.1
WeakSet ( [
iterable
] )
23.4.2
Properties of the WeakSet Constructor
23.4.2.1
WeakSet.prototype
23.4.3
Properties of the WeakSet Prototype Object
23.4.3.1
WeakSet.prototype.add (
value
23.4.3.2
WeakSet.prototype.constructor
23.4.3.3
WeakSet.prototype.delete (
value
23.4.3.4
WeakSet.prototype.has (
value
23.4.3.5
WeakSet.prototype [ @@toStringTag ]
23.4.4
Properties of WeakSet Instances
24
Structured Data
24.1
ArrayBuffer Objects
24.1.1
Abstract Operations For ArrayBuffer Objects
24.1.1.1
AllocateArrayBuffer (
constructor
byteLength
24.1.1.2
IsDetachedBuffer (
arrayBuffer
24.1.1.3
DetachArrayBuffer (
arrayBuffer
[ ,
key
] )
24.1.1.4
CloneArrayBuffer (
srcBuffer
srcByteOffset
srcLength
cloneConstructor
24.1.1.5
RawBytesToNumber (
type
rawBytes
isLittleEndian
24.1.1.6
GetValueFromBuffer (
arrayBuffer
byteIndex
type
isTypedArray
order
[ ,
isLittleEndian
] )
24.1.1.7
NumberToRawBytes (
type
value
isLittleEndian
24.1.1.8
SetValueInBuffer (
arrayBuffer
byteIndex
type
value
isTypedArray
order
[ ,
isLittleEndian
] )
24.1.1.9
GetModifySetValueInBuffer (
arrayBuffer
byteIndex
type
value
op
[ ,
isLittleEndian
] )
24.1.2
The ArrayBuffer Constructor
24.1.2.1
ArrayBuffer (
length
24.1.3
Properties of the ArrayBuffer Constructor
24.1.3.1
ArrayBuffer.isView (
arg
24.1.3.2
ArrayBuffer.prototype
24.1.3.3
get ArrayBuffer [ @@species ]
24.1.4
Properties of the ArrayBuffer Prototype Object
24.1.4.1
get ArrayBuffer.prototype.byteLength
24.1.4.2
ArrayBuffer.prototype.constructor
24.1.4.3
ArrayBuffer.prototype.slice (
start
end
24.1.4.4
ArrayBuffer.prototype [ @@toStringTag ]
24.1.5
Properties of ArrayBuffer Instances
24.2
SharedArrayBuffer Objects
24.2.1
Abstract Operations for SharedArrayBuffer Objects
24.2.1.1
AllocateSharedArrayBuffer (
constructor
byteLength
24.2.1.2
IsSharedArrayBuffer (
obj
24.2.2
The SharedArrayBuffer Constructor
24.2.2.1
SharedArrayBuffer ( [
length
] )
24.2.3
Properties of the SharedArrayBuffer Constructor
24.2.3.1
SharedArrayBuffer.prototype
24.2.3.2
get SharedArrayBuffer [ @@species ]
24.2.4
Properties of the SharedArrayBuffer Prototype Object
24.2.4.1
get SharedArrayBuffer.prototype.byteLength
24.2.4.2
SharedArrayBuffer.prototype.constructor
24.2.4.3
SharedArrayBuffer.prototype.slice (
start
end
24.2.4.4
SharedArrayBuffer.prototype [ @@toStringTag ]
24.2.5
Properties of SharedArrayBuffer Instances
24.3
DataView Objects
24.3.1
Abstract Operations For DataView Objects
24.3.1.1
GetViewValue (
view
requestIndex
isLittleEndian
type
24.3.1.2
SetViewValue (
view
requestIndex
isLittleEndian
type
value
24.3.2
The DataView Constructor
24.3.2.1
DataView (
buffer
[ ,
byteOffset
[ ,
byteLength
] ] )
24.3.3
Properties of the DataView Constructor
24.3.3.1
DataView.prototype
24.3.4
Properties of the DataView Prototype Object
24.3.4.1
get DataView.prototype.buffer
24.3.4.2
get DataView.prototype.byteLength
24.3.4.3
get DataView.prototype.byteOffset
24.3.4.4
DataView.prototype.constructor
24.3.4.5
DataView.prototype.getFloat32 (
byteOffset
[ ,
littleEndian
] )
24.3.4.6
DataView.prototype.getFloat64 (
byteOffset
[ ,
littleEndian
] )
24.3.4.7
DataView.prototype.getInt8 (
byteOffset
24.3.4.8
DataView.prototype.getInt16 (
byteOffset
[ ,
littleEndian
] )
24.3.4.9
DataView.prototype.getInt32 (
byteOffset
[ ,
littleEndian
] )
24.3.4.10
DataView.prototype.getUint8 (
byteOffset
24.3.4.11
DataView.prototype.getUint16 (
byteOffset
[ ,
littleEndian
] )
24.3.4.12
DataView.prototype.getUint32 (
byteOffset
[ ,
littleEndian
] )
24.3.4.13
DataView.prototype.setFloat32 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.14
DataView.prototype.setFloat64 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.15
DataView.prototype.setInt8 (
byteOffset
value
24.3.4.16
DataView.prototype.setInt16 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.17
DataView.prototype.setInt32 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.18
DataView.prototype.setUint8 (
byteOffset
value
24.3.4.19
DataView.prototype.setUint16 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.20
DataView.prototype.setUint32 (
byteOffset
value
[ ,
littleEndian
] )
24.3.4.21
DataView.prototype [ @@toStringTag ]
24.3.5
Properties of DataView Instances
24.4
The Atomics Object
24.4.1
Abstract Operations for Atomics
24.4.1.1
ValidateSharedIntegerTypedArray (
typedArray
[ ,
onlyInt32
] )
24.4.1.2
ValidateAtomicAccess (
typedArray
requestIndex
24.4.1.3
GetWaiterList (
block
24.4.1.4
EnterCriticalSection (
WL
24.4.1.5
LeaveCriticalSection (
WL
24.4.1.6
AddWaiter (
WL
24.4.1.7
RemoveWaiter (
WL
24.4.1.8
RemoveWaiters (
WL
24.4.1.9
Suspend (
WL
timeout
24.4.1.10
NotifyWaiter (
WL
24.4.1.11
AtomicReadModifyWrite (
typedArray
index
value
op
24.4.1.12
AtomicLoad (
typedArray
index
24.4.2
Atomics.add (
typedArray
index
value
24.4.3
Atomics.and (
typedArray
index
value
24.4.4
Atomics.compareExchange (
typedArray
index
expectedValue
replacementValue
24.4.5
Atomics.exchange (
typedArray
index
value
24.4.6
Atomics.isLockFree (
size
24.4.7
Atomics.load (
typedArray
index
24.4.8
Atomics.or (
typedArray
index
value
24.4.9
Atomics.store (
typedArray
index
value
24.4.10
Atomics.sub (
typedArray
index
value
24.4.11
Atomics.wait (
typedArray
index
value
timeout
24.4.12
Atomics.notify (
typedArray
index
count
24.4.13
Atomics.xor (
typedArray
index
value
24.4.14
Atomics [ @@toStringTag ]
24.5
The JSON Object
24.5.1
JSON.parse (
text
[ ,
reviver
] )
24.5.1.1
RS: InternalizeJSONProperty (
holder
name
24.5.2
JSON.stringify (
value
[ ,
replacer
[ ,
space
] ] )
24.5.2.1
RS: SerializeJSONProperty (
key
holder
24.5.2.2
RS: QuoteJSONString (
value
24.5.2.3
RS: UnicodeEscape (
24.5.2.4
RS: SerializeJSONObject (
value
24.5.2.5
RS: SerializeJSONArray (
value
24.5.3
JSON [ @@toStringTag ]
25
Control Abstraction Objects
25.1
Iteration
25.1.1
Common Iteration Interfaces
25.1.1.1
The
Iterable
Interface
25.1.1.2
The
Iterator
Interface
25.1.1.3
The
AsyncIterable
Interface
25.1.1.4
The
AsyncIterator
Interface
25.1.1.5
The IteratorResult Interface
25.1.2
The %IteratorPrototype% Object
25.1.2.1
%IteratorPrototype% [ @@iterator ] ( )
25.1.3
The %AsyncIteratorPrototype% Object
25.1.3.1
%AsyncIteratorPrototype% [ @@asyncIterator ] ( )
25.1.4
Async-from-Sync Iterator Objects
25.1.4.1
CreateAsyncFromSyncIterator (
syncIteratorRecord
25.1.4.2
The %AsyncFromSyncIteratorPrototype% Object
25.1.4.2.1
%AsyncFromSyncIteratorPrototype%.next (
value
25.1.4.2.2
%AsyncFromSyncIteratorPrototype%.return (
value
25.1.4.2.3
%AsyncFromSyncIteratorPrototype%.throw (
value
25.1.4.2.4
%AsyncFromSyncIteratorPrototype% [ @@toStringTag ]
25.1.4.2.5
Async-from-Sync Iterator Value Unwrap Functions
25.1.4.3
Properties of Async-from-Sync Iterator Instances
25.1.4.4
AsyncFromSyncIteratorContinuation (
result
promiseCapability
25.2
GeneratorFunction Objects
25.2.1
The GeneratorFunction Constructor
25.2.1.1
GeneratorFunction (
p1
p2
, … ,
pn
body
25.2.2
Properties of the GeneratorFunction Constructor
25.2.2.1
GeneratorFunction.length
25.2.2.2
GeneratorFunction.prototype
25.2.3
Properties of the GeneratorFunction Prototype Object
25.2.3.1
GeneratorFunction.prototype.constructor
25.2.3.2
GeneratorFunction.prototype.prototype
25.2.3.3
GeneratorFunction.prototype [ @@toStringTag ]
25.2.4
GeneratorFunction Instances
25.2.4.1
length
25.2.4.2
name
25.2.4.3
prototype
25.3
AsyncGeneratorFunction Objects
25.3.1
The AsyncGeneratorFunction Constructor
25.3.1.1
AsyncGeneratorFunction (
p1
p2
, ...,
pn
body
25.3.2
Properties of the AsyncGeneratorFunction Constructor
25.3.2.1
AsyncGeneratorFunction.length
25.3.2.2
AsyncGeneratorFunction.prototype
25.3.3
Properties of the AsyncGeneratorFunction Prototype Object
25.3.3.1
AsyncGeneratorFunction.prototype.constructor
25.3.3.2
AsyncGeneratorFunction.prototype.prototype
25.3.3.3
AsyncGeneratorFunction.prototype [ @@toStringTag ]
25.3.4
AsyncGeneratorFunction Instances
25.3.4.1
length
25.3.4.2
name
25.3.4.3
prototype
25.4
Generator Objects
25.4.1
Properties of the Generator Prototype Object
25.4.1.1
Generator.prototype.constructor
25.4.1.2
Generator.prototype.next (
value
25.4.1.3
Generator.prototype.return (
value
25.4.1.4
Generator.prototype.throw (
exception
25.4.1.5
Generator.prototype [ @@toStringTag ]
25.4.2
Properties of Generator Instances
25.4.3
Generator Abstract Operations
25.4.3.1
GeneratorStart (
generator
generatorBody
25.4.3.2
GeneratorValidate (
generator
25.4.3.3
GeneratorResume (
generator
value
25.4.3.4
GeneratorResumeAbrupt (
generator
abruptCompletion
25.4.3.5
GetGeneratorKind ( )
25.4.3.6
GeneratorYield (
iterNextObj
25.5
AsyncGenerator Objects
25.5.1
Properties of the AsyncGenerator Prototype Object
25.5.1.1
AsyncGenerator.prototype.constructor
25.5.1.2
AsyncGenerator.prototype.next (
value
25.5.1.3
AsyncGenerator.prototype.return (
value
25.5.1.4
AsyncGenerator.prototype.throw (
exception
25.5.1.5
AsyncGenerator.prototype [ @@toStringTag ]
25.5.2
Properties of AsyncGenerator Instances
25.5.3
AsyncGenerator Abstract Operations
25.5.3.1
AsyncGeneratorRequest Records
25.5.3.2
AsyncGeneratorStart (
generator
generatorBody
25.5.3.3
AsyncGeneratorResolve (
generator
value
done
25.5.3.4
AsyncGeneratorReject (
generator
exception
25.5.3.5
AsyncGeneratorResumeNext (
generator
25.5.3.5.1
AsyncGeneratorResumeNext Return Processor Fulfilled Functions
25.5.3.5.2
AsyncGeneratorResumeNext Return Processor Rejected Functions
25.5.3.6
AsyncGeneratorEnqueue (
generator
completion
25.5.3.7
AsyncGeneratorYield (
value
25.6
Promise Objects
25.6.1
Promise Abstract Operations
25.6.1.1
PromiseCapability Records
25.6.1.1.1
IfAbruptRejectPromise (
value
capability
25.6.1.2
PromiseReaction Records
25.6.1.3
CreateResolvingFunctions (
promise
25.6.1.3.1
Promise Reject Functions
25.6.1.3.2
Promise Resolve Functions
25.6.1.4
FulfillPromise (
promise
value
25.6.1.5
NewPromiseCapability (
25.6.1.5.1
GetCapabilitiesExecutor Functions
25.6.1.6
IsPromise (
25.6.1.7
RejectPromise (
promise
reason
25.6.1.8
TriggerPromiseReactions (
reactions
argument
25.6.1.9
HostPromiseRejectionTracker (
promise
operation
25.6.2
Promise Jobs
25.6.2.1
PromiseReactionJob (
reaction
argument
25.6.2.2
PromiseResolveThenableJob (
promiseToResolve
thenable
then
25.6.3
The Promise Constructor
25.6.3.1
Promise (
executor
25.6.4
Properties of the Promise Constructor
25.6.4.1
Promise.all (
iterable
25.6.4.1.1
RS: PerformPromiseAll (
iteratorRecord
constructor
resultCapability
25.6.4.1.2
Promise.all
Resolve Element Functions
25.6.4.2
Promise.prototype
25.6.4.3
Promise.race (
iterable
25.6.4.3.1
RS: PerformPromiseRace (
iteratorRecord
constructor
resultCapability
25.6.4.4
Promise.reject (
25.6.4.5
Promise.resolve (
25.6.4.5.1
PromiseResolve (
25.6.4.6
get Promise [ @@species ]
25.6.5
Properties of the Promise Prototype Object
25.6.5.1
Promise.prototype.catch (
onRejected
25.6.5.2
Promise.prototype.constructor
25.6.5.3
Promise.prototype.finally (
onFinally
25.6.5.3.1
Then Finally Functions
25.6.5.3.2
Catch Finally Functions
25.6.5.4
Promise.prototype.then (
onFulfilled
onRejected
25.6.5.4.1
PerformPromiseThen (
promise
onFulfilled
onRejected
[ ,
resultCapability
] )
25.6.5.5
Promise.prototype [ @@toStringTag ]
25.6.6
Properties of Promise Instances
25.7
AsyncFunction Objects
25.7.1
The AsyncFunction Constructor
25.7.1.1
AsyncFunction (
p1
p2
, … ,
pn
body
25.7.2
Properties of the AsyncFunction Constructor
25.7.2.1
AsyncFunction.length
25.7.2.2
AsyncFunction.prototype
25.7.3
Properties of the AsyncFunction Prototype Object
25.7.3.1
AsyncFunction.prototype.constructor
25.7.3.2
AsyncFunction.prototype [ @@toStringTag ]
25.7.4
AsyncFunction Instances
25.7.4.1
length
25.7.4.2
name
25.7.5
Async Functions Abstract Operations
25.7.5.1
AsyncFunctionStart (
promiseCapability
asyncFunctionBody
26
Reflection
26.1
The Reflect Object
26.1.1
Reflect.apply (
target
thisArgument
argumentsList
26.1.2
Reflect.construct (
target
argumentsList
[ ,
newTarget
] )
26.1.3
Reflect.defineProperty (
target
propertyKey
attributes
26.1.4
Reflect.deleteProperty (
target
propertyKey
26.1.5
Reflect.get (
target
propertyKey
[ ,
receiver
] )
26.1.6
Reflect.getOwnPropertyDescriptor (
target
propertyKey
26.1.7
Reflect.getPrototypeOf (
target
26.1.8
Reflect.has (
target
propertyKey
26.1.9
Reflect.isExtensible (
target
26.1.10
Reflect.ownKeys (
target
26.1.11
Reflect.preventExtensions (
target
26.1.12
Reflect.set (
target
propertyKey
[ ,
receiver
] )
26.1.13
Reflect.setPrototypeOf (
target
proto
26.2
Proxy Objects
26.2.1
The Proxy Constructor
26.2.1.1
Proxy (
target
handler
26.2.2
Properties of the Proxy Constructor
26.2.2.1
Proxy.revocable (
target
handler
26.2.2.1.1
Proxy Revocation Functions
26.3
Module Namespace Objects
26.3.1
@@toStringTag
27
Memory Model
27.1
Memory Model Fundamentals
27.2
Agent Events Records
27.3
Chosen Value Records
27.4
Candidate Executions
27.5
Abstract Operations for the Memory Model
27.5.1
EventSet (
execution
27.5.2
SharedDataBlockEventSet (
execution
27.5.3
SynchronizeEventSet (
execution
27.5.4
HostEventSet (
execution
27.5.5
ComposeWriteEventBytes (
execution
byteIndex
Ws
27.5.6
ValueOfReadEvent (
execution
27.6
Relations of Candidate Executions
27.6.1
agent-order
27.6.2
reads-bytes-from
27.6.3
reads-from
27.6.4
host-synchronizes-with
27.6.5
synchronizes-with
27.6.6
happens-before
27.7
Properties of Valid Executions
27.7.1
Valid Chosen Reads
27.7.2
Coherent Reads
27.7.3
Tear Free Reads
27.7.4
Sequentially Consistent Atomics
27.7.5
Valid Executions
27.8
Races
27.9
Data Races
27.10
Data Race Freedom
27.11
Shared Memory Guidelines
Grammar Summary
A.1
Lexical Grammar
A.2
Expressions
A.3
Statements
A.4
Functions and Classes
A.5
Scripts and Modules
A.6
Number Conversions
A.7
Universal Resource Identifier Character Classes
A.8
Regular Expressions
Additional ECMAScript Features for Web Browsers
B.1
Additional Syntax
B.1.1
Numeric Literals
B.1.1.1
Static Semantics
B.1.2
String Literals
B.1.2.1
Static Semantics
B.1.3
HTML-like Comments
B.1.4
Regular Expressions Patterns
B.1.4.1
SS: Early Errors
B.1.4.2
SS: IsCharacterClass
B.1.4.3
SS: CharacterValue
B.1.4.4
Pattern Semantics
B.1.4.4.1
RS: CharacterRangeOrUnion (
B.2
Additional Built-in Properties
B.2.1
Additional Properties of the Global Object
B.2.1.1
escape (
string
B.2.1.2
unescape (
string
B.2.2
Additional Properties of the Object.prototype Object
B.2.2.1
Object.prototype.__proto__
B.2.2.1.1
get Object.prototype.__proto__
B.2.2.1.2
set Object.prototype.__proto__
B.2.2.2
Object.prototype.__defineGetter__ (
getter
B.2.2.3
Object.prototype.__defineSetter__ (
setter
B.2.2.4
Object.prototype.__lookupGetter__ (
B.2.2.5
Object.prototype.__lookupSetter__ (
B.2.3
Additional Properties of the String.prototype Object
B.2.3.1
String.prototype.substr (
start
length
B.2.3.2
String.prototype.anchor (
name
B.2.3.2.1
RS: CreateHTML (
string
tag
attribute
value
B.2.3.3
String.prototype.big ( )
B.2.3.4
String.prototype.blink ( )
B.2.3.5
String.prototype.bold ( )
B.2.3.6
String.prototype.fixed ( )
B.2.3.7
String.prototype.fontcolor (
color
B.2.3.8
String.prototype.fontsize (
size
B.2.3.9
String.prototype.italics ( )
B.2.3.10
String.prototype.link (
url
B.2.3.11
String.prototype.small ( )
B.2.3.12
String.prototype.strike ( )
B.2.3.13
String.prototype.sub ( )
B.2.3.14
String.prototype.sup ( )
B.2.3.15
String.prototype.trimLeft ( )
B.2.3.16
String.prototype.trimRight ( )
B.2.4
Additional Properties of the Date.prototype Object
B.2.4.1
Date.prototype.getYear ( )
B.2.4.2
Date.prototype.setYear (
year
B.2.4.3
Date.prototype.toGMTString ( )
B.2.5
Additional Properties of the RegExp.prototype Object
B.2.5.1
RegExp.prototype.compile (
pattern
flags
B.3
Other Additional Features
B.3.1
__proto__ Property Names in Object Initializers
B.3.2
Labelled Function Declarations
B.3.3
Block-Level Function Declarations Web Legacy Compatibility Semantics
B.3.3.1
Changes to FunctionDeclarationInstantiation
B.3.3.2
Changes to GlobalDeclarationInstantiation
B.3.3.3
Changes to EvalDeclarationInstantiation
B.3.3.4
Changes to Block SS: Early Errors
B.3.3.5
Changes to
switch
Statement SS: Early Errors
B.3.3.6
Changes to BlockDeclarationInstantiation
B.3.4
FunctionDeclarations in IfStatement Statement Clauses
B.3.5
VariableStatements in Catch Blocks
B.3.6
Initializers in ForIn Statement Heads
B.3.7
The [[IsHTMLDDA]] Internal Slot
B.3.7.1
Changes to ToBoolean
B.3.7.2
Changes to Abstract Equality Comparison
B.3.7.3
Changes to the
typeof
Operator
The Strict Mode of ECMAScript
Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
Additions and Changes That Introduce Incompatibilities with Prior Editions
Colophon
Bibliography
Copyright & Software License
ECMA-262, 10
th
edition, June 2019
ECMAScript® 2019 Language Specification
Contributing to this Specification
This specification is developed on GitHub with the help of the
ECMAScript community. There are a number of ways to contribute to the
development of this specification:
GitHub Repository:
Issues:
All Issues
File a New Issue
Pull Requests:
All Pull Requests
Create a New Pull Request
Test Suite:
Test262
Editors:
Brian Terlson
@bterlson
Bradley Farias
@bradleymeck
Jordan Harband
@ljharb
Community:
Mailing list:
es-discuss
IRC:
#tc39
on
freenode
Refer to the
colophon
for more information on how this document is created.
Introduction
This Ecma Standard defines the ECMAScript 2019 Language. It is the
tenth edition of the ECMAScript Language Specification. Since
publication of the first edition in 1997, ECMAScript has grown to be one
of the world's most widely used general-purpose programming languages.
It is best known as the language embedded in web browsers but has also
been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most
well-known being JavaScript (Netscape) and JScript (Microsoft). The
language was invented by Brendan Eich at Netscape and first appeared in
that company's Navigator 2.0 browser. It has appeared in all subsequent
browsers from Netscape and in all browsers from Microsoft starting with
Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in
November 1996. The first edition of this Ecma Standard was adopted by
the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption
under the fast-track procedure, and approved as international standard
ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998
approved the second edition of ECMA-262 to keep it fully aligned with
ISO/IEC 16262. Changes between the first and the second edition are
editorial in nature.
The third edition of the Standard introduced powerful regular
expressions, better string handling, new control statements, try/catch
exception handling, tighter definition of errors, formatting for numeric
output and minor changes in anticipation of future language growth. The
third edition of the ECMAScript standard was adopted by the Ecma
General Assembly of December 1999 and published as ISO/IEC 16262:2002 in
June 2002.
After publication of the third edition, ECMAScript achieved massive
adoption in conjunction with the World Wide Web where it has become the
programming language that is supported by essentially all web browsers.
Significant work was done to develop a fourth edition of ECMAScript.
However, that work was not completed and not published as the fourth
edition of ECMAScript but some of it was incorporated into the
development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5
th
edition) codified de facto interpretations of the language
specification that have become common among browser implementations and
added support for new features that had emerged since the publication of
the third edition. Such features include accessor properties,
reflective creation and inspection of objects, program control of
property attributes, additional array manipulation functions, support
for the JSON object encoding format, and a strict mode that provides
enhanced error checking and program security. The fifth edition was
adopted by the Ecma General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under
the fast-track procedure, and approved as international standard
ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated
minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1
Edition was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the
fifth edition was being prepared for publication. However, this was
preceded by significant experimentation and language enhancement design
efforts dating to the publication of the third edition in 1999. In a
very real sense, the completion of the sixth edition is the culmination
of a fifteen year effort. The goals for this addition included providing
better support for large applications, library creation, and for use of
ECMAScript as a compilation target for other languages. Some of its
major enhancements included modules, class declarations, lexical block
scoping, iterators and generators, promises for asynchronous
programming, destructuring patterns, and proper tail calls. The
ECMAScript library of built-ins was expanded to support additional data
abstractions including maps, sets, and arrays of binary numeric values
as well as additional support for Unicode supplemental characters in
strings and regular expressions. The built-ins were also made extensible
via subclassing. The sixth edition provides the foundation for regular,
incremental language and library enhancements. The sixth edition was
adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under
Ecma TC39's new yearly release cadence and open development process. A
plain-text source document was built from the ECMAScript 2015 source
document to serve as the base for further development entirely on
GitHub. Over the year of this standard's development, hundreds of pull
requests and issues were filed representing thousands of bug fixes,
editorial fixes and other improvements. Additionally, numerous software
tools were developed to aid in this effort including Ecmarkup,
Ecmarkdown, and Grammarkdown. ES2016 also included support for a new
exponentiation operator and adds a new method to Array.prototype called
includes
ECMAScript 2017 introduced Async Functions, Shared Memory, and
Atomics along with smaller language and library enhancements, bug fixes,
and editorial updates. Async functions improve the asynchronous
programming experience by providing syntax for promise-returning
functions. Shared Memory and Atomics introduce a new
memory model
that allows multi-
agent
programs to communicate using atomic operations that ensure a
well-defined execution order even on parallel CPUs. This specification
also includes new static methods on Object:
Object.values
Object.entries
, and
Object.getOwnPropertyDescriptors
ECMAScript 2018 introduced support for asynchronous iteration via
the AsyncIterator protocol and async generators. It also included four
new regular expression features: the dotAll flag, named capture groups,
Unicode property escapes, and look-behind assertions. Lastly it included
rest parameter and spread operator support for object properties.
This specification, the 10
th
edition, introduces a few new built-in functions:
flat
and
flatMap
on
Array.prototype
for flattening arrays,
Object.fromEntries
for directly turning the return value of
Object.entries
into a new Object, and
trimStart
and
trimEnd
on
String.prototype
as better-named alternatives to the widely implemented but non-standard
String.prototype.trimLeft
and
trimRight
built-ins. In addition, this specification includes a few minor updates
to syntax and semantics. Updated syntax includes optional catch binding
parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH
SEPARATOR) in string literals to align with JSON. Other updates include
requiring that
Array.prototype.sort
be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying
Function.prototype.toString
by requiring that it either return the corresponding original source text or a standard placeholder.
Dozens of individuals representing many organizations have made
very significant contributions within Ecma TC39 to the development of
this edition and to the prior editions. In addition, a vibrant community
has emerged supporting TC39's ECMAScript efforts. This community has
reviewed numerous drafts, filed thousands of bug reports, performed
implementation experiments, contributed test suites, and educated the
world-wide developer community about ECMAScript. Unfortunately, it is
impossible to identify and acknowledge every person and organization who
has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6
th
Edition
Brian Terlson
ECMA-262, Project Editor, 7
th
through 10
th
Editions
Scope
This Standard defines the ECMAScript 2019 general-purpose programming language.
Conformance
A conforming implementation of ECMAScript must provide and support
all the types, values, objects, properties, functions, and program
syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source
text input in conformance with the latest version of the Unicode
Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an
application programming interface (API) that supports programs that need
to adapt to the linguistic and cultural conventions used by different
human languages and countries must implement the interface defined by
the most recent edition of ECMA-402 that is compatible with this
specification.
A conforming implementation of ECMAScript may provide additional
types, values, objects, properties, and functions beyond those described
in this specification. In particular, a conforming implementation of
ECMAScript may provide properties not described in this specification,
and values for those properties, for objects that are described in this
specification.
A conforming implementation of ECMAScript may support program and
regular expression syntax not described in this specification. In
particular, a conforming implementation of ECMAScript may support
program syntax that makes use of the “future reserved words” listed in
subclause
11.6.2.2
of this specification.
A conforming implementation of ECMAScript must not implement any
extension that is listed as a Forbidden Extension in subclause
16.2
Normative References
The following referenced documents are indispensable for the
application of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
ISO/IEC 10646
Information Technology – Universal Multiple-Octet
Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006,
Amendment 3:2008, and Amendment 4:2008
, plus additional amendments and corrigenda, or successor
ECMA-402,
ECMAScript 2015 Internationalization API Specification
ECMA-404,
The JSON Data Interchange Format
Overview
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for
performing computations and manipulating computational objects within a
host environment. ECMAScript as defined here is not intended to be
computationally self-sufficient; indeed, there are no provisions in this
specification for input of external data or output of computed results.
Instead, it is expected that the computational environment of an
ECMAScript program will provide not only the objects and other
facilities described in this specification but also certain
environment-specific objects, whose description and behaviour are beyond
the scope of this specification except to indicate that they may
provide certain properties that can be accessed and certain functions
that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting
language, but has become widely used as a general-purpose programming
language. A
scripting language
is a programming language that
is used to manipulate, customize, and automate the facilities of an
existing system. In such systems, useful functionality is already
available through a user interface, and the scripting language is a
mechanism for exposing that functionality to program control. In this
way, the existing system is said to provide a host environment of
objects and facilities, which completes the capabilities of the
scripting language. A scripting language is intended for use by both
professional and non-professional programmers.
ECMAScript was originally designed to be a
Web scripting language
providing a mechanism to enliven Web pages in browsers and to perform
server computation as part of a Web-based client-server architecture.
ECMAScript is now used to provide core scripting capabilities for a
variety of host environments. Therefore the core language is specified
in this document apart from any particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now
used for the full spectrum of programming tasks in many different
environments and scales. As the usage of ECMAScript has expanded, so has
the features and facilities it provides. ECMAScript is now a fully
featured general-purpose programming language.
Some of the facilities of ECMAScript are similar to those used in
other programming languages; in particular C, Java™, Self, and Scheme as
described in:
ISO/IEC 9899:1996,
Programming Languages – C
Gosling, James, Bill Joy and Guy Steele.
The Java
Language Specification
. Addison Wesley Publishing Co., 1996.
Ungar, David, and Smith, Randall B. Self: The Power of Simplicity.
OOPSLA '87 Conference Proceedings
, pp. 227-241, Orlando, FL, October 1987.
IEEE Standard for the Scheme Programming Language
. IEEE Std 1178-1990.
4.1
Web Scripting
A web browser provides an ECMAScript host environment for
client-side computation including, for instance, objects that represent
windows, menus, pop-ups, dialog boxes, text areas, anchors, frames,
history, cookies, and input/output. Further, the host environment
provides a means to attach scripting code to events such as change of
focus, page and image loading, unloading, error and abort, selection,
form submission, and mouse actions. Scripting code appears within the
HTML and the displayed page is a combination of user interface elements
and fixed and computed text and images. The scripting code is reactive
to user interaction, and there is no need for a main program.
A web server provides a different host environment for
server-side computation including objects representing requests,
clients, and files; and mechanisms to lock and share data. By using
browser-side and server-side scripting together, it is possible to
distribute computation between the client and server while providing a
customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its
own host environment, completing the ECMAScript execution environment.
4.2
ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts
of the language are described. This overview is not part of the
standard proper.
ECMAScript is object-based: basic language and host facilities
are provided by objects, and an ECMAScript program is a cluster of
communicating objects. In ECMAScript, an
object
is a collection of zero or more
properties
each with
attributes
that determine how each property can be used—for example, when the Writable attribute for a property is set to
false
any attempt by executed ECMAScript code to assign a different value to
the property fails. Properties are containers that hold other objects,
primitive values
, or
functions
. A primitive value is a member of one of the following built-in types:
Undefined
Null
Boolean
Number
String
, and
Symbol;
an object is a member of the built-in type
Object
; and a function is a callable object. A function that is associated with an object via a property is called a
method
ECMAScript defines a collection of
built-in objects
that round out the definition of ECMAScript entities. These built-in objects include the
global object
; objects that are fundamental to the
runtime semantics
of the language including
Object
Function
Boolean
Symbol
, and various
Error
objects; objects that represent and manipulate numeric values including
Math
Number
, and
Date
; the text processing objects
String
and
RegExp
; objects that are indexed collections of values including
Array
and nine different kinds of Typed Arrays whose elements all have a
specific numeric data representation; keyed collections including
Map
and
Set
objects; objects supporting structured data including the
JSON
object,
ArrayBuffer
SharedArrayBuffer
, and
DataView
; objects supporting control abstractions including generator functions and
Promise
objects; and reflection objects including
Proxy
and
Reflect
ECMAScript also defines a set of built-in
operators
ECMAScript operators include various unary operations, multiplicative
operators, additive operators, bitwise shift operators, relational
operators, equality operators, binary bitwise operators, binary logical
operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by
modules
which allow a program to be divided into multiple sequences of
statements and declarations. Each module explicitly identifies
declarations it uses that need to be provided by other modules and which
of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript
syntax is relaxed to enable it to serve as an easy-to-use scripting
language. For example, a variable is not required to have its type
declared nor are types associated with properties, and defined functions
are not required to have their declarations appear textually before
calls to them.
4.2.1
Objects
Even though ECMAScript includes syntax for class definitions,
ECMAScript objects are not fundamentally class-based such as those in
C++, Smalltalk, or Java. Instead objects may be created in various ways
including via a literal notation or via
constructors
which
create objects and then execute code that initializes all or part of
them by assigning initial values to their properties. Each
constructor
is a function that has a property named
"prototype"
that is used to implement
prototype-based inheritance
and
shared properties
. Objects are created by using constructors in
new
expressions; for example,
new Date(2009, 11)
creates a new Date object. Invoking a
constructor
without using
new
has consequences that depend on the
constructor
. For example,
Date()
produces a string representation of the current date and time rather than an object.
Every object created by a
constructor
has an implicit reference (called the object's
prototype
) to the value of its
constructor
's
"prototype"
property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the
prototype chain
When a reference is made to a property in an object, that reference is
to the property of that name in the first object in the prototype chain
that contains a property of that name. In other words, first the object
mentioned directly is examined for such a property; if that object
contains the named property, that is the property to which the reference
refers; if that object does not contain the named property, the
prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is
carried by instances, methods are carried by classes, and inheritance
is only of structure and behaviour. In ECMAScript, the state and methods
are carried by objects, while structure, behaviour, and state are all
inherited.
All objects that do not directly contain a particular property
that their prototype contains share that property and its value. Figure 1
illustrates this:
CF
is a
constructor
(and also an object). Five objects have been created by using
new
expressions:
cf
cf
cf
cf
, and
cf
. Each of these objects contains properties named
q1
and
q2
. The dashed lines represent the implicit prototype relationship; so, for example,
cf
's prototype is
CF
. The
constructor
CF
, has two properties itself, named
P1
and
P2
, which are not visible to
CF
cf
cf
cf
cf
, or
cf
. The property named
CFP1
in
CF
is shared by
cf
cf
cf
cf
, and
cf
(but not by
CF
), as are any properties found in
CF
's implicit prototype chain that are not named
q1
q2
, or
CFP1
. Notice that there is no implicit prototype link between
CF
and
CF
Unlike most class-based object languages, properties can be
added to objects dynamically by assigning values to them. That is,
constructors are not required to name or assign values to all or any of
the constructed object's properties. In the above diagram, one could add
a new shared property for
cf
cf
cf
cf
, and
cf
by assigning a new value to the property in
CF
Although ECMAScript objects are not inherently class-based, it
is often convenient to define class-like abstractions based upon a
common pattern of
constructor
functions, prototype objects, and methods. The ECMAScript built-in
objects themselves follow such a class-like pattern. Beginning with
ECMAScript 2015, the ECMAScript language includes syntactic class
definitions that permit programmers to concisely define objects that
conform to the same class-like abstraction pattern used by the built-in
objects.
4.2.2
The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some
users of the language may wish to restrict their usage of some features
available in the language. They might do so in the interests of
security, to avoid what they consider to be error-prone features, to get
enhanced error checking, or for other reasons of their choosing. In
support of this possibility, ECMAScript defines a strict variant of the
language. The strict variant of the language excludes some specific
syntactic and semantic features of the regular ECMAScript language and
modifies the detailed semantics of some features. The strict variant
also specifies additional error conditions that must be reported by
throwing error exceptions in situations that are not specified as errors
by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the
strict mode
of the language. Strict mode selection and use of the strict mode
syntax and semantics of ECMAScript is explicitly made at the level of
individual ECMAScript source text units. Because strict mode is selected
at the level of a syntactic source text unit, strict mode only imposes
restrictions that have local effect within such a source text unit.
Strict mode does not restrict or modify any aspect of the ECMAScript
semantics that must operate consistently across multiple source text
units. A complete ECMAScript program may be composed of both strict mode
and non-strict mode ECMAScript source text units. In this case, strict
mode only applies when actually executing code that is defined within a
strict mode source text unit.
In order to conform to this specification, an ECMAScript
implementation must implement both the full unrestricted ECMAScript
language and the strict variant of the ECMAScript language as defined by
this specification. In addition, an implementation must support the
combination of unrestricted and strict mode source text units into a
single composite program.
4.3
Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.3.1
type
set of data values as defined in clause
of this specification
4.3.2
primitive value
member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause
Note
A primitive value is a datum that is represented directly at the lowest level of the language implementation.
4.3.3
object
member of the type Object
Note
An object is a collection of properties and has a single prototype object. The prototype may be the null value.
4.3.4
constructor
function object
that creates and initializes objects
Note
The value of a
constructor
's
prototype
property is a prototype object that is used to implement inheritance and shared properties.
4.3.5
prototype
object that provides shared properties for other objects
Note
When a
constructor
creates an object, that object implicitly references the
constructor
's
prototype
property for the purpose of resolving property references. The
constructor
's
prototype
property can be referenced by the program expression
constructor
.prototype
and properties added to an object's prototype are shared, through
inheritance, by all objects sharing the prototype. Alternatively, a new
object may be created with an explicitly specified prototype by using
the
Object.create
built-in function.
4.3.6
ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all objects
4.3.7
exotic object
object that does not have the default behaviour for one or more of the essential internal methods
Note
Any object that is not an ordinary object is an
exotic object
4.3.8
standard object
object whose semantics are defined by this specification
4.3.9
built-in object
object specified and supplied by an ECMAScript implementation
Note
Standard built-in objects are defined in this specification.
An ECMAScript implementation may specify and supply additional kinds of
built-in objects. A
built-in
constructor
is a built-in object that is also a
constructor
4.3.10
undefined value
primitive value used when a variable has not been assigned a value
4.3.11
Undefined type
type whose sole value is the
undefined
value
4.3.12
null value
primitive value that represents the intentional absence of any object value
4.3.13
Null type
type whose sole value is the
null
value
4.3.14
Boolean value
member of the Boolean type
Note
There are only two Boolean values,
true
and
false
4.3.15
Boolean type
type consisting of the primitive values
true
and
false
4.3.16
Boolean object
member of the Object type that is an instance of the standard built-in
Boolean
constructor
Note
A Boolean object is created by using the
Boolean
constructor
in a
new
expression, supplying a Boolean value as an argument. The resulting
object has an internal slot whose value is the Boolean value. A Boolean
object can be coerced to a Boolean value.
4.3.17
String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer values
Note
A String value is a member of the String type. Each integer
value in the sequence usually represents a single 16-bit unit of UTF-16
text. However, ECMAScript does not place any restrictions or
requirements on the values except that they must be 16-bit unsigned
integers.
4.3.18
String type
set of all possible String values
4.3.19
String object
member of the Object type that is an instance of the standard built-in
String
constructor
Note
A String object is created by using the
String
constructor
in a
new
expression, supplying a String value as an argument. The resulting
object has an internal slot whose value is the String value. A String
object can be coerced to a String value by calling the
String
constructor
as a function (
21.1.1.1
).
4.3.20
Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2008 value
Note
A Number value is a member of the Number type and is a direct representation of a number.
4.3.21
Number type
set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity
4.3.22
Number object
member of the Object type that is an instance of the standard built-in
Number
constructor
Note
A Number object is created by using the
Number
constructor
in a
new
expression, supplying a number value as an argument. The resulting
object has an internal slot whose value is the number value. A Number
object can be coerced to a number value by calling the
Number
constructor
as a function (
20.1.1.1
).
4.3.23
Infinity
number value that is the positive infinite number value
4.3.24
NaN
number value that is an IEEE 754-2008 “Not-a-Number” value
4.3.25
Symbol value
primitive value that represents a unique, non-String Object property key
4.3.26
Symbol type
set of all possible Symbol values
4.3.27
Symbol object
member of the Object type that is an instance of the standard built-in
Symbol
constructor
4.3.28
function
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable
code and state that determine how it behaves when invoked. A function's
code may or may not be written in ECMAScript.
4.3.29
built-in function
built-in object that is a function
Note
Examples of built-in functions include
parseInt
and
Math.exp
. An implementation may provide implementation-dependent built-in functions that are not described in this specification.
4.3.30
property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be
represented either directly as a data value (a primitive value, an
object, or a
function object
) or indirectly by a pair of accessor functions.
4.3.31
method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function as its
this
value.
4.3.32
built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification,
and an ECMAScript implementation may specify and provide other
additional built-in methods.
4.3.33
attribute
internal value that defines some characteristic of a property
4.3.34
own property
property that is directly contained by its object
4.3.35
inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object's prototype
4.4
Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the specification.
Clauses 6-9 define the execution environment within which ECMAScript programs operate.
Clauses 10-16 define the actual ECMAScript programming language
including its syntactic encoding and the execution semantics of all
language features.
Clauses 17-26 define the ECMAScript standard library. They
include the definitions of all of the standard objects that are
available for use by ECMAScript programs as they execute.
Clause 27 describes the memory consistency model of accesses on
SharedArrayBuffer-backed memory and methods of the Atomics object.
Notational Conventions
5.1
Syntactic and Lexical Grammars
5.1.1
Context-Free Grammars
context-free grammar
consists of a number of
productions
. Each production has an abstract symbol called a
nonterminal
as its
left-hand side
, and a sequence of zero or more nonterminal and
terminal
symbols as its
right-hand side
. For each grammar, the terminal symbols are drawn from a specified alphabet.
chain production
is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the
goal symbol
, a given context-free grammar specifies a
language
namely, the (perhaps infinite) set of possible sequences of terminal
symbols that can result from repeatedly replacing any nonterminal in the
sequence with a right-hand side of a production for which the
nonterminal is the left-hand side.
5.1.2
The Lexical and RegExp Grammars
lexical grammar
for ECMAScript is given in clause
11
. This grammar has as its terminal symbols Unicode code points that conform to the rules for
SourceCharacter
defined in
10.1
. It defines a set of productions, starting from the
goal symbol
InputElementDiv
InputElementTemplateTail
, or
InputElementRegExp
, or
InputElementRegExpOrTemplateTail
, that describe how sequences of such code points are translated into a sequence of input elements.
Input elements other than white space and comments form the
terminal symbols for the syntactic grammar for ECMAScript and are called
ECMAScript
tokens
. These tokens are the reserved words,
identifiers, literals, and punctuators of the ECMAScript language.
Moreover, line terminators, although not considered to be tokens, also
become part of the stream of input elements and guide the process of
automatic semicolon insertion (
11.9
).
Simple white space and single-line comments are discarded and do not
appear in the stream of input elements for the syntactic grammar. A
MultiLineComment
(that is, a comment of the form
/*
*/
regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a
MultiLineComment
contains one or more line terminators, then it is replaced by a single
line terminator, which becomes part of the stream of input elements for
the syntactic grammar.
RegExp grammar
for ECMAScript is given in
21.2.1
. This grammar also has as its terminal symbols the code points as defined by
SourceCharacter
. It defines a set of productions, starting from the
goal symbol
Pattern
, that describe how sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “
::
” as separating punctuation. The lexical and RegExp grammars share some productions.
5.1.3
The Numeric String Grammar
Another grammar is used for translating Strings into numeric
values. This grammar is similar to the part of the lexical grammar
having to do with numeric literals and has as its terminal symbols
SourceCharacter
. This grammar appears in
7.1.3.1
Productions of the numeric string grammar are distinguished by having three colons “
:::
” as punctuation.
5.1.4
The Syntactic Grammar
The
syntactic grammar
for ECMAScript is given in
clauses 11, 12, 13, 14, and 15. This grammar has ECMAScript tokens
defined by the lexical grammar as its terminal symbols (
5.1.2
). It defines a set of productions, starting from two alternative goal symbols
Script
and
Module
, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript
Script
or
Module
it is first converted to a stream of input elements by repeated
application of the lexical grammar; this stream of input elements is
then parsed by a single application of the syntactic grammar. The input
stream is syntactically in error if the tokens in the stream of input
elements cannot be parsed as a single instance of the goal nonterminal (
Script
or
Module
), with no tokens left over.
When a parse is successful, it constructs a
parse tree
, a rooted tree structure in which each node is a
Parse Node
. Each Parse Node is an
instance
of a symbol in the grammar; it represents a span of the source text
that can be derived from that symbol. The root node of the parse tree,
representing the whole of the source text, is an instance of the parse's
goal symbol
When a Parse Node is an instance of a nonterminal, it is also an
instance of some production that has that nonterminal as its left-hand
side. Moreover, it has zero or more
children
, one for each
symbol on the production's right-hand side: each child is a Parse Node
that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the
parser and never reused between parses even of identical source text.
Parse Nodes are considered
the same Parse Node
if and only
if they represent the same span of source text, are instances of the
same grammar symbol, and resulted from the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes, e.g., as occurs in:
eval(str); eval(str);
Note 2
Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “
” as punctuation.
The syntactic grammar as presented in clauses 12, 13, 14 and 15
is not a complete account of which token sequences are accepted as a
correct ECMAScript
Script
or
Module
Certain additional token sequences are also accepted, namely, those
that would be described by the grammar if only semicolons were added to
the sequence in certain places (such as before line terminator
characters). Furthermore, certain token sequences that are described by
the grammar are not considered acceptable if a line terminator character
appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic
grammar uses generalized productions that permit token sequences that do
not form a valid ECMAScript
Script
or
Module
. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive
supplemental grammar
is provided that further restricts the acceptable token sequences. Typically, an
early error
rule will then define an error condition if "
is not
covering
an
", where
is a Parse Node (an instance of the generalized production) and
is a nonterminal from the supplemental grammar. Here, the sequence of tokens originally matched by
is parsed again using
as the
goal symbol
. (If
takes grammatical parameters, then they are set to the same values used when
was originally parsed.) An error occurs if the sequence of tokens cannot be parsed as a single instance of
, with no tokens left over. Subsequently, algorithms access the result of the parse using a phrase of the form "the
that is
covered
by
". This will always be a Parse Node (an instance of
, unique for a given
), since any parsing failure would have been detected by an
early error
rule.
5.1.5
Grammar Notation
Terminal symbols of the lexical, RegExp, and numeric string grammars are shown in
fixed width
font, both in the productions of the grammars and throughout this
specification whenever the text directly refers to such a terminal
symbol. These are to appear in a script exactly as written. All terminal
symbol code points specified in this way are to be understood as the
appropriate Unicode code points from the Basic Latin range, as opposed
to any similar-looking code points from other Unicode ranges.
Nonterminal symbols are shown in
italic
type. The
definition of a nonterminal (also called a “production”) is introduced
by the name of the nonterminal being defined followed by one or more
colons. (The number of colons indicates to which grammar the production
belongs.) One or more alternative right-hand sides for the nonterminal
then follow on succeeding lines. For example, the syntactic definition:
WhileStatement
while
Expression
Statement
states that the nonterminal
WhileStatement
represents the token
while
, followed by a left parenthesis token, followed by an
Expression
, followed by a right parenthesis token, followed by a
Statement
. The occurrences of
Expression
and
Statement
are themselves nonterminals. As another example, the syntactic definition:
ArgumentList
AssignmentExpression
ArgumentList
AssignmentExpression
states that an
ArgumentList
may represent either a single
AssignmentExpression
or an
ArgumentList
, followed by a comma, followed by an
AssignmentExpression
. This definition of
ArgumentList
is recursive, that is, it is defined in terms of itself. The result is that an
ArgumentList
may contain any positive number of arguments, separated by commas, where each argument expression is an
AssignmentExpression
. Such recursive definitions of nonterminals are common.
The subscripted suffix “
opt
”, which may appear after
a terminal or nonterminal, indicates an optional symbol. The
alternative containing the optional symbol actually specifies two
right-hand sides, one that omits the optional element and one that
includes it. This means that:
VariableDeclaration
BindingIdentifier
Initializer
opt
is a convenient abbreviation for:
VariableDeclaration
BindingIdentifier
BindingIdentifier
Initializer
and that:
IterationStatement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
is a convenient abbreviation for:
IterationStatement
for
LexicalDeclaration
Expression
opt
Statement
for
LexicalDeclaration
Expression
Expression
opt
Statement
which in turn is an abbreviation for:
IterationStatement
for
LexicalDeclaration
Statement
for
LexicalDeclaration
Expression
Statement
for
LexicalDeclaration
Expression
Statement
for
LexicalDeclaration
Expression
Expression
Statement
so, in this example, the nonterminal
IterationStatement
actually has four alternative right-hand sides.
A production may be parameterized by a subscripted annotation of the form “
[parameters]
”, which may appear as a suffix to the nonterminal symbol defined by the production. “
parameters
may be either a single name or a comma separated list of names. A
parameterized production is shorthand for a set of productions defining
all combinations of the parameter names, preceded by an underscore,
appended to the parameterized nonterminal symbol. This means that:
StatementList
[Return]
ReturnStatement
ExpressionStatement
is a convenient abbreviation for:
StatementList
ReturnStatement
ExpressionStatement
StatementList_Return
ReturnStatement
ExpressionStatement
and that:
StatementList
[Return, In]
ReturnStatement
ExpressionStatement
is an abbreviation for:
StatementList
ReturnStatement
ExpressionStatement
StatementList_Return
ReturnStatement
ExpressionStatement
StatementList_In
ReturnStatement
ExpressionStatement
StatementList_Return_In
ReturnStatement
ExpressionStatement
Multiple parameters produce a combinatory number of
productions, not all of which are necessarily referenced in a complete
grammar.
References to nonterminals on the right-hand side of a production can also be parameterized. For example:
StatementList
ReturnStatement
ExpressionStatement
[+In]
is equivalent to saying:
StatementList
ReturnStatement
ExpressionStatement_In
and:
StatementList
ReturnStatement
ExpressionStatement
[~In]
is equivalent to:
StatementList
ReturnStatement
ExpressionStatement
A nonterminal reference may have both a parameter list and an “
opt
” suffix. For example:
VariableDeclaration
BindingIdentifier
Initializer
[+In]
opt
is an abbreviation for:
VariableDeclaration
BindingIdentifier
BindingIdentifier
Initializer_In
Prefixing a parameter name with “
” on a right-hand
side nonterminal reference makes that parameter value dependent upon the
occurrence of the parameter name on the reference to the current
production's left-hand side symbol. For example:
VariableDeclaration
[In]
BindingIdentifier
Initializer
[?In]
is an abbreviation for:
VariableDeclaration
BindingIdentifier
Initializer
VariableDeclaration_In
BindingIdentifier
Initializer_In
If a right-hand side alternative is prefixed with
“[+parameter]” that alternative is only available if the named parameter
was used in referencing the production's nonterminal symbol. If a
right-hand side alternative is prefixed with “[~parameter]” that
alternative is only available if the named parameter was
not
used in referencing the production's nonterminal symbol. This means that:
StatementList
[Return]
[+Return]
ReturnStatement
ExpressionStatement
is an abbreviation for:
StatementList
ExpressionStatement
StatementList_Return
ReturnStatement
ExpressionStatement
and that:
StatementList
[Return]
[~Return]
ReturnStatement
ExpressionStatement
is an abbreviation for:
StatementList
ReturnStatement
ExpressionStatement
StatementList_Return
ExpressionStatement
When the words “
one of
” follow the colon(s) in a grammar
definition, they signify that each of the terminal symbols on the
following line or lines is an alternative definition. For example, the
lexical grammar for ECMAScript contains the production:
NonZeroDigit
::
one of
which is merely a convenient abbreviation for:
NonZeroDigit
::
If the phrase “[empty]” appears as the right-hand side of a
production, it indicates that the production's right-hand side contains
no terminals or nonterminals.
If the phrase “[lookahead ∉
set
]” appears in the
right-hand side of a production, it indicates that the production may
not be used if the immediately following input token sequence is a
member of the given
set
. The
set
can be written as
a comma separated list of one or two element terminal sequences
enclosed in curly brackets. For convenience, the set can also be written
as a nonterminal, in which case it represents the set of all terminals
to which that nonterminal could expand. If the
set
consists of a single terminal the phrase “[lookahead ≠
terminal
]” may be used.
For example, given the definitions:
DecimalDigit
::
one of
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
the definition:
LookaheadExample
::
[lookahead ∉ {
}]
DecimalDigits
DecimalDigit
[lookahead ∉
DecimalDigit
matches either the letter
followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.
Similarly, if the phrase “[lookahead ∈
set
]” appears
in the right-hand side of a production, it indicates that the
production may only be used if the immediately following input token
sequence is a member of the given
set
. If the
set
consists of a single terminal the phrase “[lookahead =
terminal
]” may be used.
If the phrase “[no
LineTerminator
here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is
a restricted production
: it may not be used if a
LineTerminator
occurs in the input stream at the indicated position. For example, the production:
ThrowStatement
throw
[no
LineTerminator
here]
Expression
indicates that the production may not be used if a
LineTerminator
occurs in the script between the
throw
token and the
Expression
Unless the presence of a
LineTerminator
is forbidden by a restricted production, any number of occurrences of
LineTerminator
may appear between any two consecutive tokens in the stream of input
elements without affecting the syntactic acceptability of the script.
When an alternative in a production of the lexical grammar or
the numeric string grammar appears to be a multi-code point token, it
represents the sequence of code points that would make up such a token.
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “
but not
” and then indicating the expansions to be excluded. For example, the production:
Identifier
::
IdentifierName
but not
ReservedWord
means that the nonterminal
Identifier
may be replaced by any sequence of code points that could replace
IdentifierName
provided that the same sequence of code points could not replace
ReservedWord
Finally, a few nonterminal symbols are described by a
descriptive phrase in sans-serif type in cases where it would be
impractical to list all the alternatives:
SourceCharacter
::
any Unicode code point
5.2
Algorithm Conventions
The specification often uses a numbered list to specify steps in
an algorithm. These algorithms are used to precisely specify the
required semantics of ECMAScript language constructs. The algorithms are
not intended to imply the use of any specific implementation technique.
In practice, there may be more efficient algorithms available to
implement a given feature.
Algorithms may be explicitly parameterized, in which case the
names and usage of the parameters must be provided as part of the
algorithm's definition.
Algorithm steps may be subdivided into sequential substeps.
Substeps are indented and may themselves be further divided into
indented substeps. Outline numbering conventions are used to identify
substeps with the first level of substeps labelled with lower case
alphabetic characters and the second level of substeps labelled with
lower case roman numerals. If more than three levels are required these
rules repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that
conditions its substeps. In this case, the substeps are only applied if
the predicate is true. If a step or substep begins with the word “else”,
it is a predicate that is the negation of the preceding “if” predicate
step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “
Assert
:” asserts
an invariant condition of its algorithm. Such assertions are used to
make explicit algorithmic invariants that would otherwise be implicit.
Such assertions add no additional semantic requirements and hence need
not be checked by an implementation. They are used simply to clarify
algorithms.
Algorithm steps may declare named aliases for any value using the form “Let
be
someValue
”. These aliases are reference-like in that both
and
someValue
refer to the same underlying data and modifications to either are
visible to both. Algorithm steps that want to avoid this reference-like
behaviour should explicitly make a copy of the right-hand side: “Let
be a copy of
someValue
” creates a shallow copy of
someValue
Once declared, an alias may be referenced in any subsequent steps
and must not be referenced from steps prior to the alias's declaration.
Aliases may be modified using the form “Set
to
someOtherValue
”.
5.2.1
Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called
abstract operations
are named and written in parameterized functional form so that they may
be referenced by name from within other algorithms. Abstract operations
are typically referenced using a functional application style such as
OperationName(
arg1
arg2
). Some abstract
operations are treated as polymorphically dispatched methods of
class-like specification abstractions. Such method-like abstract
operations are typically referenced using a method application style
such as
someValue
.OperationName(
arg1
arg2
).
5.2.2
Syntax-Directed Operations
syntax-directed operation
is a named operation
whose definition consists of algorithms, each of which is associated
with one or more productions from one of the ECMAScript grammars. A
production that has multiple alternative definitions will typically have
a distinct algorithm for each alternative. When an algorithm is
associated with a grammar production, it may reference the terminal and
nonterminal symbols of the production alternative as if they were
parameters of the algorithm. When used in this manner, nonterminal
symbols refer to the actual alternative definition that is matched when
parsing the source text. The
source text matched by
grammar production is the portion of the source text that starts at the
beginning of the first terminal that participated in the match and ends
at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative,
the alternative is typically shown without any “[ ]” grammar
annotations. Such annotations should only affect the syntactic
recognition of the alternative and have no effect on the associated
semantics for the alternative.
Syntax-directed operations are invoked with a parse node and,
optionally, other parameters by using the conventions on steps 1, 3, and
4 in the following algorithm:
Let
status
be the result of performing SyntaxDirectedOperation of
SomeNonTerminal
Let
someParseNode
be the parse of some source text.
Perform SyntaxDirectedOperation of
someParseNode
Perform SyntaxDirectedOperation of
someParseNode
passing
"value"
as the argument.
Unless explicitly specified otherwise, all chain productions
have an implicit definition for every operation that might be applied to
that production's left-hand side nonterminal. The implicit definition
simply reapplies the same operation with the same parameters, if any, to
the
chain production
's
sole right-hand side nonterminal and then returns the result. For
example, assume that some algorithm has a step of the form: “Return the
result of evaluating
Block
” and that there is a production:
Block
StatementList
but the Evaluation operation does not associate an algorithm
with that production. In that case, the Evaluation operation implicitly
includes an association of the form:
Runtime Semantics: Evaluation
Block
StatementList
Return the result of evaluating
StatementList
5.2.3
Runtime Semantics
Algorithms which specify semantics that must be called at runtime are called
runtime semantics
. Runtime semantics are defined by
abstract operations
or syntax-directed operations. Such algorithms always return a completion record.
5.2.3.1
Implicit Completion Values
The algorithms of this specification often implicitly return
Completion
Records whose [[Type]] is
normal
. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a
Completion Record
, such as:
Return
"Infinity"
means the same thing as:
Return
NormalCompletion
"Infinity"
).
However, if the value expression of a “return” statement is a
Completion Record
construction literal, the resulting
Completion Record
is returned. If the value expression is a call to an abstract operation, the “return” statement simply returns the
Completion Record
produced by the abstract operation.
The abstract operation
Completion
completionRecord
) is used to emphasize that a previously computed
Completion Record
is being returned. The
Completion
abstract operation takes a single argument,
completionRecord
, and performs the following steps:
Assert
completionRecord
is a
Completion Record
Return
completionRecord
as the
Completion Record
of this abstract operation.
A “return” statement without a value in an algorithm step means the same thing as:
Return
NormalCompletion
undefined
).
Any reference to a
Completion Record
value that is in a context that does not explicitly require a complete
Completion Record
value is equivalent to an explicit reference to the [[Value]] field of the
Completion Record
value unless the
Completion Record
is an
abrupt completion
5.2.3.2
Throw an Exception
Algorithms steps that say to throw an exception, such as
Throw a
TypeError
exception.
mean the same things as:
Return
ThrowCompletion
(a newly created
TypeError
object).
5.2.3.3
ReturnIfAbrupt
Algorithms steps that say or are otherwise equivalent to:
ReturnIfAbrupt
argument
).
mean the same thing as:
If
argument
is an
abrupt completion
, return
argument
Else if
argument
is a
Completion Record
, set
argument
to
argument
.[[Value]].
Algorithms steps that say or are otherwise equivalent to:
ReturnIfAbrupt
(AbstractOperation()).
mean the same thing as:
Let
hygienicTemp
be AbstractOperation().
If
hygienicTemp
is an
abrupt completion
, return
hygienicTemp
Else if
hygienicTemp
is a
Completion Record
, set
hygienicTemp
to
hygienicTemp
.[[Value]].
Where
hygienicTemp
is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.
Algorithms steps that say or are otherwise equivalent to:
Let
result
be AbstractOperation(
ReturnIfAbrupt
argument
)).
mean the same thing as:
If
argument
is an
abrupt completion
, return
argument
If
argument
is a
Completion Record
, set
argument
to
argument
.[[Value]].
Let
result
be AbstractOperation(
argument
).
5.2.3.4
ReturnIfAbrupt Shorthands
Invocations of
abstract operations
and syntax-directed operations that are prefixed by
indicate that
ReturnIfAbrupt
should be applied to the resulting
Completion Record
. For example, the step:
? OperationName().
is equivalent to the following step:
ReturnIfAbrupt
(OperationName()).
Similarly, for method application style, the step:
someValue
.OperationName().
is equivalent to:
ReturnIfAbrupt
someValue
.OperationName()).
Similarly, prefix
is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an
abrupt completion
and that the resulting
Completion Record
's [[Value]] field should be used in place of the return value of the operation. For example, the step:
Let
val
be ! OperationName().
is equivalent to the following steps:
Let
val
be OperationName().
Assert
val
is never an
abrupt completion
If
val
is a
Completion Record
, set
val
to
val
.[[Value]].
Syntax-directed operations for
runtime semantics
make use of this shorthand by placing
or
before the invocation of the operation:
Perform ! SyntaxDirectedOperation of
NonTerminal
5.2.4
Static Semantics
Context-free grammars are not sufficiently powerful to express
all the rules that define whether a stream of input elements form a
valid ECMAScript
Script
or
Module
that may be evaluated. In some situations additional rules are needed
that may be expressed using either ECMAScript algorithm conventions or
prose requirements. Such rules are always associated with a production
of a grammar and are called the
static semantics
of the production.
Static Semantic Rules have names and typically are defined
using an algorithm. Named Static Semantic Rules are associated with
grammar productions and a production that has multiple alternative
definitions will typically have for each alternative a distinct
algorithm for each applicable named static semantic rule.
Unless otherwise specified every grammar production alternative
in this specification implicitly has a definition for a static semantic
rule named Contains which takes an argument named
symbol
whose value is a terminal or nonterminal of the grammar that includes
the associated production. The default definition of Contains is:
For each child node
child
of this
Parse Node
, do
If
child
is an instance of
symbol
, return
true
If
child
is an instance of a nonterminal, then
Let
contained
be the result of
child
Contains
symbol
If
contained
is
true
, return
true
Return
false
The above definition is explicitly over-ridden for specific productions.
A special kind of static semantic rule is an
Early Error Rule
Early error
rules define
early error
conditions (see clause
16
) that are associated with specific grammar productions. Evaluation of most
early error
rules are not explicitly invoked within the algorithms of this
specification. A conforming implementation must, prior to the first
evaluation of a
Script
or
Module
, validate all of the
early error
rules of the productions used to parse that
Script
or
Module
. If any of the
early error
rules are violated the
Script
or
Module
is invalid and cannot be evaluated.
5.2.5
Mathematical Operations
Mathematical operations such as addition, subtraction,
negation, multiplication, division, and the mathematical functions
defined later in this clause should always be understood as computing
exact mathematical results on mathematical real numbers, which unless
otherwise noted do not include infinities and do not include a negative
zero that is distinguished from positive zero. Algorithms in this
standard that model floating-point arithmetic include explicit steps,
where necessary, to handle infinities and signed zero and to perform
rounding. If a mathematical operation or function is applied to a
floating-point number, it should be understood as being applied to the
exact mathematical value represented by that floating-point number; such
a floating-point number must be finite, and if it is
+0
or
-0
then the corresponding mathematical value is simply 0.
The mathematical function
abs(
produces the absolute value of
, which is
if
is negative (less than zero) and otherwise is
itself.
The mathematical function
min(
x1
x2
, ...,
xN
produces the mathematically smallest of
x1
through
xN
. The mathematical function
max(
x1
x2
, ...,
xN
produces the mathematically largest of
x1
through
xN
. The domain and range of these mathematical functions include
+∞
and
-∞
The notation “
modulo
” (
must be finite and nonzero) computes a value
of the same sign as
(or zero) such that
abs
) <
abs
) and
for some integer
The mathematical function
floor(
produces the largest integer (closest to positive infinity) that is not larger than
Note
floor
) =
- (
modulo
1)
ECMAScript Data Types and Values
Algorithms within this specification manipulate values each of
which has an associated type. The possible value types are exactly those
defined in this clause. Types are further subclassified into ECMAScript
language types and specification types.
Within this specification, the notation “Type(
)” is used as shorthand for “the
type
of
where “type” refers to the ECMAScript language and specification types
defined in this clause. When the term “empty” is used as if it was
naming a value, it is equivalent to saying “no value of any type”.
6.1
ECMAScript Language Types
An
ECMAScript language type
corresponds to values
that are directly manipulated by an ECMAScript programmer using the
ECMAScript language. The ECMAScript language types are Undefined, Null,
Boolean, String, Symbol, Number, and Object. An
ECMAScript language value
is a value that is characterized by an ECMAScript language type.
6.1.1
The Undefined Type
The Undefined type has exactly one value, called
undefined
. Any variable that has not been assigned a value has the value
undefined
6.1.2
The Null Type
The Null type has exactly one value, called
null
6.1.3
The Boolean Type
The Boolean type represents a logical entity having two values, called
true
and
false
6.1.4
The String Type
The String type is the set of all ordered sequences of zero or
more 16-bit unsigned integer values (“elements”) up to a maximum length
of 2
53
- 1 elements. The String type is generally used to
represent textual data in a running ECMAScript program, in which case
each element in the String is treated as a UTF-16 code unit value. Each
element is regarded as occupying a position within the sequence. These
positions are indexed with nonnegative integers. The first element (if
any) is at index 0, the next element (if any) at index 1, and so on. The
length of a String is the number of elements (i.e., 16-bit values)
within it. The empty String has length zero and therefore contains no
elements.
ECMAScript operations that do not interpret String contents
apply no further semantics. Operations that do interpret String values
treat each element as a single UTF-16 code unit. However, ECMAScript
does not restrict the value of or relationships between these code
units, so operations that further interpret String contents as sequences
of Unicode code points encoded in UTF-16 must account for ill-formed
subsequences. Such operations apply special treatment to every code unit
with a numeric value in the inclusive range 0xD800 to 0xDBFF (defined
by the Unicode Standard as a
leading surrogate
, or more formally as a
high-surrogate code unit
) and every code unit with a numeric value in the inclusive range 0xDC00 to 0xDFFF (defined as a
trailing surrogate
, or more formally as a
low-surrogate code unit
) using the following rules:
A code unit that is not a
leading surrogate
and not a
trailing surrogate
is interpreted as a code point with the same value.
A sequence of two code units, where the first code unit
c1
is a
leading surrogate
and the second code unit
c2
trailing surrogate
, is a
surrogate pair
and is interpreted as a code point with the value (
c1
- 0xD800) × 0x400 + (
c2
- 0xDC00) + 0x10000. (See
10.1.2
A code unit that is a
leading surrogate
or
trailing surrogate
, but is not part of a
surrogate pair
, is interpreted as a code point with the same value.
The function
String.prototype.normalize
(see
21.1.3.12
) can be used to explicitly normalize a String value.
String.prototype.localeCompare
(see
21.1.3.10
internally normalizes String values, but no other operations implicitly
normalize the strings upon which they operate. Only operations that are
explicitly specified to be language or locale sensitive produce
language-sensitive results.
Note
The rationale behind this design was to keep the
implementation of Strings as simple and high-performing as possible. If
ECMAScript source text is in Normalized Form C, string literals are
guaranteed to also be normalized, as long as they do not contain any
Unicode escape sequences.
In this specification, the phrase "the
string-concatenation
of
..." (where each argument is a String value, a code unit, or a sequence
of code units) denotes the String value whose sequence of code units is
the concatenation of the code units (in order) of each of the arguments
(in order).
6.1.5
The Symbol Type
The Symbol type is the set of all non-String values that may be used as the key of an Object property (
6.1.7
).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that is either
undefined
or a String value.
6.1.5.1
Well-Known Symbols
Well-known symbols are built-in Symbol values that are
explicitly referenced by algorithms of this specification. They are
typically used as the keys of properties whose values serve as extension
points of a specification algorithm. Unless otherwise specified,
well-known symbols values are shared by all realms (
8.2
).
Within this specification a well-known symbol is referred to
by using a notation of the form @@name, where “name” is one of the
values listed in
Table 1
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
@@asyncIterator
"Symbol.asyncIterator"
A method that returns the default AsyncIterator for an object. Called by the semantics of the
for
await
of
statement.
@@hasInstance
"Symbol.hasInstance"
A method that determines if a
constructor
object recognizes an object as one of the
constructor
's instances. Called by the semantics of the
instanceof
operator.
@@isConcatSpreadable
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array elements by
Array.prototype.concat
@@iterator
"Symbol.iterator"
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by the
String.prototype.match
method.
@@replace
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by the
String.prototype.replace
method.
@@search
"Symbol.search"
A regular expression method that returns the index
within a string that matches the regular expression. Called by the
String.prototype.search
method.
@@species
"Symbol.species"
A function valued property that is the
constructor
function that is used to create derived objects.
@@split
"Symbol.split"
A regular expression method that splits a string at the
indices that match the regular expression. Called by the
String.prototype.split
method.
@@toPrimitive
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by the
ToPrimitive
abstract operation.
@@toStringTag
"Symbol.toStringTag"
A String valued property that is used in the creation of
the default string description of an object. Accessed by the built-in
method
Object.prototype.toString
@@unscopables
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are excluded from the
with
environment bindings of the associated object.
6.1.6
The Number Type
The Number type has exactly 18437736874454810627 (that is,
64
- 2
53
+ 3
values, representing the double-precision 64-bit format IEEE 754-2008
values as specified in the IEEE Standard for Binary Floating-Point
Arithmetic, except that the 9007199254740990 (that is,
53
- 2
) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special
NaN
value. (Note that the
NaN
value is produced by the program expression
NaN
.)
In some implementations, external code might be able to detect a
difference between various Not-a-Number values, but such behaviour is
implementation-dependent; to ECMAScript code, all
NaN
values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see
24.1
) or a SharedArrayBuffer (see
24.2
after a Number value has been stored into it is not necessarily the
same as the internal representation of that Number value used by the
ECMAScript implementation.
There are two other special values, called
positive Infinity
and
negative Infinity
. For brevity, these values are also referred to for expository purposes by the symbols
+∞
and
-∞
, respectively. (Note that these two infinite Number values are produced by the program expressions
+Infinity
(or simply
Infinity
) and
-Infinity
.)
The other 18437736874454810624 (that is,
64
- 2
53
values are called the finite numbers. Half of these are positive
numbers and half are negative numbers; for every finite positive Number
value there is a corresponding negative value having the same magnitude.
Note that there is both a
positive zero
and a
negative zero
. For brevity, these values are also referred to for expository purposes by the symbols
+0
and
-0
, respectively. (Note that these two different zero Number values are produced by the program expressions
+0
(or simply
) and
-0
.)
The 18437736874454810622 (that is,
64
- 2
53
- 2
) finite nonzero values are of two kinds:
18428729675200069632 (that is,
64
- 2
54
) of them are normalized, having the form
× 2
where
is +1 or -1,
is a positive integer less than 2
53
but not less than 2
52
, and
is an integer ranging from -1074 to 971, inclusive.
The remaining 9007199254740990 (that is,
53
- 2
) values are denormalized, having the form
× 2
where
is +1 or -1,
is a positive integer less than 2
52
, and
is -1074.
Note that all the positive and negative integers whose magnitude is no greater than 2
53
are representable in the Number type (indeed, the integer 0 has two representations,
+0
and
-0
).
A finite number has an
odd significand
if it is nonzero and the integer
used to express it (in one of the two forms shown above) is odd. Otherwise, it has an
even significand
In this specification, the phrase “the Number value for
” where
represents an exact real mathematical quantity (which might even be an
irrational number such as π) means a Number value chosen in the
following manner. Consider the set of all finite values of the Number
type, with
-0
removed and with two additional values added to it that are not representable in the Number type, namely 2
1024
(which is
+1 × 2
53
× 2
971
) and
-2
1024
(which is
-1 × 2
53
× 2
971
). Choose the member of this set that is closest in value to
If two values of the set are equally close, then the one with an even
significand is chosen; for this purpose, the two extra values 2
1024
and
-2
1024
are considered to have even significands. Finally, if 2
1024
was chosen, replace it with
+∞
; if
-2
1024
was chosen, replace it with
-∞
; if
+0
was chosen, replace it with
-0
if and only if
is less than zero; any other chosen value is used unchanged. The result is the Number value for
. (This procedure corresponds exactly to the behaviour of the IEEE 754-2008 “round to nearest, ties to even” mode.)
Some ECMAScript operators deal only with integers in specific ranges such as
-2
31
through
31
- 1
, inclusive, or in the range 0 through
16
- 1
inclusive. These operators accept any value of the Number type but
first convert each such value to an integer value in the expected range.
See the descriptions of the numeric conversion operations in
7.1
6.1.7
The Object Type
An Object is logically a collection of properties. Each property is either a data property, or an accessor property:
data property
associates a key value with an
ECMAScript language value
and a set of Boolean attributes.
An
accessor property
associates a key value with
one or two accessor functions, and a set of Boolean attributes. The
accessor functions are used to store or retrieve an
ECMAScript language value
that is associated with the property.
Properties are identified using key values. A property key
value is either an ECMAScript String value or a Symbol value. All String
and Symbol values, including the empty string, are valid as property
keys. A
property name
is a property key that is a String value.
An
integer index
is a String-valued property key that is a canonical numeric String (see
7.1.16
) and whose numeric value is either
+0
or a positive integer ≤ 2
53
- 1. An
array index
is an
integer index
whose numeric value
is in the range
+0 ≤
< 2
32
- 1
Property keys are used to access properties and their values. There are two kinds of access for properties:
get
and
set
corresponding to value retrieval and assignment, respectively. The
properties accessible via get and set access includes both
own properties
that are a direct part of an object and
inherited properties
which are provided by another associated object via a property
inheritance relationship. Inherited properties may be either own or
inherited properties of the associated object. Each own property of an
object must each have a key value that is distinct from the key values
of the other own properties of that object.
All objects are logically collections of properties, but there
are multiple forms of objects that differ in their semantics for
accessing and manipulating their properties.
Ordinary objects
are the most common form of objects and have the default object semantics. An
exotic object
is any form of object whose property semantics differ in any way from the default semantics.
6.1.7.1
Property Attributes
Attributes are used in this specification to define and explain the state of Object properties. A
data property
associates a key value with the attributes listed in
Table 2
Table 2: Attributes of a Data Property
Attribute Name
Value Domain
Description
[[Value]]
Any
ECMAScript language type
The value retrieved by a get access of the property.
[[Writable]]
Boolean
If
false
, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]
Boolean
If
true
, the property will be enumerated by a for-in enumeration (see
13.7.5
). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
If
false
, attempts to delete the property, change the property to be an
accessor property
, or change its attributes (other than [[Value]], or changing [[Writable]] to
false
) will fail.
An
accessor property
associates a key value with the attributes listed in
Table 3
Table 3: Attributes of an Accessor Property
Attribute Name
Value Domain
Description
[[Get]]
Object | Undefined
If the value is an Object it must be a
function object
. The function's [[Call]] internal method (
Table 6
) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]
Object | Undefined
If the value is an Object it must be a
function object
. The function's [[Call]] internal method (
Table 6
is called with an arguments list containing the assigned value as its
sole argument each time a set access of the property is performed. The
effect of a property's [[Set]] internal method may, but is not required
to, have an effect on the value returned by subsequent calls to the
property's [[Get]] internal method.
[[Enumerable]]
Boolean
If
true
, the property is to be enumerated by a for-in enumeration (see
13.7.5
). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
If
false
, attempts to delete the property, change the property to be a
data property
, or change its attributes will fail.
If the initial values of a property's attributes are not
explicitly specified by this specification, the default value defined in
Table 4
is used.
Table 4: Default Attribute Values
Attribute Name
Default Value
[[Value]]
undefined
[[Get]]
undefined
[[Set]]
undefined
[[Writable]]
false
[[Enumerable]]
false
[[Configurable]]
false
6.1.7.2
Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called
internal methods
Each object in an ECMAScript engine is associated with a set of
internal methods that defines its runtime behaviour. These internal
methods are not part of the ECMAScript language. They are defined by
this specification purely for expository purposes. However, each object
within an implementation of ECMAScript must behave as specified by the
internal methods associated with it. The exact manner in which this is
accomplished is determined by the implementation.
Internal method names are polymorphic. This means that
different object values may perform different algorithms when a common
internal method name is invoked upon them. That actual object upon which
an internal method is invoked is the “target” of the invocation. If, at
runtime, the implementation of an algorithm attempts to use an internal
method of an object that the object does not support, a
TypeError
exception is thrown.
Internal slots correspond to internal state that is
associated with objects and used by various ECMAScript specification
algorithms. Internal slots are not object properties and they are not
inherited. Depending upon the specific internal slot specification, such
state may consist of values of any
ECMAScript language type
or of specific ECMAScript specification type values. Unless explicitly
specified otherwise, internal slots are allocated as part of the process
of creating an object and may not be dynamically added to an object.
Unless specified otherwise, the initial value of an internal slot is the
value
undefined
. Various algorithms within this
specification create objects that have internal slots. However, the
ECMAScript language provides no direct way to associate internal slots
with an object.
Internal methods and internal slots are identified within
this specification using names enclosed in double square brackets [[ ]].
Table 5
summarizes the
essential internal methods
used by this specification that are applicable to all objects created
or manipulated by ECMAScript code. Every object must have algorithms for
all of the essential internal methods. However, all objects do not
necessarily use the same algorithms for those methods.
The “Signature” column of
Table 5
and other similar tables describes the invocation pattern for each
internal method. The invocation pattern always includes a parenthesized
list of descriptive parameter names. If a parameter name is the same as
an ECMAScript type name then the name describes the required type of the
parameter value. If an internal method explicitly returns a value, its
parameter list is followed by the symbol “→” and the type name of the
returned value. The type names used in signatures refer to the types
defined in clause
augmented by the following additional names. “
any
” means the value may be any
ECMAScript language type
. An internal method implicitly returns a
Completion Record
. In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.
Table 5: Essential Internal Methods
Internal Method
Signature
Description
[[GetPrototypeOf]]
( )
Object | Null
Determine the object that provides inherited properties for this object. A
null
value indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null)
Boolean
Associate this object with another object that provides inherited properties. Passing
null
indicates that there are no inherited properties. Returns
true
indicating that the operation was completed successfully or
false
indicating that the operation was not successful.
[[IsExtensible]]
( )
Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( )
Boolean
Control whether new properties may be added to this object. Returns
true
if the operation was successful or
false
if the operation was unsuccessful.
[[GetOwnProperty]]
propertyKey
Undefined |
Property Descriptor
Return a
Property Descriptor
for the own property of this object whose key is
propertyKey
, or
undefined
if no such property exists.
[[DefineOwnProperty]]
propertyKey
PropertyDescriptor
Boolean
Create or alter the own property, whose key is
propertyKey
, to have the state described by
PropertyDescriptor
. Return
true
if that property was successfully created/updated or
false
if the property could not be created or updated.
[[HasProperty]]
propertyKey
Boolean
Return a Boolean value indicating whether this object
already has either an own or inherited property whose key is
propertyKey
[[Get]]
propertyKey
Receiver
any
Return the value of the property whose key is
propertyKey
from this object. If any ECMAScript code must be executed to retrieve the property value,
Receiver
is used as the
this
value when evaluating the code.
[[Set]]
propertyKey
value
Receiver
Boolean
Set the value of the property whose key is
propertyKey
to
value
. If any ECMAScript code must be executed to set the property value,
Receiver
is used as the
this
value when evaluating the code. Returns
true
if the property value was set or
false
if it could not be set.
[[Delete]]
propertyKey
Boolean
Remove the own property whose key is
propertyKey
from this object. Return
false
if the property was not deleted and is still present. Return
true
if the property was deleted or is not present.
[[OwnPropertyKeys]]
( )
List
of propertyKey
Return a
List
whose elements are all of the own property keys for the object.
Table 6
summarizes additional essential internal methods that are supported by objects that may be called as functions. A
function object
is an object that supports the [[Call]] internal method. A
constructor
is an object that supports the [[Construct]] internal method. Every
object that supports [[Construct]] must support [[Call]]; that is, every
constructor
must be a
function object
. Therefore, a
constructor
may also be referred to as a
constructor
function
or
constructor
function object
Table 6: Additional Essential Internal Methods of Function Objects
Internal Method
Signature
Description
[[Call]]
any
, a
List
of
any
any
Executes code associated with this object. Invoked via a
function call expression. The arguments to the internal method are a
this
value and a list containing the arguments passed to the function by a
call expression. Objects that implement this internal method are
callable
[[Construct]]
(a
List
of
any
, Object)
Object
Creates an object. Invoked via the
new
or
super
operators. The first argument to the internal method is a list
containing the arguments of the operator. The second argument is the
object to which the
new
operator was initially applied. Objects that implement this internal method are called
constructors
. A
function object
is not necessarily a
constructor
and such non-
constructor
function objects do not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause
. If any specified use of an internal method of an
exotic object
is not supported by an implementation, that usage must throw a
TypeError
exception when attempted.
6.1.7.3
Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must
conform to the list of invariants specified below. Ordinary ECMAScript
Objects as well as all standard exotic objects in this specification
maintain these invariants. ECMAScript Proxy objects maintain these
invariants by means of runtime checks on the result of traps invoked on
the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain
these invariants for those objects. Violation of these invariants may
cause ECMAScript code to have unpredictable behaviour and create
security issues. However, violation of these invariants must never
compromise the memory safety of an implementation.
An implementation must not allow these invariants to be
circumvented in any manner such as by providing alternative interfaces
that implement the functionality of the essential internal methods
without enforcing their invariants.
Definitions:
The
target
of an internal method is the object upon which the internal method is called.
A target is
non-extensible
if it has been observed to return
false
from its [[IsExtensible]] internal method, or
true
from its [[PreventExtensions]] internal method.
non-existent
property is a property that does not exist as an own property on a non-extensible target.
All references to
SameValue
are according to the definition of the
SameValue
algorithm.
[[GetPrototypeOf]] ( )
The Type of the return value must be either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a value
, then any future calls to [[GetPrototypeOf]] should return the
SameValue
as
Note 1
An object's prototype chain should have finite length (that
is, starting from any object, recursively applying the
[[GetPrototypeOf]] internal method to its result should eventually lead
to the value
null
). However, this requirement is not
enforceable as an object level invariant if the prototype chain includes
any exotic objects that do not use the ordinary object definition of
[[GetPrototypeOf]]. Such a circular prototype chain may result in
infinite loops when accessing object properties.
[[SetPrototypeOf]] (
The Type of the return value must be Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return
false
, unless
is the
SameValue
as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The Type of the return value must be Boolean.
If [[IsExtensible]] returns
false
, all future calls to [[IsExtensible]] on the target must return
false
[[PreventExtensions]] ( )
The Type of the return value must be Boolean.
If [[PreventExtensions]] returns
true
, all future calls to [[IsExtensible]] on the target must return
false
and the target is now considered non-extensible.
[[GetOwnProperty]] (
The Type of the return value must be either
Property Descriptor
or Undefined.
If the Type of the return value is
Property Descriptor
, the return value must be a
complete property descriptor
If
is described as a non-configurable, non-writable own
data property
, all future calls to [[GetOwnProperty]] (
) must return Property Descritor whose [[Value]] is
SameValue
as
's [[Value]] attribute.
If
's attributes other than [[Writable]] may change over time or if the property might be deleted, then
's [[Configurable]] attribute must be
true
If the [[Writable]] attribute may change from
false
to
true
, then the [[Configurable]] attribute must be
true
If the target is non-extensible and
is non-existent, then all future calls to [[GetOwnProperty]] (
) on the target must describe
as non-existent (i.e. [[GetOwnProperty]] (
) must return
undefined
).
Note 2
As a consequence of the third invariant, if a property is described as a
data property
and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be
true
even if no mechanism to change the value is exposed via the other internal methods.
[[DefineOwnProperty]] (
Desc
The Type of the return value must be Boolean.
[[DefineOwnProperty]] must return
false
if
has previously been observed as a non-configurable own property of the target, unless either:
is a writable
data property
. A non-configurable writable
data property
can be changed into a non-configurable non-writable
data property
All attributes of
Desc
are the
SameValue
as
's attributes.
[[DefineOwnProperty]] (
Desc
) must return
false
if target is non-extensible and
is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] (
The Type of the return value must be Boolean.
If
was previously observed as a non-configurable own data or
accessor property
of the target, [[HasProperty]] must return
true
[[Get]] (
Receiver
If
was previously observed as a non-configurable, non-writable own
data property
of the target with value
, then [[Get]] must return the
SameValue
as
If
was previously observed as a non-configurable own
accessor property
of the target whose [[Get]] attribute is
undefined
, the [[Get]] operation must return
undefined
[[Set]] (
Receiver
The Type of the return value must be Boolean.
If
was previously observed as a non-configurable, non-writable own
data property
of the target, then [[Set]] must return
false
unless
is the
SameValue
as
's [[Value]] attribute.
If
was previously observed as a non-configurable own
accessor property
of the target whose [[Set]] attribute is
undefined
, the [[Set]] operation must return
false
[[Delete]] (
The Type of the return value must be Boolean.
If
was previously observed as a non-configurable own data or
accessor property
of the target, [[Delete]] must return
false
[[OwnPropertyKeys]] ( )
The return value must be a
List
The returned
List
must not contain any duplicate entries.
The Type of each element of the returned
List
is either String or Symbol.
The returned
List
must contain at least the keys of all non-configurable own properties that have previously been observed.
If the object is non-extensible, the returned
List
must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].
[[Construct]] ( )
The Type of the return value must be Object.
6.1.7.4
Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are
explicitly referenced by the algorithms of this specification and which
usually have
realm
-specific
identities. Unless otherwise specified each intrinsic object actually
corresponds to a set of similar objects, one per
realm
Within this specification a reference such as %name% means the intrinsic object, associated with the current
realm
, corresponding to the name. Determination of the current
realm
and its intrinsics is described in
8.3
. The well-known intrinsics are listed in
Table 7
Table 7: Well-Known Intrinsic Objects
Intrinsic Name
Global Name
ECMAScript Language Association
%Array%
Array
The
Array
constructor
22.1.1
%ArrayBuffer%
ArrayBuffer
The
ArrayBuffer
constructor
24.1.2
%ArrayBufferPrototype%
ArrayBuffer.prototype
The initial value of the
prototype
data property
of
%ArrayBuffer%
%ArrayIteratorPrototype%
The prototype of Array iterator objects (
22.1.5
%ArrayPrototype%
Array.prototype
The initial value of the
prototype
data property
of
%Array%
22.1.3
%ArrayProto_entries%
Array.prototype.entries
The initial value of the
entries
data property
of
%ArrayPrototype%
22.1.3.4
%ArrayProto_forEach%
Array.prototype.forEach
The initial value of the
forEach
data property
of
%ArrayPrototype%
22.1.3.12
%ArrayProto_keys%
Array.prototype.keys
The initial value of the
keys
data property
of
%ArrayPrototype%
22.1.3.16
%ArrayProto_values%
Array.prototype.values
The initial value of the
values
data property
of
%ArrayPrototype%
22.1.3.32
%AsyncFromSyncIteratorPrototype%
The prototype of async-from-sync iterator objects (
25.1.4
%AsyncFunction%
The
constructor
of async function objects (
25.7.1
%AsyncFunctionPrototype%
The initial value of the
prototype
data property
of
%AsyncFunction%
%AsyncGenerator%
The initial value of the
prototype
property of
%AsyncGeneratorFunction%
%AsyncGeneratorFunction%
The
constructor
of async iterator objects (
25.3.1
%AsyncGeneratorPrototype%
The initial value of the
prototype
property of
%AsyncGenerator%
%AsyncIteratorPrototype%
An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%
Atomics
The
Atomics
object (
24.4
%Boolean%
Boolean
The
Boolean
constructor
19.3.1
%BooleanPrototype%
Boolean.prototype
The initial value of the
prototype
data property
of
%Boolean%
19.3.3
%DataView%
DataView
The
DataView
constructor
24.3.2
%DataViewPrototype%
DataView.prototype
The initial value of the
prototype
data property
of
%DataView%
%Date%
Date
The
Date
constructor
20.3.2
%DatePrototype%
Date.prototype
The initial value of the
prototype
data property
of
%Date%
%decodeURI%
decodeURI
The
decodeURI
function (
18.2.6.2
%decodeURIComponent%
decodeURIComponent
The
decodeURIComponent
function (
18.2.6.3
%encodeURI%
encodeURI
The
encodeURI
function (
18.2.6.4
%encodeURIComponent%
encodeURIComponent
The
encodeURIComponent
function (
18.2.6.5
%Error%
Error
The
Error
constructor
19.5.1
%ErrorPrototype%
Error.prototype
The initial value of the
prototype
data property
of
%Error%
%eval%
eval
The
eval
function (
18.2.1
%EvalError%
EvalError
The
EvalError
constructor
19.5.5.1
%EvalErrorPrototype%
EvalError.prototype
The initial value of the
prototype
data property
of %EvalError%
%Float32Array%
Float32Array
The
Float32Array
constructor
22.2
%Float32ArrayPrototype%
Float32Array.prototype
The initial value of the
prototype
data property
of %Float32Array%
%Float64Array%
Float64Array
The
Float64Array
constructor
22.2
%Float64ArrayPrototype%
Float64Array.prototype
The initial value of the
prototype
data property
of %Float64Array%
%Function%
Function
The
Function
constructor
19.2.1
%FunctionPrototype%
Function.prototype
The initial value of the
prototype
data property
of
%Function%
%Generator%
The initial value of the
prototype
data property
of
%GeneratorFunction%
%GeneratorFunction%
The
constructor
of generator objects (
25.2.1
%GeneratorPrototype%
The initial value of the
prototype
data property
of
%Generator%
%Int8Array%
Int8Array
The
Int8Array
constructor
22.2
%Int8ArrayPrototype%
Int8Array.prototype
The initial value of the
prototype
data property
of %Int8Array%
%Int16Array%
Int16Array
The
Int16Array
constructor
22.2
%Int16ArrayPrototype%
Int16Array.prototype
The initial value of the
prototype
data property
of %Int16Array%
%Int32Array%
Int32Array
The
Int32Array
constructor
22.2
%Int32ArrayPrototype%
Int32Array.prototype
The initial value of the
prototype
data property
of %Int32Array%
%isFinite%
isFinite
The
isFinite
function (
18.2.2
%isNaN%
isNaN
The
isNaN
function (
18.2.3
%IteratorPrototype%
An object that all standard built-in iterator objects indirectly inherit from
%JSON%
JSON
The
JSON
object (
24.5
%JSONParse%
JSON.parse
The initial value of the
parse
data property
of
%JSON%
%JSONStringify%
JSON.stringify
The initial value of the
stringify
data property
of
%JSON%
%Map%
Map
The
Map
constructor
23.1.1
%MapIteratorPrototype%
The prototype of Map iterator objects (
23.1.5
%MapPrototype%
Map.prototype
The initial value of the
prototype
data property
of
%Map%
%Math%
Math
The
Math
object (
20.2
%Number%
Number
The
Number
constructor
20.1.1
%NumberPrototype%
Number.prototype
The initial value of the
prototype
data property
of
%Number%
%Object%
Object
The
Object
constructor
19.1.1
%ObjectPrototype%
Object.prototype
The initial value of the
prototype
data property
of
%Object%
19.1.3
%ObjProto_toString%
Object.prototype.toString
The initial value of the
toString
data property
of
%ObjectPrototype%
19.1.3.6
%ObjProto_valueOf%
Object.prototype.valueOf
The initial value of the
valueOf
data property
of
%ObjectPrototype%
19.1.3.7
%parseFloat%
parseFloat
The
parseFloat
function (
18.2.4
%parseInt%
parseInt
The
parseInt
function (
18.2.5
%Promise%
Promise
The
Promise
constructor
25.6.3
%PromisePrototype%
Promise.prototype
The initial value of the
prototype
data property
of
%Promise%
%PromiseProto_then%
Promise.prototype.then
The initial value of the
then
data property
of
%PromisePrototype%
25.6.5.4
%Promise_all%
Promise.all
The initial value of the
all
data property
of
%Promise%
25.6.4.1
%Promise_reject%
Promise.reject
The initial value of the
reject
data property
of
%Promise%
25.6.4.4
%Promise_resolve%
Promise.resolve
The initial value of the
resolve
data property
of
%Promise%
25.6.4.5
%Proxy%
Proxy
The
Proxy
constructor
26.2.1
%RangeError%
RangeError
The
RangeError
constructor
19.5.5.2
%RangeErrorPrototype%
RangeError.prototype
The initial value of the
prototype
data property
of %RangeError%
%ReferenceError%
ReferenceError
The
ReferenceError
constructor
19.5.5.3
%ReferenceErrorPrototype%
ReferenceError.prototype
The initial value of the
prototype
data property
of %ReferenceError%
%Reflect%
Reflect
The
Reflect
object (
26.1
%RegExp%
RegExp
The
RegExp
constructor
21.2.3
%RegExpPrototype%
RegExp.prototype
The initial value of the
prototype
data property
of
%RegExp%
%Set%
Set
The
Set
constructor
23.2.1
%SetIteratorPrototype%
The prototype of Set iterator objects (
23.2.5
%SetPrototype%
Set.prototype
The initial value of the
prototype
data property
of
%Set%
%SharedArrayBuffer%
SharedArrayBuffer
The
SharedArrayBuffer
constructor
24.2.2
%SharedArrayBufferPrototype%
SharedArrayBuffer.prototype
The initial value of the
prototype
data property
of
%SharedArrayBuffer%
%String%
String
The
String
constructor
21.1.1
%StringIteratorPrototype%
The prototype of String iterator objects (
21.1.5
%StringPrototype%
String.prototype
The initial value of the
prototype
data property
of
%String%
%Symbol%
Symbol
The
Symbol
constructor
19.4.1
%SymbolPrototype%
Symbol.prototype
The initial value of the
prototype
data property
of
%Symbol%
19.4.3
%SyntaxError%
SyntaxError
The
SyntaxError
constructor
19.5.5.4
%SyntaxErrorPrototype%
SyntaxError.prototype
The initial value of the
prototype
data property
of %SyntaxError%
%ThrowTypeError%
function object
that unconditionally throws a new instance of %TypeError%
%TypedArray%
The super class of all typed Array constructors (
22.2.1
%TypedArrayPrototype%
The initial value of the
prototype
data property
of
%TypedArray%
%TypeError%
TypeError
The
TypeError
constructor
19.5.5.5
%TypeErrorPrototype%
TypeError.prototype
The initial value of the
prototype
data property
of %TypeError%
%Uint8Array%
Uint8Array
The
Uint8Array
constructor
22.2
%Uint8ArrayPrototype%
Uint8Array.prototype
The initial value of the
prototype
data property
of %Uint8Array%
%Uint8ClampedArray%
Uint8ClampedArray
The
Uint8ClampedArray
constructor
22.2
%Uint8ClampedArrayPrototype%
Uint8ClampedArray.prototype
The initial value of the
prototype
data property
of %Uint8ClampedArray%
%Uint16Array%
Uint16Array
The
Uint16Array
constructor
22.2
%Uint16ArrayPrototype%
Uint16Array.prototype
The initial value of the
prototype
data property
of %Uint16Array%
%Uint32Array%
Uint32Array
The
Uint32Array
constructor
22.2
%Uint32ArrayPrototype%
Uint32Array.prototype
The initial value of the
prototype
data property
of %Uint32Array%
%URIError%
URIError
The
URIError
constructor
19.5.5.6
%URIErrorPrototype%
URIError.prototype
The initial value of the
prototype
data property
of %URIError%
%WeakMap%
WeakMap
The
WeakMap
constructor
23.3.1
%WeakMapPrototype%
WeakMap.prototype
The initial value of the
prototype
data property
of
%WeakMap%
%WeakSet%
WeakSet
The
WeakSet
constructor
23.4.1
%WeakSetPrototype%
WeakSet.prototype
The initial value of the
prototype
data property
of
%WeakSet%
6.2
ECMAScript Specification Types
A specification type corresponds to meta-values that are used
within algorithms to describe the semantics of ECMAScript language
constructs and ECMAScript language types. The specification types
include
Reference
List
Completion
Property Descriptor
Lexical Environment
Environment Record
, and
Data Block
Specification type values are specification artefacts that do not
necessarily correspond to any specific entity within an ECMAScript
implementation. Specification type values may be used to describe
intermediate results of ECMAScript expression evaluation but such values
cannot be stored as properties of objects or values of ECMAScript
language variables.
6.2.1
The List and Record Specification Types
The
List
type is used to explain the evaluation of argument lists (see
12.3.6
) in
new
expressions, in function calls, and in other algorithms where a simple
ordered list of values is needed. Values of the List type are simply
ordered sequences of list elements containing the individual values.
These sequences may be of any length. The elements of a list may be
randomly accessed using 0-origin indices. For notational convenience an
array-like syntax can be used to access List elements. For example,
arguments
[2] is shorthand for saying the 3
rd
element of the List
arguments
For notational convenience within this specification, a literal
syntax can be used to express a new List value. For example, « 1, 2 »
defines a List value that has two elements each of which is initialized
to a specific value. A new empty List can be expressed as « ».
The
Record
type is used to describe data
aggregations within the algorithms of this specification. A Record type
value consists of one or more named fields. The value of each field is
either an ECMAScript value or an abstract value represented by a name
associated with the Record type. Field names are always enclosed in
double brackets, for example [[Value]].
For notational convenience within this specification, an object
literal-like syntax can be used to express a Record value. For example,
{ [[Field1]]: 42, [[Field2]]:
false
, [[Field3]]:
empty
} defines a Record value that has three fields, each of which is
initialized to a specific value. Field name order is not significant.
Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used
to refer to a specific field of a Record value. For example, if R is the
record shown in the previous paragraph then R.[[Field2]] is shorthand
for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be
named, and that name may be used as a prefix to a literal Record value
to identify the specific kind of aggregations that is being described.
For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]:
false
, [[Configurable]]:
true
}.
6.2.2
The Set and Relation Specification Types
The
Set
type is used to explain a collection of unordered elements for use in the
memory model
Values of the Set type are simple collections of elements, where no
element appears more than once. Elements may be added to and removed
from Sets. Sets may be unioned, intersected, or subtracted from each
other.
The
Relation
type is used to explain constraints on
Sets. Values of the Relation type are Sets of ordered pairs of values
from its value domain. For example, a Relation on events is a set of
ordered pairs of events. For a Relation
and two values
and
in the value domain of
is shorthand for saying the ordered pair (
) is a member of
. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.
strict partial order
is a Relation value
that satisfies the following.
For all
, and
in
's domain:
It is not the case that
, and
If
and
, then
Note 1
The two properties above are called, in order, irreflexivity and transitivity.
strict total order
is a Relation value
that satisfies the following.
For all
, and
in
's domain:
is identical to
or
or
, and
It is not the case that
, and
If
and
, then
Note 2
The three properties above are called, in order, totality, irreflexivity, and transitivity.
6.2.3
The Completion Record Specification Type
The Completion type is a
Record
used to explain the runtime propagation of values and control flow such as the behaviour of statements (
break
continue
return
and
throw
) that perform nonlocal transfers of control.
Values of the Completion type are
Record
values whose fields are defined as by
Table 8
. Such values are referred to as
Completion Record
s.
Table 8:
Completion Record
Fields
Field Name
Value
Meaning
[[Type]]
One of
normal
break
continue
return
, or
throw
The type of completion that occurred.
[[Value]]
any
ECMAScript language value
or
empty
The value that was produced.
[[Target]]
any ECMAScript string or
empty
The target label for directed control transfers.
The term “
abrupt completion
” refers to any completion with a [[Type]] value other than
normal
6.2.3.1
Await
Algorithm steps that say
Let
completion
be
Await
value
).
mean the same thing as:
Let
asyncContext
be the
running execution context
Let
promise
be ?
PromiseResolve
%Promise%
, «
value
»).
Let
stepsFulfilled
be the algorithm steps defined in
Await Fulfilled Functions
Let
onFulfilled
be
CreateBuiltinFunction
stepsFulfilled
, « [[AsyncContext]] »).
Set
onFulfilled
.[[AsyncContext]] to
asyncContext
Let
stepsRejected
be the algorithm steps defined in
Await Rejected Functions
Let
onRejected
be
CreateBuiltinFunction
stepsRejected
, « [[AsyncContext]] »).
Set
onRejected
.[[AsyncContext]] to
asyncContext
Perform !
PerformPromiseThen
promise
onFulfilled
onRejected
).
Remove
asyncContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
Set the code evaluation state of
asyncContext
such that when evaluation is resumed with a
Completion
completion
, the following steps of the algorithm that invoked
Await
will be performed, with
completion
available.
Return.
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of
asyncContext
where all variables in the above steps, with the exception of
completion
, are ephemeral and visible only in the steps pertaining to Await.
Note
Await can be combined with the
and
prefixes, so that for example
Let
result
be ?
Await
value
).
means the same thing as:
Let
result
be
Await
value
).
ReturnIfAbrupt
result
).
6.2.3.1.1
Await Fulfilled Functions
An
Await
fulfilled function is an anonymous built-in function that is used as part of the
Await
specification device to deliver the promise fulfillment value to the caller as a normal completion. Each
Await
fulfilled function has an [[AsyncContext]] internal slot.
When an
Await
fulfilled function is called with argument
value
, the following steps are taken:
Let
be the
active function object
Let
asyncContext
be
.[[AsyncContext]].
Let
prevContext
be the
running execution context
Suspend
prevContext
Push
asyncContext
onto the
execution context stack
asyncContext
is now the
running execution context
Resume the suspended evaluation of
asyncContext
using
NormalCompletion
value
) as the result of the operation that suspended it.
Assert
: When we reach this step,
asyncContext
has already been removed from the
execution context stack
and
prevContext
is the currently
running execution context
Return
undefined
The
"length"
property of an
Await
fulfilled function is 1.
6.2.3.1.2
Await Rejected Functions
An
Await
rejected function is an anonymous built-in function that is used as part of the
Await
specification device to deliver the promise rejection reason to the caller as an abrupt throw completion. Each
Await
rejected function has an [[AsyncContext]] internal slot.
When an
Await
rejected function is called with argument
reason
, the following steps are taken:
Let
be the
active function object
Let
asyncContext
be
.[[AsyncContext]].
Let
prevContext
be the
running execution context
Suspend
prevContext
Push
asyncContext
onto the
execution context stack
asyncContext
is now the
running execution context
Resume the suspended evaluation of
asyncContext
using
ThrowCompletion
reason
) as the result of the operation that suspended it.
Assert
: When we reach this step,
asyncContext
has already been removed from the
execution context stack
and
prevContext
is the currently
running execution context
Return
undefined
The
"length"
property of an
Await
rejected function is 1.
6.2.3.2
NormalCompletion
The abstract operation NormalCompletion with a single
argument
, such as:
Return
NormalCompletion
argument
).
Is a shorthand that is defined as follows:
Return
Completion
{ [[Type]]:
normal
, [[Value]]:
argument
, [[Target]]:
empty
}.
6.2.3.3
ThrowCompletion
The abstract operation ThrowCompletion with a single
argument
, such as:
Return
ThrowCompletion
argument
).
Is a shorthand that is defined as follows:
Return
Completion
{ [[Type]]:
throw
, [[Value]]:
argument
, [[Target]]:
empty
}.
6.2.3.4
UpdateEmpty (
completionRecord
value
The abstract operation UpdateEmpty with arguments
completionRecord
and
value
performs the following steps:
Assert
: If
completionRecord
.[[Type]] is either
return
or
throw
, then
completionRecord
.[[Value]] is not
empty
If
completionRecord
.[[Value]] is not
empty
, return
Completion
completionRecord
).
Return
Completion
{ [[Type]]:
completionRecord
.[[Type]], [[Value]]:
value
, [[Target]]:
completionRecord
.[[Target]] }.
6.2.4
The Reference Specification Type
Note
The Reference type is used to explain the behaviour of such operators as
delete
typeof
, the assignment operators, the
super
keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a reference.
Reference
is a resolved name or property binding.
A Reference consists of three components, the base value component, the
referenced name component, and the Boolean-valued strict reference
flag. The base value component is either
undefined
, an Object, a Boolean, a String, a Symbol, a Number, or an
Environment Record
. A base value component of
undefined
indicates that the Reference could not be resolved to a binding. The referenced name component is a String or Symbol value.
Super Reference
is a Reference that is used to represent a name binding that was expressed using the super keyword. A
Super Reference
has an additional thisValue component, and its base value component will never be an
Environment Record
The following
abstract operations
are used in this specification to operate on references:
6.2.4.1
GetBase (
Assert
Type
) is
Reference
Return the base value component of
6.2.4.2
GetReferencedName (
Assert
Type
) is
Reference
Return the referenced name component of
6.2.4.3
IsStrictReference (
Assert
Type
) is
Reference
Return the strict reference flag of
6.2.4.4
HasPrimitiveBase (
Assert
Type
) is
Reference
If
Type
's base value component) is Boolean, String, Symbol, or Number, return
true
; otherwise return
false
6.2.4.5
IsPropertyReference (
Assert
Type
) is
Reference
If either the base value component of
is an Object or
HasPrimitiveBase
) is
true
, return
true
; otherwise return
false
6.2.4.6
IsUnresolvableReference (
Assert
Type
) is
Reference
If the base value component of
is
undefined
, return
true
; otherwise return
false
6.2.4.7
IsSuperReference (
Assert
Type
) is
Reference
If
has a thisValue component, return
true
; otherwise return
false
6.2.4.8
GetValue (
ReturnIfAbrupt
).
If
Type
) is not
Reference
, return
Let
base
be
GetBase
).
If
IsUnresolvableReference
) is
true
, throw a
ReferenceError
exception.
If
IsPropertyReference
) is
true
, then
If
HasPrimitiveBase
) is
true
, then
Assert
: In this case,
base
will never be
undefined
or
null
Set
base
to !
ToObject
base
).
Return ?
base
.[[Get]](
GetReferencedName
),
GetThisValue
)).
Else
base
must be an
Environment Record
Return ?
base
.GetBindingValue(
GetReferencedName
),
IsStrictReference
)) (see
8.1.1
).
Note
The object that may be created in step 5.a.ii is not
accessible outside of the above abstract operation and the ordinary
object [[Get]] internal method. An implementation might choose to avoid
the actual creation of the object.
6.2.4.9
PutValue (
ReturnIfAbrupt
).
ReturnIfAbrupt
).
If
Type
) is not
Reference
, throw a
ReferenceError
exception.
Let
base
be
GetBase
).
If
IsUnresolvableReference
) is
true
, then
If
IsStrictReference
) is
true
, then
Throw a
ReferenceError
exception.
Let
globalObj
be
GetGlobalObject
().
Return ?
Set
globalObj
GetReferencedName
),
false
).
Else if
IsPropertyReference
) is
true
, then
If
HasPrimitiveBase
) is
true
, then
Assert
: In this case,
base
will never be
undefined
or
null
Set
base
to !
ToObject
base
).
Let
succeeded
be ?
base
.[[Set]](
GetReferencedName
),
GetThisValue
)).
If
succeeded
is
false
and
IsStrictReference
) is
true
, throw a
TypeError
exception.
Return.
Else
base
must be an
Environment Record
Return ?
base
.SetMutableBinding(
GetReferencedName
),
IsStrictReference
)) (see
8.1.1
).
Note
The object that may be created in step 6.a.ii is not
accessible outside of the above algorithm and the ordinary object
[[Set]] internal method. An implementation might choose to avoid the
actual creation of that object.
6.2.4.10
GetThisValue (
Assert
IsPropertyReference
) is
true
If
IsSuperReference
) is
true
, then
Return the value of the thisValue component of the reference
Return
GetBase
).
6.2.4.11
InitializeReferencedBinding (
ReturnIfAbrupt
).
ReturnIfAbrupt
).
Assert
Type
) is
Reference
Assert
IsUnresolvableReference
) is
false
Let
base
be
GetBase
).
Assert
base
is an
Environment Record
Return
base
.InitializeBinding(
GetReferencedName
),
).
6.2.5
The Property Descriptor Specification Type
The
Property Descriptor
type is used to explain the
manipulation and reification of Object property attributes. Values of
the Property Descriptor type are Records. Each field's name is an
attribute name and its value is a corresponding attribute value as
specified in
6.1.7.1
In addition, any field may be present or absent. The schema name used
within this specification to tag literal descriptions of Property
Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data
Property Descriptors and accessor Property Descriptors based upon the
existence or use of certain fields. A data Property Descriptor is one
that includes any fields named either [[Value]] or [[Writable]]. An
accessor Property Descriptor is one that includes any fields named
either [[Get]] or [[Set]]. Any Property Descriptor may have fields named
[[Enumerable]] and [[Configurable]]. A Property Descriptor value may
not be both a data Property Descriptor and an accessor Property
Descriptor; however, it may be neither. A generic Property Descriptor is
a Property Descriptor value that is neither a data Property Descriptor
nor an accessor Property Descriptor. A fully populated Property
Descriptor is one that is either an accessor Property Descriptor or a
data Property Descriptor and that has all of the fields that correspond
to the property attributes defined in either
Table 2
or
Table 3
The following
abstract operations
are used in this specification to operate upon Property Descriptor values:
6.2.5.1
IsAccessorDescriptor (
Desc
When the abstract operation IsAccessorDescriptor is called with
Property Descriptor
Desc
, the following steps are taken:
If
Desc
is
undefined
, return
false
If both
Desc
.[[Get]] and
Desc
.[[Set]] are absent, return
false
Return
true
6.2.5.2
IsDataDescriptor (
Desc
When the abstract operation IsDataDescriptor is called with
Property Descriptor
Desc
, the following steps are taken:
If
Desc
is
undefined
, return
false
If both
Desc
.[[Value]] and
Desc
.[[Writable]] are absent, return
false
Return
true
6.2.5.3
IsGenericDescriptor (
Desc
When the abstract operation IsGenericDescriptor is called with
Property Descriptor
Desc
, the following steps are taken:
If
Desc
is
undefined
, return
false
If
IsAccessorDescriptor
Desc
) and
IsDataDescriptor
Desc
) are both
false
, return
true
Return
false
6.2.5.4
FromPropertyDescriptor (
Desc
When the abstract operation FromPropertyDescriptor is called with
Property Descriptor
Desc
, the following steps are taken:
If
Desc
is
undefined
, return
undefined
Let
obj
be
ObjectCreate
%ObjectPrototype%
).
Assert
obj
is an extensible ordinary object with no own properties.
If
Desc
has a [[Value]] field, then
Perform
CreateDataProperty
obj
"value"
Desc
.[[Value]]).
If
Desc
has a [[Writable]] field, then
Perform
CreateDataProperty
obj
"writable"
Desc
.[[Writable]]).
If
Desc
has a [[Get]] field, then
Perform
CreateDataProperty
obj
"get"
Desc
.[[Get]]).
If
Desc
has a [[Set]] field, then
Perform
CreateDataProperty
obj
"set"
Desc
.[[Set]]).
If
Desc
has an [[Enumerable]] field, then
Perform
CreateDataProperty
obj
"enumerable"
Desc
.[[Enumerable]]).
If
Desc
has a [[Configurable]] field, then
Perform
CreateDataProperty
obj
"configurable"
Desc
.[[Configurable]]).
Assert
: All of the above
CreateDataProperty
operations return
true
Return
obj
6.2.5.5
ToPropertyDescriptor (
Obj
When the abstract operation ToPropertyDescriptor is called with object
Obj
, the following steps are taken:
If
Type
Obj
) is not Object, throw a
TypeError
exception.
Let
desc
be a new
Property Descriptor
that initially has no fields.
Let
hasEnumerable
be ?
HasProperty
Obj
"enumerable"
).
If
hasEnumerable
is
true
, then
Let
enumerable
be
ToBoolean
(?
Get
Obj
"enumerable"
)).
Set
desc
.[[Enumerable]] to
enumerable
Let
hasConfigurable
be ?
HasProperty
Obj
"configurable"
).
If
hasConfigurable
is
true
, then
Let
configurable
be
ToBoolean
(?
Get
Obj
"configurable"
)).
Set
desc
.[[Configurable]] to
configurable
Let
hasValue
be ?
HasProperty
Obj
"value"
).
If
hasValue
is
true
, then
Let
value
be ?
Get
Obj
"value"
).
Set
desc
.[[Value]] to
value
Let
hasWritable
be ?
HasProperty
Obj
"writable"
).
If
hasWritable
is
true
, then
Let
writable
be
ToBoolean
(?
Get
Obj
"writable"
)).
Set
desc
.[[Writable]] to
writable
Let
hasGet
be ?
HasProperty
Obj
"get"
).
If
hasGet
is
true
, then
Let
getter
be ?
Get
Obj
"get"
).
If
IsCallable
getter
) is
false
and
getter
is not
undefined
, throw a
TypeError
exception.
Set
desc
.[[Get]] to
getter
Let
hasSet
be ?
HasProperty
Obj
"set"
).
If
hasSet
is
true
, then
Let
setter
be ?
Get
Obj
"set"
).
If
IsCallable
setter
) is
false
and
setter
is not
undefined
, throw a
TypeError
exception.
Set
desc
.[[Set]] to
setter
If
desc
.[[Get]] is present or
desc
.[[Set]] is present, then
If
desc
.[[Value]] is present or
desc
.[[Writable]] is present, throw a
TypeError
exception.
Return
desc
6.2.5.6
CompletePropertyDescriptor (
Desc
When the abstract operation CompletePropertyDescriptor is called with
Property Descriptor
Desc
, the following steps are taken:
Assert
Desc
is a
Property Descriptor
Let
like
be
Record
{ [[Value]]:
undefined
, [[Writable]]:
false
, [[Get]]:
undefined
, [[Set]]:
undefined
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
If
IsGenericDescriptor
Desc
) is
true
or
IsDataDescriptor
Desc
) is
true
, then
If
Desc
does not have a [[Value]] field, set
Desc
.[[Value]] to
like
.[[Value]].
If
Desc
does not have a [[Writable]] field, set
Desc
.[[Writable]] to
like
.[[Writable]].
Else,
If
Desc
does not have a [[Get]] field, set
Desc
.[[Get]] to
like
.[[Get]].
If
Desc
does not have a [[Set]] field, set
Desc
.[[Set]] to
like
.[[Set]].
If
Desc
does not have an [[Enumerable]] field, set
Desc
.[[Enumerable]] to
like
.[[Enumerable]].
If
Desc
does not have a [[Configurable]] field, set
Desc
.[[Configurable]] to
like
.[[Configurable]].
Return
Desc
6.2.6
The Lexical Environment and Environment Record Specification Types
The
Lexical Environment
and
Environment Record
types are used to explain the behaviour of name resolution in nested
functions and blocks. These types and the operations upon them are
defined in
8.1
6.2.7
Data Blocks
The
Data Block
specification type is used to
describe a distinct and mutable sequence of byte-sized (8 bit) numeric
values. A Data Block value is created with a fixed number of bytes that
each have the initial value 0.
For notational convenience within this specification, an
array-like syntax can be used to access the individual bytes of a Data
Block value. This notation presents a Data Block value as a 0-origined
integer-indexed sequence of bytes. For example, if
db
is a 5 byte Data Block value then
db
[2] can be used to access its 3
rd
byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a
Shared Data Block
. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is
address-free
it is tied not to the virtual addresses the block is mapped to in any
process, but to the set of locations in memory that the block
represents. Two data blocks are equal only if the sets of the locations
they contain are equal; otherwise, they are not equal and the
intersection of the sets of locations they contain is empty. Finally,
Shared Data Blocks can be distinguished from Data Blocks.
The semantics of Shared Data Blocks is defined using Shared Data Block events by the
memory model
Abstract operations
below introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of the
memory model
. The events form a
candidate execution
, on which the
memory model
acts as a filter. Please consult the
memory model
for full semantics.
Shared Data Block events are modeled by Records, defined in the
memory model
The following
abstract operations
are used in this specification to operate upon Data Block values:
6.2.7.1
CreateByteDataBlock (
size
When the abstract operation CreateByteDataBlock is called with integer argument
size
, the following steps are taken:
Assert
size
≥ 0.
Let
db
be a new
Data Block
value consisting of
size
bytes. If it is impossible to create such a
Data Block
, throw a
RangeError
exception.
Set all of the bytes of
db
to 0.
Return
db
6.2.7.2
CreateSharedByteDataBlock (
size
When the abstract operation CreateSharedByteDataBlock is called with integer argument
size
, the following steps are taken:
Assert
size
≥ 0.
Let
db
be a new
Shared Data Block
value consisting of
size
bytes. If it is impossible to create such a
Shared Data Block
, throw a
RangeError
exception.
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
Let
zero
be « 0 ».
For each index
of
db
, do
Append
WriteSharedMemory
{ [[Order]]:
"Init"
, [[NoTear]]:
true
, [[Block]]:
db
, [[ByteIndex]]:
, [[ElementSize]]: 1, [[Payload]]:
zero
} to
eventList
Return
db
6.2.7.3
CopyDataBlockBytes (
toBlock
toIndex
fromBlock
fromIndex
count
When the abstract operation CopyDataBlockBytes is called, the following steps are taken:
Assert
fromBlock
and
toBlock
are distinct
Data Block
or
Shared Data Block
values.
Assert
fromIndex
toIndex
, and
count
are integer values ≥ 0.
Let
fromSize
be the number of bytes in
fromBlock
Assert
fromIndex
count
fromSize
Let
toSize
be the number of bytes in
toBlock
Assert
toIndex
count
toSize
Repeat, while
count
> 0
If
fromBlock
is a
Shared Data Block
, then
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
Let
bytes
be a
List
of length 1 that contains a nondeterministically chosen byte value.
NOTE: In implementations,
bytes
is the result of a non-atomic read instruction on the underlying
hardware. The nondeterminism is a semantic prescription of the
memory model
to describe observable behaviour of hardware with weak consistency.
Let
readEvent
be
ReadSharedMemory
{ [[Order]]:
"Unordered"
, [[NoTear]]:
true
, [[Block]]:
fromBlock
, [[ByteIndex]]:
fromIndex
, [[ElementSize]]: 1 }.
Append
readEvent
to
eventList
Append
Chosen Value Record
{ [[Event]]:
readEvent
, [[ChosenValue]]:
bytes
} to
execution
.[[ChosenValues]].
If
toBlock
is a
Shared Data Block
, then
Append
WriteSharedMemory
{ [[Order]]:
"Unordered"
, [[NoTear]]:
true
, [[Block]]:
toBlock
, [[ByteIndex]]:
toIndex
, [[ElementSize]]: 1, [[Payload]]:
bytes
} to
eventList
Else,
Set
toBlock
toIndex
] to
bytes
[0].
Else,
Assert
toBlock
is not a
Shared Data Block
Set
toBlock
toIndex
] to
fromBlock
fromIndex
].
Increment
toIndex
and
fromIndex
each by 1.
Decrement
count
by 1.
Return
NormalCompletion
empty
).
Abstract Operations
These operations are not a part of the ECMAScript language; they
are defined here to solely to aid the specification of the semantics of
the ECMAScript language. Other, more specialized
abstract operations
are defined throughout this specification.
7.1
Type Conversion
The ECMAScript language implicitly performs automatic type
conversion as needed. To clarify the semantics of certain constructs it
is useful to define a set of conversion
abstract operations
. The conversion
abstract operations
are polymorphic; they can accept a value of any
ECMAScript language type
. But no other specification types are used with these operations.
7.1.1
ToPrimitive (
input
[ ,
PreferredType
] )
The abstract operation ToPrimitive takes an
input
argument and an optional argument
PreferredType
. The abstract operation ToPrimitive converts its
input
argument to a non-Object type. If an object is capable of converting to
more than one primitive type, it may use the optional hint
PreferredType
to favour that type. Conversion occurs according to the following algorithm:
Assert
input
is an
ECMAScript language value
If
Type
input
) is Object, then
If
PreferredType
is not present, let
hint
be
"default"
Else if
PreferredType
is hint String, let
hint
be
"string"
Else
PreferredType
is hint Number, let
hint
be
"number"
Let
exoticToPrim
be ?
GetMethod
input
, @@toPrimitive).
If
exoticToPrim
is not
undefined
, then
Let
result
be ?
Call
exoticToPrim
input
, «
hint
»).
If
Type
result
) is not Object, return
result
Throw a
TypeError
exception.
If
hint
is
"default"
, set
hint
to
"number"
Return ?
OrdinaryToPrimitive
input
hint
).
Return
input
Note
When ToPrimitive is called with no hint, then it generally
behaves as if the hint were Number. However, objects may over-ride this
behaviour by defining a @@toPrimitive method. Of the objects defined in
this specification only Date objects (see
20.3.4.45
) and Symbol objects (see
19.4.3.5
) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.
7.1.1.1
OrdinaryToPrimitive (
hint
When the abstract operation OrdinaryToPrimitive is called with arguments
and
hint
, the following steps are taken:
Assert
Type
) is Object.
Assert
Type
hint
) is String and its value is either
"string"
or
"number"
If
hint
is
"string"
, then
Let
methodNames
be «
"toString"
"valueOf"
».
Else,
Let
methodNames
be «
"valueOf"
"toString"
».
For each
name
in
methodNames
in
List
order, do
Let
method
be ?
Get
name
).
If
IsCallable
method
) is
true
, then
Let
result
be ?
Call
method
).
If
Type
result
) is not Object, return
result
Throw a
TypeError
exception.
7.1.2
ToBoolean (
argument
The abstract operation ToBoolean converts
argument
to a value of type Boolean according to
Table 9
Table 9:
ToBoolean
Conversions
Argument Type
Result
Undefined
Return
false
Null
Return
false
Boolean
Return
argument
Number
If
argument
is
+0
-0
, or
NaN
, return
false
; otherwise return
true
String
If
argument
is the empty String (its length is zero), return
false
; otherwise return
true
Symbol
Return
true
Object
Return
true
7.1.3
ToNumber (
argument
The abstract operation ToNumber converts
argument
to a value of type Number according to
Table 10
Table 10:
ToNumber
Conversions
Argument Type
Result
Undefined
Return
NaN
Null
Return
+0
Boolean
If
argument
is
true
, return 1. If
argument
is
false
, return
+0
Number
Return
argument
(no conversion).
String
See grammar and conversion algorithm below.
Symbol
Throw a
TypeError
exception.
Object
Apply the following steps:
Let
primValue
be ?
ToPrimitive
argument
, hint Number).
Return ?
ToNumber
primValue
).
7.1.3.1
ToNumber Applied to the String Type
ToNumber
applied to Strings applies the following grammar to the input String interpreted as a sequence of UTF-16 encoded code points (
6.1.4
). If the grammar cannot interpret the String as an expansion of
StringNumericLiteral
, then the result of
ToNumber
is
NaN
Note 1
The terminal symbols of this grammar are all composed of
characters in the Unicode Basic Multilingual Plane (BMP). Therefore, the
result of
ToNumber
will be
NaN
if the string contains any
leading surrogate
or
trailing surrogate
code units, whether paired or unpaired.
Syntax
StringNumericLiteral
:::
StrWhiteSpace
opt
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
StrWhiteSpace
:::
StrWhiteSpaceChar
StrWhiteSpace
opt
StrWhiteSpaceChar
:::
WhiteSpace
LineTerminator
StrNumericLiteral
:::
StrDecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
:::
Infinity
DecimalDigits
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
ExponentPart
opt
All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (
11.8.3
Note 2
Some differences should be noted between the syntax of a
StringNumericLiteral
and a
NumericLiteral
StringNumericLiteral
may include leading and/or trailing white space and/or line terminators.
StringNumericLiteral
that is decimal may have any number of leading
digits.
StringNumericLiteral
that is decimal may include a
or
to indicate its sign.
StringNumericLiteral
that is empty or contains only white space is converted to
+0
Infinity
and
-Infinity
are recognized as a
StringNumericLiteral
but not as a
NumericLiteral
7.1.3.1.1
Runtime Semantics: MV
The conversion of a String to a Number value is similar
overall to the determination of the Number value for a numeric literal
(see
11.8.3
),
but some of the details are different, so the process for converting a
String numeric literal to a value of Number type is given here. This
value is determined in two steps: first, a mathematical value (MV) is
derived from the String numeric literal; second, this mathematical value
is rounded as described below. The MV on any grammar symbol, not
provided below, is the MV for that symbol defined in
11.8.3.1
The MV of
StringNumericLiteral
:::
[empty]
is 0.
The MV of
StringNumericLiteral
:::
StrWhiteSpace
is 0.
The MV of
StringNumericLiteral
:::
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
is the MV of
StrNumericLiteral
, no matter whether white space is present or not.
The MV of
StrNumericLiteral
:::
StrDecimalLiteral
is the MV of
StrDecimalLiteral
The MV of
StrNumericLiteral
:::
BinaryIntegerLiteral
is the MV of
BinaryIntegerLiteral
The MV of
StrNumericLiteral
:::
OctalIntegerLiteral
is the MV of
OctalIntegerLiteral
The MV of
StrNumericLiteral
:::
HexIntegerLiteral
is the MV of
HexIntegerLiteral
The MV of
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
is the MV of
StrUnsignedDecimalLiteral
The MV of
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
is the MV of
StrUnsignedDecimalLiteral
The MV of
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
is the negative of the MV of
StrUnsignedDecimalLiteral
. (Note that if the MV of
StrUnsignedDecimalLiteral
is 0, the negative of this MV is also 0. The rounding rule described
below handles the conversion of this signless mathematical zero to a
floating-point
+0
or
-0
as appropriate.)
The MV of
StrUnsignedDecimalLiteral
:::
Infinity
is 10
10000
(a value so large that it will round to
+∞
).
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
DecimalDigits
is the MV of the first
DecimalDigits
plus (the MV of the second
DecimalDigits
times 10
), where
is the number of code points in the second
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
DecimalDigits
ExponentPart
is (the MV of the first
DecimalDigits
plus (the MV of the second
DecimalDigits
times 10
)) times 10
, where
is the number of code points in the second
DecimalDigits
and
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
times 10
, where
is the number of code points in
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the number of code points in
DecimalDigits
and
is the MV of
ExponentPart
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
is the MV of
DecimalDigits
The MV of
StrUnsignedDecimalLiteral
:::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
times 10
, where
is the MV of
ExponentPart
Once the exact MV for a String numeric literal has been
determined, it is then rounded to a value of the Number type. If the MV
is 0, then the rounded value is
+0
unless the first non white space code point in the String numeric literal is
"-"
, in which case the rounded value is
-0
. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in
6.1.6
), unless the literal includes a
StrUnsignedDecimalLiteral
and the literal has more than 20 significant digits, in which case the
Number value may be either the Number value for the MV of a literal
produced by replacing each significant digit after the 20th with a 0
digit or the Number value for the MV of a literal produced by replacing
each significant digit after the 20th with a 0 digit and then
incrementing the literal at the 20th digit position. A digit is
significant if it is not part of an
ExponentPart
and
it is not
; or
there is a nonzero digit to its left and there is a nonzero digit, not in the
ExponentPart
, to its right.
7.1.4
ToInteger (
argument
The abstract operation ToInteger converts
argument
to an integral numeric value. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
, return
+0
If
number
is
+0
-0
+∞
, or
-∞
, return
number
Return the number value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
7.1.5
ToInt32 (
argument
The abstract operation ToInt32 converts
argument
to one of 2
32
integer values in the range
-2
31
through
31
- 1
, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int32bit
be
int
modulo
32
If
int32bit
≥ 2
31
, return
int32bit
- 2
32
; otherwise return
int32bit
Note
Given the above definition of ToInt32:
The ToInt32 abstract operation is idempotent: if applied to a
result that it produced, the second application leaves that value
unchanged.
ToInt32(
ToUint32
)) is equal to ToInt32(
) for all values of
. (It is to preserve this latter property that
+∞
and
-∞
are mapped to
+0
.)
ToInt32 maps
-0
to
+0
7.1.6
ToUint32 (
argument
The abstract operation ToUint32 converts
argument
to one of 2
32
integer values in the range 0 through
32
- 1
, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int32bit
be
int
modulo
32
Return
int32bit
Note
Given the above definition of ToUint32:
Step 5 is the only difference between ToUint32 and
ToInt32
The ToUint32 abstract operation is idempotent: if applied to
a result that it produced, the second application leaves that value
unchanged.
ToUint32(
ToInt32
)) is equal to ToUint32(
) for all values of
. (It is to preserve this latter property that
+∞
and
-∞
are mapped to
+0
.)
ToUint32 maps
-0
to
+0
7.1.7
ToInt16 (
argument
The abstract operation ToInt16 converts
argument
to one of 2
16
integer values in the range -32768 through 32767, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int16bit
be
int
modulo
16
If
int16bit
≥ 2
15
, return
int16bit
- 2
16
; otherwise return
int16bit
7.1.8
ToUint16 (
argument
The abstract operation ToUint16 converts
argument
to one of 2
16
integer values in the range 0 through
16
- 1
, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int16bit
be
int
modulo
16
Return
int16bit
Note
Given the above definition of ToUint16:
The substitution of 2
16
for 2
32
in step 4 is the only difference between
ToUint32
and ToUint16.
ToUint16 maps
-0
to
+0
7.1.9
ToInt8 (
argument
The abstract operation ToInt8 converts
argument
to one of 2
integer values in the range -128 through 127, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int8bit
be
int
modulo
If
int8bit
≥ 2
, return
int8bit
- 2
; otherwise return
int8bit
7.1.10
ToUint8 (
argument
The abstract operation ToUint8 converts
argument
to one of 2
integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
+0
-0
+∞
, or
-∞
, return
+0
Let
int
be the mathematical value that is the same sign as
number
and whose magnitude is
floor
abs
number
)).
Let
int8bit
be
int
modulo
Return
int8bit
7.1.11
ToUint8Clamp (
argument
The abstract operation ToUint8Clamp converts
argument
to one of 2
integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:
Let
number
be ?
ToNumber
argument
).
If
number
is
NaN
, return
+0
If
number
≤ 0, return
+0
If
number
≥ 255, return 255.
Let
be
floor
number
).
If
+ 0.5 <
number
, return
+ 1.
If
number
+ 0.5, return
If
is odd, return
+ 1.
Return
Note
Unlike the other ECMAScript integer conversion abstract
operation, ToUint8Clamp rounds rather than truncates non-integer values
and does not convert
+∞
to 0. ToUint8Clamp does “round half to even” tie-breaking. This differs from
Math.round
which does “round half up” tie-breaking.
7.1.12
ToString (
argument
The abstract operation ToString converts
argument
to a value of type String according to
Table 11
Table 11:
ToString
Conversions
Argument Type
Result
Undefined
Return
"undefined"
Null
Return
"null"
Boolean
If
argument
is
true
, return
"true"
If
argument
is
false
, return
"false"
Number
Return
NumberToString
argument
).
String
Return
argument
Symbol
Throw a
TypeError
exception.
Object
Apply the following steps:
Let
primValue
be ?
ToPrimitive
argument
, hint String).
Return ?
ToString
primValue
).
7.1.12.1
NumberToString (
The abstract operation NumberToString converts a Number
to String format as follows:
If
is
NaN
, return the String
"NaN"
If
is
+0
or
-0
, return the String
"0"
If
is less than zero, return the
string-concatenation
of
"-"
and !
NumberToString
(-
).
If
is
+∞
, return the String
"Infinity"
Otherwise, let
, and
be integers such that
≥ 1, 10
- 1
< 10
, the Number value for
× 10
is
, and
is as small as possible. Note that
is the number of digits in the decimal representation of
, that
is not divisible by 10, and that the least significant digit of
is not necessarily uniquely determined by these criteria.
If
≤ 21, return the
string-concatenation
of:
the code units of the
digits of the decimal representation of
(in order, with no leading zeroes)
occurrences of the code unit 0x0030 (DIGIT ZERO)
If 0 <
≤ 21, return the
string-concatenation
of:
the code units of the most significant
digits of the decimal representation of
the code unit 0x002E (FULL STOP)
the code units of the remaining
digits of the decimal representation of
If -6 <
≤ 0, return the
string-concatenation
of:
the code unit 0x0030 (DIGIT ZERO)
the code unit 0x002E (FULL STOP)
occurrences of the code unit 0x0030 (DIGIT ZERO)
the code units of the
digits of the decimal representation of
Otherwise, if
= 1, return the
string-concatenation
of:
the code unit of the single digit of
the code unit 0x0065 (LATIN SMALL LETTER E)
the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether
- 1 is positive or negative
the code units of the decimal representation of the integer
abs
- 1) (with no leading zeroes)
Return the
string-concatenation
of:
the code units of the most significant digit of the decimal representation of
the code unit 0x002E (FULL STOP)
the code units of the remaining
- 1 digits of the decimal representation of
the code unit 0x0065 (LATIN SMALL LETTER E)
the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether
- 1 is positive or negative
the code units of the decimal representation of the integer
abs
- 1) (with no leading zeroes)
Note 1
The following observations may be useful as guidelines for
implementations, but are not part of the normative requirements of this
Standard:
If x is any Number value other than
-0
, then
ToNumber
ToString
(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions
than required by the rules above, it is recommended that the following
alternative version of step 5 be used as a guideline:
Otherwise, let
, and
be integers such that
≥ 1, 10
- 1
< 10
, the Number value for
× 10
is
, and
is as small as possible. If there are multiple possibilities for
, choose the value of
for which
× 10
is closest in value to
. If there are two such possible values of
, choose the one that is even. Note that
is the number of digits in the decimal representation of
and that
is not divisible by 10.
Note 3
Implementers of ECMAScript may find useful the paper and
code written by David M. Gay for binary-to-decimal conversion of
floating-point numbers:
Gay, David M. Correctly Rounded Binary-Decimal and
Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10.
AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990.
Available as
. Associated code available as
and as
and may also be found at the various
netlib
mirror sites.
7.1.13
ToObject (
argument
The abstract operation ToObject converts
argument
to a value of type Object according to
Table 12
Table 12:
ToObject
Conversions
Argument Type
Result
Undefined
Throw a
TypeError
exception.
Null
Throw a
TypeError
exception.
Boolean
Return a new Boolean object whose [[BooleanData]] internal slot is set to
argument
. See
19.3
for a description of Boolean objects.
Number
Return a new Number object whose [[NumberData]] internal slot is set to
argument
. See
20.1
for a description of Number objects.
String
Return a new String object whose [[StringData]] internal slot is set to
argument
. See
21.1
for a description of String objects.
Symbol
Return a new Symbol object whose [[SymbolData]] internal slot is set to
argument
. See
19.4
for a description of Symbol objects.
Object
Return
argument
7.1.14
ToPropertyKey (
argument
The abstract operation ToPropertyKey converts
argument
to a value that can be used as a property key by performing the following steps:
Let
key
be ?
ToPrimitive
argument
, hint String).
If
Type
key
) is Symbol, then
Return
key
Return !
ToString
key
).
7.1.15
ToLength (
argument
The abstract operation ToLength converts
argument
to an integer suitable for use as the length of an array-like object. It performs the following steps:
Let
len
be ?
ToInteger
argument
).
If
len
+0
, return
+0
Return
min
len
, 2
53
- 1).
7.1.16
CanonicalNumericIndexString (
argument
The abstract operation CanonicalNumericIndexString returns
argument
converted to a numeric value if it is a String representation of a Number that would be produced by
ToString
, or the string
"-0"
. Otherwise, it returns
undefined
. This abstract operation functions as follows:
Assert
Type
argument
) is String.
If
argument
is
"-0"
, return
-0
Let
be !
ToNumber
argument
).
If
SameValue
(!
ToString
),
argument
) is
false
, return
undefined
Return
canonical numeric string
is any String value for which the CanonicalNumericIndexString abstract operation does not return
undefined
7.1.17
ToIndex (
value
The abstract operation ToIndex returns
value
argument converted to a numeric value if it is a valid
integer index
value. This abstract operation functions as follows:
If
value
is
undefined
, then
Let
index
be 0.
Else,
Let
integerIndex
be ?
ToInteger
value
).
If
integerIndex
< 0, throw a
RangeError
exception.
Let
index
be !
ToLength
integerIndex
).
If
SameValueZero
integerIndex
index
) is
false
, throw a
RangeError
exception.
Return
index
7.2
Testing and Comparison Operations
7.2.1
RequireObjectCoercible (
argument
The abstract operation RequireObjectCoercible throws an error if
argument
is a value that cannot be converted to an Object using
ToObject
. It is defined by
Table 13
Table 13:
RequireObjectCoercible
Results
Argument Type
Result
Undefined
Throw a
TypeError
exception.
Null
Throw a
TypeError
exception.
Boolean
Return
argument
Number
Return
argument
String
Return
argument
Symbol
Return
argument
Object
Return
argument
7.2.2
IsArray (
argument
The abstract operation IsArray takes one argument
argument
, and performs the following steps:
If
Type
argument
) is not Object, return
false
If
argument
is an Array
exotic object
, return
true
If
argument
is a Proxy
exotic object
, then
If
argument
.[[ProxyHandler]] is
null
, throw a
TypeError
exception.
Let
target
be
argument
.[[ProxyTarget]].
Return ?
IsArray
target
).
Return
false
7.2.3
IsCallable (
argument
The abstract operation IsCallable determines if
argument
, which must be an
ECMAScript language value
, is a callable function with a [[Call]] internal method.
If
Type
argument
) is not Object, return
false
If
argument
has a [[Call]] internal method, return
true
Return
false
7.2.4
IsConstructor (
argument
The abstract operation IsConstructor determines if
argument
, which must be an
ECMAScript language value
, is a
function object
with a [[Construct]] internal method.
If
Type
argument
) is not Object, return
false
If
argument
has a [[Construct]] internal method, return
true
Return
false
7.2.5
IsExtensible (
The abstract operation IsExtensible is used to determine whether additional properties can be added to the object that is
. A Boolean value is returned. This abstract operation performs the following steps:
Assert
Type
) is Object.
Return ?
.[[IsExtensible]]().
7.2.6
IsInteger (
argument
The abstract operation IsInteger determines if
argument
is a finite integer numeric value.
If
Type
argument
) is not Number, return
false
If
argument
is
NaN
+∞
, or
-∞
, return
false
If
floor
abs
argument
)) ≠
abs
argument
), return
false
Return
true
7.2.7
IsPropertyKey (
argument
The abstract operation IsPropertyKey determines if
argument
, which must be an
ECMAScript language value
, is a value that may be used as a property key.
If
Type
argument
) is String, return
true
If
Type
argument
) is Symbol, return
true
Return
false
7.2.8
IsRegExp (
argument
The abstract operation IsRegExp with argument
argument
performs the following steps:
If
Type
argument
) is not Object, return
false
Let
matcher
be ?
Get
argument
, @@match).
If
matcher
is not
undefined
, return
ToBoolean
matcher
).
If
argument
has a [[RegExpMatcher]] internal slot, return
true
Return
false
7.2.9
IsStringPrefix (
The abstract operation IsStringPrefix determines if String
is a prefix of String
Assert
Type
) is String.
Assert
Type
) is String.
If
can be the
string-concatenation
of
and some other String
, return
true
. Otherwise, return
false
NOTE: Any String is a prefix of itself, because
may be the empty String.
7.2.10
SameValue (
The internal comparison abstract operation SameValue(
), where
and
are ECMAScript language values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is different from
Type
), return
false
If
Type
) is Number, then
If
is
NaN
and
is
NaN
, return
true
If
is
+0
and
is
-0
, return
false
If
is
-0
and
is
+0
, return
false
If
is the same Number value as
, return
true
Return
false
Return
SameValueNonNumber
).
Note
This algorithm differs from the
Strict Equality Comparison
Algorithm in its treatment of signed zeroes and NaNs.
7.2.11
SameValueZero (
The internal comparison abstract operation SameValueZero(
), where
and
are ECMAScript language values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is different from
Type
), return
false
If
Type
) is Number, then
If
is
NaN
and
is
NaN
, return
true
If
is
+0
and
is
-0
, return
true
If
is
-0
and
is
+0
, return
true
If
is the same Number value as
, return
true
Return
false
Return
SameValueNonNumber
).
Note
SameValueZero differs from
SameValue
only in its treatment of
+0
and
-0
7.2.12
SameValueNonNumber (
The internal comparison abstract operation SameValueNonNumber(
), where neither
nor
are Number values, produces
true
or
false
. Such a comparison is performed as follows:
Assert
Type
) is not Number.
Assert
Type
) is the same as
Type
).
If
Type
) is Undefined, return
true
If
Type
) is Null, return
true
If
Type
) is String, then
If
and
are exactly the same sequence of code units (same length and same code units at corresponding indices), return
true
; otherwise, return
false
If
Type
) is Boolean, then
If
and
are both
true
or both
false
, return
true
; otherwise, return
false
If
Type
) is Symbol, then
If
and
are both the same Symbol value, return
true
; otherwise, return
false
If
and
are the same Object value, return
true
. Otherwise, return
false
7.2.13
Abstract Relational Comparison
The comparison
, where
and
are values, produces
true
false
, or
undefined
(which indicates that at least one operand is
NaN
). In addition to
and
the algorithm takes a Boolean flag named
LeftFirst
as a parameter. The flag is used to control the order in which
operations with potentially visible side-effects are performed upon
and
. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of
LeftFirst
is
true
and indicates that the
parameter corresponds to an expression that occurs to the left of the
parameter's corresponding expression. If
LeftFirst
is
false
, the reverse is the case and operations must be performed upon
before
. Such a comparison is performed as follows:
If the
LeftFirst
flag is
true
, then
Let
px
be ?
ToPrimitive
, hint Number).
Let
py
be ?
ToPrimitive
, hint Number).
Else the order of evaluation needs to be reversed to preserve left to right evaluation,
Let
py
be ?
ToPrimitive
, hint Number).
Let
px
be ?
ToPrimitive
, hint Number).
If
Type
px
) is String and
Type
py
) is String, then
If
IsStringPrefix
py
px
) is
true
, return
false
If
IsStringPrefix
px
py
) is
true
, return
true
Let
be the smallest nonnegative integer such that the code unit at index
within
px
is different from the code unit at index
within
py
. (There must be such a
, for neither String is a prefix of the other.)
Let
be the integer that is the numeric value of the code unit at index
within
px
Let
be the integer that is the numeric value of the code unit at index
within
py
If
, return
true
. Otherwise, return
false
Else,
NOTE: Because
px
and
py
are primitive values evaluation order is not important.
Let
nx
be ?
ToNumber
px
).
Let
ny
be ?
ToNumber
py
).
If
nx
is
NaN
, return
undefined
If
ny
is
NaN
, return
undefined
If
nx
and
ny
are the same Number value, return
false
If
nx
is
+0
and
ny
is
-0
, return
false
If
nx
is
-0
and
ny
is
+0
, return
false
If
nx
is
+∞
, return
false
If
ny
is
+∞
, return
true
If
ny
is
-∞
, return
false
If
nx
is
-∞
, return
true
If the mathematical value of
nx
is less than the mathematical value of
ny
—note that these mathematical values are both finite and not both zero—return
true
. Otherwise, return
false
Note 1
Step 3 differs from step 7 in the algorithm for the addition operator
12.8.3
) by using the logical-and operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic
ordering on sequences of code unit values. There is no attempt to use
the more complex, semantically oriented definitions of character or
string equality and collating order defined in the Unicode
specification. Therefore String values that are canonically equal
according to the Unicode standard could test as unequal. In effect this
algorithm assumes that both Strings are already in normalized form.
Also, note that for strings containing supplementary characters,
lexicographic ordering on sequences of UTF-16 code unit values differs
from that on sequences of code point values.
7.2.14
Abstract Equality Comparison
The comparison
==
, where
and
are values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is the same as
Type
), then
Return the result of performing
Strict Equality Comparison
===
If
is
null
and
is
undefined
, return
true
If
is
undefined
and
is
null
, return
true
If
Type
) is Number and
Type
) is String, return the result of the comparison
== !
ToNumber
).
If
Type
) is String and
Type
) is Number, return the result of the comparison !
ToNumber
) ==
If
Type
) is Boolean, return the result of the comparison !
ToNumber
) ==
If
Type
) is Boolean, return the result of the comparison
== !
ToNumber
).
If
Type
) is either String, Number, or Symbol and
Type
) is Object, return the result of the comparison
==
ToPrimitive
).
If
Type
) is Object and
Type
) is either String, Number, or Symbol, return the result of the comparison
ToPrimitive
) ==
Return
false
7.2.15
Strict Equality Comparison
The comparison
===
, where
and
are values, produces
true
or
false
. Such a comparison is performed as follows:
If
Type
) is different from
Type
), return
false
If
Type
) is Number, then
If
is
NaN
, return
false
If
is
NaN
, return
false
If
is the same Number value as
, return
true
If
is
+0
and
is
-0
, return
true
If
is
-0
and
is
+0
, return
true
Return
false
Return
SameValueNonNumber
).
Note
This algorithm differs from the
SameValue
Algorithm in its treatment of signed zeroes and NaNs.
7.3
Operations on Objects
7.3.1
Get (
The abstract operation Get is used to retrieve the value of a
specific property of an object. The operation is called with arguments
and
where
is the object and
is the property key. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Return ?
.[[Get]](
).
7.3.2
GetV (
The abstract operation GetV is used to retrieve the value of a specific property of an
ECMAScript language value
If the value is not an object, the property lookup is performed using a
wrapper object appropriate for the type of the value. The operation is
called with arguments
and
where
is the value and
is the property key. This abstract operation performs the following steps:
Assert
IsPropertyKey
) is
true
Let
be ?
ToObject
).
Return ?
.[[Get]](
).
7.3.3
Set (
Throw
The abstract operation Set is used to set the value of a specific property of an object. The operation is called with arguments
, and
Throw
where
is the object,
is the property key,
is the new value for the property and
Throw
is a Boolean flag. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Assert
Type
Throw
) is Boolean.
Let
success
be ?
.[[Set]](
).
If
success
is
false
and
Throw
is
true
, throw a
TypeError
exception.
Return
success
7.3.4
CreateDataProperty (
The abstract operation CreateDataProperty is used to create a
new own property of an object. The operation is called with arguments
, and
where
is the object,
is the property key, and
is the value for the property. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
newDesc
be the PropertyDescriptor { [[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
Return ?
.[[DefineOwnProperty]](
newDesc
).
Note
This abstract operation creates a property whose attributes
are set to the same defaults used for properties created by the
ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if
is not extensible, [[DefineOwnProperty]] will return
false
7.3.5
CreateMethodProperty (
The abstract operation CreateMethodProperty is used to create a
new own property of an object. The operation is called with arguments
, and
where
is the object,
is the property key, and
is the value for the property. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
newDesc
be the PropertyDescriptor { [[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Return ?
.[[DefineOwnProperty]](
newDesc
).
Note
This abstract operation creates a property whose attributes
are set to the same defaults used for built-in methods and methods
defined using class declaration syntax. Normally, the property will not
already exist. If it does exist and is not configurable or if
is not extensible, [[DefineOwnProperty]] will return
false
7.3.6
CreateDataPropertyOrThrow (
The abstract operation CreateDataPropertyOrThrow is used to create a new own property of an object. It throws a
TypeError
exception if the requested property update cannot be performed. The operation is called with arguments
, and
where
is the object,
is the property key, and
is the value for the property. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
success
be ?
CreateDataProperty
).
If
success
is
false
, throw a
TypeError
exception.
Return
success
Note
This abstract operation creates a property whose attributes
are set to the same defaults used for properties created by the
ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if
is not extensible, [[DefineOwnProperty]] will return
false
causing this operation to throw a
TypeError
exception.
7.3.7
DefinePropertyOrThrow (
desc
The abstract operation DefinePropertyOrThrow is used to call
the [[DefineOwnProperty]] internal method of an object in a manner that
will throw a
TypeError
exception if the requested property update cannot be performed. The operation is called with arguments
, and
desc
where
is the object,
is the property key, and
desc
is the
Property Descriptor
for the property. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
success
be ?
.[[DefineOwnProperty]](
desc
).
If
success
is
false
, throw a
TypeError
exception.
Return
success
7.3.8
DeletePropertyOrThrow (
The abstract operation DeletePropertyOrThrow is used to remove a
specific own property of an object. It throws an exception if the
property is not configurable. The operation is called with arguments
and
where
is the object and
is the property key. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
success
be ?
.[[Delete]](
).
If
success
is
false
, throw a
TypeError
exception.
Return
success
7.3.9
GetMethod (
The abstract operation GetMethod is used to get the value of a specific property of an
ECMAScript language value
when the value of the property is expected to be a function. The operation is called with arguments
and
where
is the
ECMAScript language value
is the property key. This abstract operation performs the following steps:
Assert
IsPropertyKey
) is
true
Let
func
be ?
GetV
).
If
func
is either
undefined
or
null
, return
undefined
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
Return
func
7.3.10
HasProperty (
The abstract operation HasProperty is used to determine whether
an object has a property with the specified property key. The property
may be either an own or inherited. A Boolean value is returned. The
operation is called with arguments
and
where
is the object and
is the property key. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Return ?
.[[HasProperty]](
).
7.3.11
HasOwnProperty (
The abstract operation HasOwnProperty is used to determine
whether an object has an own property with the specified property key. A
Boolean value is returned. The operation is called with arguments
and
where
is the object and
is the property key. This abstract operation performs the following steps:
Assert
Type
) is Object.
Assert
IsPropertyKey
) is
true
Let
desc
be ?
.[[GetOwnProperty]](
).
If
desc
is
undefined
, return
false
Return
true
7.3.12
Call (
[ ,
argumentsList
] )
The abstract operation Call is used to call the [[Call]] internal method of a
function object
. The operation is called with arguments
, and optionally
argumentsList
where
is the
function object
is an
ECMAScript language value
that is the
this
value of the [[Call]], and
argumentsList
is the value passed to the corresponding argument of the internal method. If
argumentsList
is not present, a new empty
List
is used as its value. This abstract operation performs the following steps:
If
argumentsList
is not present, set
argumentsList
to a new empty
List
If
IsCallable
) is
false
, throw a
TypeError
exception.
Return ?
.[[Call]](
argumentsList
).
7.3.13
Construct (
[ ,
argumentsList
[ ,
newTarget
] ] )
The abstract operation Construct is used to call the [[Construct]] internal method of a
function object
. The operation is called with arguments
, and optionally
argumentsList
, and
newTarget
where
is the
function object
argumentsList
and
newTarget
are the values to be passed as the corresponding arguments of the internal method. If
argumentsList
is not present, a new empty
List
is used as its value. If
newTarget
is not present,
is used as its value. This abstract operation performs the following steps:
If
newTarget
is not present, set
newTarget
to
If
argumentsList
is not present, set
argumentsList
to a new empty
List
Assert
IsConstructor
) is
true
Assert
IsConstructor
newTarget
) is
true
Return ?
.[[Construct]](
argumentsList
newTarget
).
Note
If
newTarget
is not present, this operation is equivalent to:
new F(...argumentsList)
7.3.14
SetIntegrityLevel (
level
The abstract operation SetIntegrityLevel is used to fix the set
of own properties of an object. This abstract operation performs the
following steps:
Assert
Type
) is Object.
Assert
level
is either
"sealed"
or
"frozen"
Let
status
be ?
.[[PreventExtensions]]().
If
status
is
false
, return
false
Let
keys
be ?
.[[OwnPropertyKeys]]().
If
level
is
"sealed"
, then
For each element
of
keys
, do
Perform ?
DefinePropertyOrThrow
, PropertyDescriptor { [[Configurable]]:
false
}).
Else
level
is
"frozen"
For each element
of
keys
, do
Let
currentDesc
be ?
.[[GetOwnProperty]](
).
If
currentDesc
is not
undefined
, then
If
IsAccessorDescriptor
currentDesc
) is
true
, then
Let
desc
be the PropertyDescriptor { [[Configurable]]:
false
}.
Else,
Let
desc
be the PropertyDescriptor { [[Configurable]]:
false
, [[Writable]]:
false
}.
Perform ?
DefinePropertyOrThrow
desc
).
Return
true
7.3.15
TestIntegrityLevel (
level
The abstract operation TestIntegrityLevel is used to determine
if the set of own properties of an object are fixed. This abstract
operation performs the following steps:
Assert
Type
) is Object.
Assert
level
is either
"sealed"
or
"frozen"
Let
status
be ?
IsExtensible
).
If
status
is
true
, return
false
NOTE: If the object is extensible, none of its properties are examined.
Let
keys
be ?
.[[OwnPropertyKeys]]().
For each element
of
keys
, do
Let
currentDesc
be ?
.[[GetOwnProperty]](
).
If
currentDesc
is not
undefined
, then
If
currentDesc
.[[Configurable]] is
true
, return
false
If
level
is
"frozen"
and
IsDataDescriptor
currentDesc
) is
true
, then
If
currentDesc
.[[Writable]] is
true
, return
false
Return
true
7.3.16
CreateArrayFromList (
elements
The abstract operation CreateArrayFromList is used to create an Array object whose elements are provided by a
List
. This abstract operation performs the following steps:
Assert
elements
is a
List
whose elements are all ECMAScript language values.
Let
array
be !
ArrayCreate
(0).
Let
be 0.
For each element
of
elements
, do
Let
status
be
CreateDataProperty
array
, !
ToString
),
).
Assert
status
is
true
Increment
by 1.
Return
array
7.3.17
CreateListFromArrayLike (
obj
[ ,
elementTypes
] )
The abstract operation CreateListFromArrayLike is used to create a
List
value whose elements are provided by the indexed properties of an array-like object,
obj
. The optional argument
elementTypes
is a
List
containing the names of ECMAScript Language Types that are allowed for element values of the
List
that is created. This abstract operation performs the following steps:
If
elementTypes
is not present, set
elementTypes
to « Undefined, Null, Boolean, String, Symbol, Number, Object ».
If
Type
obj
) is not Object, throw a
TypeError
exception.
Let
len
be ?
ToLength
(?
Get
obj
"length"
)).
Let
list
be a new empty
List
Let
index
be 0.
Repeat, while
index
len
Let
indexName
be !
ToString
index
).
Let
next
be ?
Get
obj
indexName
).
If
Type
next
) is not an element of
elementTypes
, throw a
TypeError
exception.
Append
next
as the last element of
list
Increase
index
by 1.
Return
list
7.3.18
Invoke (
[ ,
argumentsList
] )
The abstract operation Invoke is used to call a method property of an
ECMAScript language value
. The operation is called with arguments
, and optionally
argumentsList
where
serves as both the lookup point for the property and the
this
value of the call,
is the property key, and
argumentsList
is the list of arguments values passed to the method. If
argumentsList
is not present, a new empty
List
is used as its value. This abstract operation performs the following steps:
Assert
IsPropertyKey
) is
true
If
argumentsList
is not present, set
argumentsList
to a new empty
List
Let
func
be ?
GetV
).
Return ?
Call
func
argumentsList
).
7.3.19
OrdinaryHasInstance (
The abstract operation OrdinaryHasInstance implements the default algorithm for determining if an object
inherits from the instance object inheritance path provided by
constructor
. This abstract operation performs the following steps:
If
IsCallable
) is
false
, return
false
If
has a [[BoundTargetFunction]] internal slot, then
Let
BC
be
.[[BoundTargetFunction]].
Return ?
InstanceofOperator
BC
).
If
Type
) is not Object, return
false
Let
be ?
Get
"prototype"
).
If
Type
) is not Object, throw a
TypeError
exception.
Repeat,
Set
to ?
.[[GetPrototypeOf]]().
If
is
null
, return
false
If
SameValue
) is
true
, return
true
7.3.20
SpeciesConstructor (
defaultConstructor
The abstract operation SpeciesConstructor is used to retrieve the
constructor
that should be used to create new objects that are derived from the argument object
. The
defaultConstructor
argument is the
constructor
to use if a
constructor
@@species property cannot be found starting from
. This abstract operation performs the following steps:
Assert
Type
) is Object.
Let
be ?
Get
"constructor"
).
If
is
undefined
, return
defaultConstructor
If
Type
) is not Object, throw a
TypeError
exception.
Let
be ?
Get
, @@species).
If
is either
undefined
or
null
, return
defaultConstructor
If
IsConstructor
) is
true
, return
Throw a
TypeError
exception.
7.3.21
EnumerableOwnPropertyNames (
kind
When the abstract operation EnumerableOwnPropertyNames is called with Object
and String
kind
the following steps are taken:
Assert
Type
) is Object.
Let
ownKeys
be ?
.[[OwnPropertyKeys]]().
Let
properties
be a new empty
List
For each element
key
of
ownKeys
in
List
order, do
If
Type
key
) is String, then
Let
desc
be ?
.[[GetOwnProperty]](
key
).
If
desc
is not
undefined
and
desc
.[[Enumerable]] is
true
, then
If
kind
is
"key"
, append
key
to
properties
Else,
Let
value
be ?
Get
key
).
If
kind
is
"value"
, append
value
to
properties
Else,
Assert
kind
is
"key+value"
Let
entry
be
CreateArrayFromList
(«
key
value
»).
Append
entry
to
properties
Order the elements of
properties
so they are in the same relative order as would be produced by the Iterator that would be returned if the
EnumerateObjectProperties
internal method were invoked with
Return
properties
7.3.22
GetFunctionRealm (
obj
The abstract operation GetFunctionRealm with argument
obj
performs the following steps:
Assert
obj
is a callable object.
If
obj
has a [[Realm]] internal slot, then
Return
obj
.[[Realm]].
If
obj
is a Bound Function
exotic object
, then
Let
target
be
obj
.[[BoundTargetFunction]].
Return ?
GetFunctionRealm
target
).
If
obj
is a Proxy
exotic object
, then
If
obj
.[[ProxyHandler]] is
null
, throw a
TypeError
exception.
Let
proxyTarget
be
obj
.[[ProxyTarget]].
Return ?
GetFunctionRealm
proxyTarget
).
Return
the current Realm Record
Note
Step 5 will only be reached if
obj
is a non-standard function
exotic object
that does not have a [[Realm]] internal slot.
7.3.23
CopyDataProperties (
target
source
excludedItems
When the abstract operation CopyDataProperties is called with arguments
target
source
, and
excludedItems
, the following steps are taken:
Assert
Type
target
) is Object.
Assert
excludedItems
is a
List
of property keys.
If
source
is
undefined
or
null
, return
target
Let
from
be !
ToObject
source
).
Let
keys
be ?
from
.[[OwnPropertyKeys]]().
For each element
nextKey
of
keys
in
List
order, do
Let
excluded
be
false
For each element
of
excludedItems
in
List
order, do
If
SameValue
nextKey
) is
true
, then
Set
excluded
to
true
If
excluded
is
false
, then
Let
desc
be ?
from
.[[GetOwnProperty]](
nextKey
).
If
desc
is not
undefined
and
desc
.[[Enumerable]] is
true
, then
Let
propValue
be ?
Get
from
nextKey
).
Perform !
CreateDataProperty
target
nextKey
propValue
).
Return
target
Note
The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.
7.4
Operations on Iterator Objects
See Common Iteration Interfaces (
25.1
).
7.4.1
GetIterator (
obj
[ ,
hint
[ ,
method
] ] )
The abstract operation GetIterator with argument
obj
and optional arguments
hint
and
method
performs the following steps:
If
hint
is not present, set
hint
to
sync
Assert
hint
is either
sync
or
async
If
method
is not present, then
If
hint
is
async
, then
Set
method
to ?
GetMethod
obj
, @@asyncIterator).
If
method
is
undefined
, then
Let
syncMethod
be ?
GetMethod
obj
, @@iterator).
Let
syncIteratorRecord
be ?
GetIterator
obj
sync
syncMethod
).
Return ?
CreateAsyncFromSyncIterator
syncIteratorRecord
).
Otherwise, set
method
to ?
GetMethod
obj
, @@iterator).
Let
iterator
be ?
Call
method
obj
).
If
Type
iterator
) is not Object, throw a
TypeError
exception.
Let
nextMethod
be ?
GetV
iterator
"next"
).
Let
iteratorRecord
be
Record
{ [[Iterator]]:
iterator
, [[NextMethod]]:
nextMethod
, [[Done]]:
false
}.
Return
iteratorRecord
7.4.2
IteratorNext (
iteratorRecord
[ ,
value
] )
The abstract operation IteratorNext with argument
iteratorRecord
and optional argument
value
performs the following steps:
If
value
is not present, then
Let
result
be ?
Call
iteratorRecord
.[[NextMethod]],
iteratorRecord
.[[Iterator]], « »).
Else,
Let
result
be ?
Call
iteratorRecord
.[[NextMethod]],
iteratorRecord
.[[Iterator]], «
value
»).
If
Type
result
) is not Object, throw a
TypeError
exception.
Return
result
7.4.3
IteratorComplete (
iterResult
The abstract operation IteratorComplete with argument
iterResult
performs the following steps:
Assert
Type
iterResult
) is Object.
Return
ToBoolean
(?
Get
iterResult
"done"
)).
7.4.4
IteratorValue (
iterResult
The abstract operation IteratorValue with argument
iterResult
performs the following steps:
Assert
Type
iterResult
) is Object.
Return ?
Get
iterResult
"value"
).
7.4.5
IteratorStep (
iteratorRecord
The abstract operation IteratorStep with argument
iteratorRecord
requests the next value from
iteratorRecord
.[[Iterator]] by calling
iteratorRecord
.[[NextMethod]] and returns either
false
indicating that the iterator has reached its end or the IteratorResult
object if a next value is available. IteratorStep performs the following
steps:
Let
result
be ?
IteratorNext
iteratorRecord
).
Let
done
be ?
IteratorComplete
result
).
If
done
is
true
, return
false
Return
result
7.4.6
IteratorClose (
iteratorRecord
completion
The abstract operation IteratorClose with arguments
iteratorRecord
and
completion
is used to notify an iterator that it should perform any actions it
would normally perform when it has reached its completed state:
Assert
Type
iteratorRecord
.[[Iterator]]) is Object.
Assert
completion
is a
Completion Record
Let
iterator
be
iteratorRecord
.[[Iterator]].
Let
return
be ?
GetMethod
iterator
"return"
).
If
return
is
undefined
, return
Completion
completion
).
Let
innerResult
be
Call
return
iterator
, « »).
If
completion
.[[Type]] is
throw
, return
Completion
completion
).
If
innerResult
.[[Type]] is
throw
, return
Completion
innerResult
).
If
Type
innerResult
.[[Value]]) is not Object, throw a
TypeError
exception.
Return
Completion
completion
).
7.4.7
AsyncIteratorClose (
iteratorRecord
completion
The abstract operation AsyncIteratorClose with arguments
iteratorRecord
and
completion
is used to notify an async iterator that it should perform any actions
it would normally perform when it has reached its completed state:
Assert
Type
iteratorRecord
.[[Iterator]]) is Object.
Assert
completion
is a
Completion Record
Let
iterator
be
iteratorRecord
.[[Iterator]].
Let
return
be ?
GetMethod
iterator
"return"
).
If
return
is
undefined
, return
Completion
completion
).
Let
innerResult
be
Call
return
iterator
, « »).
If
innerResult
.[[Type]] is
normal
, set
innerResult
to
Await
innerResult
.[[Value]]).
If
completion
.[[Type]] is
throw
, return
Completion
completion
).
If
innerResult
.[[Type]] is
throw
, return
Completion
innerResult
).
If
Type
innerResult
.[[Value]]) is not Object, throw a
TypeError
exception.
Return
Completion
completion
).
7.4.8
CreateIterResultObject (
value
done
The abstract operation CreateIterResultObject with arguments
value
and
done
creates an object that supports the IteratorResult interface by performing the following steps:
Assert
Type
done
) is Boolean.
Let
obj
be
ObjectCreate
%ObjectPrototype%
).
Perform
CreateDataProperty
obj
"value"
value
).
Perform
CreateDataProperty
obj
"done"
done
).
Return
obj
7.4.9
CreateListIteratorRecord (
list
The abstract operation CreateListIteratorRecord with argument
list
creates an Iterator (
25.1.1.2
) object record whose next method returns the successive elements of
list
. It performs the following steps:
Let
iterator
be
ObjectCreate
%IteratorPrototype%
, « [[IteratedList]], [[ListIteratorNextIndex]] »).
Set
iterator
.[[IteratedList]] to
list
Set
iterator
.[[ListIteratorNextIndex]] to 0.
Let
steps
be the algorithm steps defined in ListIterator
next
7.4.9.1
).
Let
next
be
CreateBuiltinFunction
steps
, « »).
Return
Record
{ [[Iterator]]:
iterator
, [[NextMethod]]:
next
, [[Done]]:
false
}.
Note
The list iterator object is never directly accessible to ECMAScript code.
7.4.9.1
ListIterator next ( )
The ListIterator
next
method is a standard built-in
function object
(clause
17
) that performs the following steps:
Let
be the
this
value.
Assert
Type
) is Object.
Assert
has an [[IteratedList]] internal slot.
Let
list
be
.[[IteratedList]].
Let
index
be
.[[ListIteratorNextIndex]].
Let
len
be the number of elements of
list
If
index
len
, then
Return
CreateIterResultObject
undefined
true
).
Set
.[[ListIteratorNextIndex]] to
index
+ 1.
Return
CreateIterResultObject
list
index
],
false
).
Executable Code and Execution Contexts
8.1
Lexical Environments
Lexical Environment
is a specification type used to define the association of
Identifier
to specific variables and functions based upon the lexical nesting
structure of ECMAScript code. A Lexical Environment consists of an
Environment Record
and a possibly null reference to an
outer
Lexical Environment. Usually a Lexical Environment is associated with
some specific syntactic structure of ECMAScript code such as a
FunctionDeclaration
, a
BlockStatement
, or a
Catch
clause of a
TryStatement
and a new Lexical Environment is created each time such code is evaluated.
An
Environment Record
records the identifier bindings that are created within the scope of
its associated Lexical Environment. It is referred to as the Lexical
Environment's
EnvironmentRecord
The outer environment reference is used to model the logical
nesting of Lexical Environment values. The outer reference of a (inner)
Lexical Environment is a reference to the Lexical Environment that
logically surrounds the inner Lexical Environment. An outer Lexical
Environment may, of course, have its own outer Lexical Environment. A
Lexical Environment may serve as the outer environment for multiple
inner Lexical Environments. For example, if a
FunctionDeclaration
contains two nested
FunctionDeclaration
then the Lexical Environments of each of the nested functions will have
as their outer Lexical Environment the Lexical Environment of the
current evaluation of the surrounding function.
global environment
is a Lexical Environment which does not have an outer environment. The
global environment
's outer environment reference is
null
. A
global environment
's EnvironmentRecord may be prepopulated with identifier bindings and includes an associated
global object
whose properties provide some of the
global environment
's identifier bindings. As ECMAScript code is executed, additional properties may be added to the
global object
and the initial properties may be modified.
module environment
is a Lexical Environment that contains the bindings for the top level declarations of a
Module
. It also contains the bindings that are explicitly imported by the
Module
. The outer environment of a
module environment
is a
global environment
function environment
is a Lexical Environment that corresponds to the invocation of an ECMAScript
function object
. A
function environment
may establish a new
this
binding. A
function environment
also captures the state necessary to support
super
method invocations.
Lexical Environments and
Environment Record
values are purely specification mechanisms and need not correspond to
any specific artefact of an ECMAScript implementation. It is impossible
for an ECMAScript program to directly access or manipulate such values.
8.1.1
Environment Records
There are two primary kinds of
Environment Record
values used in this specification:
declarative Environment Records
and
object Environment Records
. Declarative Environment Records are used to define the effect of ECMAScript language syntactic elements such as
FunctionDeclaration
s,
VariableDeclaration
s, and
Catch
clauses that directly associate identifier bindings with ECMAScript
language values. Object Environment Records are used to define the
effect of ECMAScript elements such as
WithStatement
that associate identifier bindings with the properties of some object.
Global Environment Records and function Environment Records are
specializations that are used for specifically for
Script
global declarations and for top-level declarations within functions.
For specification purposes Environment Record values are values of the
Record
specification type and can be thought of as existing in a simple
object-oriented hierarchy where Environment Record is an abstract class
with three concrete subclasses, declarative Environment Record, object
Environment Record, and global Environment Record. Function Environment
Records and module Environment Records are subclasses of declarative
Environment Record. The abstract class includes the abstract
specification methods defined in
Table 14
. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
Table 14: Abstract Methods of Environment Records
Method
Purpose
HasBinding(N)
Determine if an
Environment Record
has a binding for the String value
. Return
true
if it does and
false
if it does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an
Environment Record
. The String value
is the text of the bound name. If the Boolean argument
is
true
the binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an
Environment Record
. The String value
is the text of the bound name. If
is
true
then attempts to set it after it has been initialized will always throw
an exception, regardless of the strict mode setting of operations that
reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an
Environment Record
. The String value
is the text of the bound name.
is the value for the binding and is a value of any
ECMAScript language type
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an
Environment Record
. The String value
is the text of the bound name.
is the value for the binding and may be a value of any
ECMAScript language type
is a Boolean flag. If
is
true
and the binding cannot be set throw a
TypeError
exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an
Environment Record
. The String value
is the text of the bound name.
is used to identify references originating in
strict mode code
or that otherwise require strict mode reference semantics. If
is
true
and the binding does not exist throw a
ReferenceError
exception. If the binding exists but is uninitialized a
ReferenceError
is thrown, regardless of the value of
DeleteBinding(N)
Delete a binding from an
Environment Record
. The String value
is the text of the bound name. If a binding for
exists, remove the binding and return
true
. If the binding exists but cannot be removed return
false
. If the binding does not exist return
true
HasThisBinding()
Determine if an
Environment Record
establishes a
this
binding. Return
true
if it does and
false
if it does not.
HasSuperBinding()
Determine if an
Environment Record
establishes a
super
method binding. Return
true
if it does and
false
if it does not.
WithBaseObject()
If this
Environment Record
is associated with a
with
statement, return the with object. Otherwise, return
undefined
8.1.1.1
Declarative Environment Records
Each declarative
Environment Record
is associated with an ECMAScript program scope containing variable,
constant, let, class, module, import, and/or function declarations. A
declarative
Environment Record
binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.
8.1.1.1.1
HasBinding (
The concrete
Environment Record
method HasBinding for declarative Environment Records simply determines
if the argument identifier is one of the identifiers bound by the
record:
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
If
envRec
has a binding for the name that is the value of
, return
true
Return
false
8.1.1.1.2
CreateMutableBinding (
The concrete
Environment Record
method CreateMutableBinding for declarative Environment Records creates a new mutable binding for the name
that is uninitialized. A binding must not already exist in this
Environment Record
for
. If Boolean argument
has the value
true
the new binding is marked as being subject to deletion.
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
Assert
envRec
does not already have a binding for
Create a mutable binding in
envRec
for
and record that it is uninitialized. If
is
true
, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return
NormalCompletion
empty
).
8.1.1.1.3
CreateImmutableBinding (
The concrete
Environment Record
method CreateImmutableBinding for declarative Environment Records creates a new immutable binding for the name
that is uninitialized. A binding must not already exist in this
Environment Record
for
. If the Boolean argument
has the value
true
the new binding is marked as a strict binding.
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
Assert
envRec
does not already have a binding for
Create an immutable binding in
envRec
for
and record that it is uninitialized. If
is
true
, record that the newly created binding is a strict binding.
Return
NormalCompletion
empty
).
8.1.1.1.4
InitializeBinding (
The concrete
Environment Record
method InitializeBinding for declarative Environment Records is used to
set the bound value of the current binding of the identifier whose name
is the value of the argument
to the value of argument
. An uninitialized binding for
must already exist.
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
Assert
envRec
must have an uninitialized binding for
Set the bound value for
in
envRec
to
Record
that the binding for
in
envRec
has been initialized.
Return
NormalCompletion
empty
).
8.1.1.1.5
SetMutableBinding (
The concrete
Environment Record
method SetMutableBinding for declarative Environment Records attempts
to change the bound value of the current binding of the identifier whose
name is the value of the argument
to the value of argument
. A binding for
normally already exists, but in rare cases it may not. If the binding is an immutable binding, a
TypeError
is thrown if
is
true
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
If
envRec
does not have a binding for
, then
If
is
true
, throw a
ReferenceError
exception.
Perform
envRec
.CreateMutableBinding(
true
).
Perform
envRec
.InitializeBinding(
).
Return
NormalCompletion
empty
).
If the binding for
in
envRec
is a strict binding, set
to
true
If the binding for
in
envRec
has not yet been initialized, throw a
ReferenceError
exception.
Else if the binding for
in
envRec
is a mutable binding, change its bound value to
Else,
Assert
: This is an attempt to change the value of an immutable binding.
If
is
true
, throw a
TypeError
exception.
Return
NormalCompletion
empty
).
Note
An example of ECMAScript code that results in a missing binding at step 2 is:
function f(){eval("var x; x = (delete x, 0);")}
8.1.1.1.6
GetBindingValue (
The concrete
Environment Record
method GetBindingValue for declarative Environment Records simply
returns the value of its bound identifier whose name is the value of the
argument
. If the binding exists but is uninitialized a
ReferenceError
is thrown, regardless of the value of
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
Assert
envRec
has a binding for
If the binding for
in
envRec
is an uninitialized binding, throw a
ReferenceError
exception.
Return the value currently bound to
in
envRec
8.1.1.1.7
DeleteBinding (
The concrete
Environment Record
method DeleteBinding for declarative Environment Records can only
delete bindings that have been explicitly designated as being subject to
deletion.
Let
envRec
be the declarative
Environment Record
for which the method was invoked.
Assert
envRec
has a binding for the name that is the value of
If the binding for
in
envRec
cannot be deleted, return
false
Remove the binding for
from
envRec
Return
true
8.1.1.1.8
HasThisBinding ( )
Regular declarative Environment Records do not provide a
this
binding.
Return
false
8.1.1.1.9
HasSuperBinding ( )
Regular declarative Environment Records do not provide a
super
binding.
Return
false
8.1.1.1.10
WithBaseObject ( )
Declarative Environment Records always return
undefined
as their WithBaseObject.
Return
undefined
8.1.1.2
Object Environment Records
Each object
Environment Record
is associated with an object called its
binding object
. An object
Environment Record
binds the set of string identifier names that directly correspond to
the property names of its binding object. Property keys that are not
strings in the form of an
IdentifierName
are not included in the set of bound identifiers. Both own and
inherited properties are included in the set regardless of the setting
of their [[Enumerable]] attribute. Because properties can be dynamically
added and deleted from objects, the set of identifiers bound by an
object
Environment Record
may potentially change as a side-effect of any operation that adds or
deletes properties. Any bindings that are created as a result of such a
side-effect are considered to be a mutable binding even if the Writable
attribute of the corresponding property has the value
false
. Immutable bindings do not exist for object Environment Records.
Object Environment Records created for
with
statements (
13.11
) can provide their binding object as an implicit
this
value for use in function calls. The capability is controlled by a
withEnvironment
Boolean value that is associated with each object
Environment Record
. By default, the value of
withEnvironment
is
false
for any object
Environment Record
The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.
8.1.1.2.1
HasBinding (
The concrete
Environment Record
method HasBinding for object Environment Records determines if its
associated binding object has a property whose name is the value of the
argument
Let
envRec
be the object
Environment Record
for which the method was invoked.
Let
bindings
be the binding object for
envRec
Let
foundBinding
be ?
HasProperty
bindings
).
If
foundBinding
is
false
, return
false
If the
withEnvironment
flag of
envRec
is
false
, return
true
Let
unscopables
be ?
Get
bindings
, @@unscopables).
If
Type
unscopables
) is Object, then
Let
blocked
be
ToBoolean
(?
Get
unscopables
)).
If
blocked
is
true
, return
false
Return
true
8.1.1.2.2
CreateMutableBinding (
The concrete
Environment Record
method CreateMutableBinding for object Environment Records creates in an
Environment Record
's associated binding object a property whose name is the String value and initializes it to the value
undefined
. If Boolean argument
has the value
true
the new property's [[Configurable]] attribute is set to
true
; otherwise it is set to
false
Let
envRec
be the object
Environment Record
for which the method was invoked.
Let
bindings
be the binding object for
envRec
Return ?
DefinePropertyOrThrow
bindings
, PropertyDescriptor { [[Value]]:
undefined
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
}).
Note
Normally
envRec
will not have a binding for
but if it does, the semantics of
DefinePropertyOrThrow
may result in an existing binding being replaced or shadowed or cause an
abrupt completion
to be returned.
8.1.1.2.3
CreateImmutableBinding (
The concrete
Environment Record
method CreateImmutableBinding is never used within this specification in association with object Environment Records.
8.1.1.2.4
InitializeBinding (
The concrete
Environment Record
method InitializeBinding for object Environment Records is used to set
the bound value of the current binding of the identifier whose name is
the value of the argument
to the value of argument
. An uninitialized binding for
must already exist.
Let
envRec
be the object
Environment Record
for which the method was invoked.
Assert
envRec
must have an uninitialized binding for
Record
that the binding for
in
envRec
has been initialized.
Return ?
envRec
.SetMutableBinding(
false
).
Note
In this specification, all uses of CreateMutableBinding
for object Environment Records are immediately followed by a call to
InitializeBinding for the same name. Hence, implementations do not need
to explicitly track the initialization state of individual object
Environment Record
bindings.
8.1.1.2.5
SetMutableBinding (
The concrete
Environment Record
method SetMutableBinding for object Environment Records attempts to set the value of the
Environment Record
's associated binding object's property whose name is the value of the argument
to the value of argument
. A property named
normally already exists but if it does not or is not currently
writable, error handling is determined by the value of the Boolean
argument
Let
envRec
be the object
Environment Record
for which the method was invoked.
Let
bindings
be the binding object for
envRec
Return ?
Set
bindings
).
8.1.1.2.6
GetBindingValue (
The concrete
Environment Record
method GetBindingValue for object Environment Records returns the value
of its associated binding object's property whose name is the String
value of the argument identifier
. The property should already exist but if it does not the result depends upon the value of the
argument:
Let
envRec
be the object
Environment Record
for which the method was invoked.
Let
bindings
be the binding object for
envRec
Let
value
be ?
HasProperty
bindings
).
If
value
is
false
, then
If
is
false
, return the value
undefined
; otherwise throw a
ReferenceError
exception.
Return ?
Get
bindings
).
8.1.1.2.7
DeleteBinding (
The concrete
Environment Record
method DeleteBinding for object Environment Records can only delete
bindings that correspond to properties of the environment object whose
[[Configurable]] attribute have the value
true
Let
envRec
be the object
Environment Record
for which the method was invoked.
Let
bindings
be the binding object for
envRec
Return ?
bindings
.[[Delete]](
).
8.1.1.2.8
HasThisBinding ( )
Regular object Environment Records do not provide a
this
binding.
Return
false
8.1.1.2.9
HasSuperBinding ( )
Regular object Environment Records do not provide a
super
binding.
Return
false
8.1.1.2.10
WithBaseObject ( )
Object Environment Records return
undefined
as their WithBaseObject unless their
withEnvironment
flag is
true
Let
envRec
be the object
Environment Record
for which the method was invoked.
If the
withEnvironment
flag of
envRec
is
true
, return the binding object for
envRec
Otherwise, return
undefined
8.1.1.3
Function Environment Records
function Environment Record
is a declarative
Environment Record
that is used to represent the top-level scope of a function and, if the function is not an
ArrowFunction
, provides a
this
binding. If a function is not an
ArrowFunction
function and references
super
, its function Environment Record also contains the state that is used to perform
super
method invocations from within the function.
Function Environment Records have the additional state fields listed in
Table 15
Table 15: Additional Fields of Function Environment Records
Field Name
Value
Meaning
[[ThisValue]]
Any
This is the
this
value used for this invocation of the function.
[[ThisBindingStatus]]
"lexical"
"initialized"
"uninitialized"
If the value is
"lexical"
, this is an
ArrowFunction
and does not have a local this value.
[[FunctionObject]]
Object
The
function object
whose invocation caused this
Environment Record
to be created.
[[HomeObject]]
Object |
undefined
If the associated function has
super
property accesses and is not an
ArrowFunction
, [[HomeObject]] is the object that the function is bound to as a method. The default value for [[HomeObject]] is
undefined
[[NewTarget]]
Object |
undefined
If this
Environment Record
was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]]
newTarget
parameter. Otherwise, its value is
undefined
Function Environment Records support all of the declarative
Environment Record
methods listed in
Table 14
and share the same specifications for all of those methods except for
HasThisBinding and HasSuperBinding. In addition, function Environment
Records support the methods listed in
Table 16
Table 16: Additional Methods of Function Environment Records
Method
Purpose
BindThisValue(V)
Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()
Return the value of this
Environment Record
's
this
binding. Throws a
ReferenceError
if the
this
binding has not been initialized.
GetSuperBase()
Return the object that is the base for
super
property accesses bound in this
Environment Record
. The object is derived from this
Environment Record
's [[HomeObject]] field. The value
undefined
indicates that
super
property accesses will produce runtime errors.
The behaviour of the additional concrete specification
methods for function Environment Records is defined by the following
algorithms:
8.1.1.3.1
BindThisValue (
Let
envRec
be the
function Environment Record
for which the method was invoked.
Assert
envRec
.[[ThisBindingStatus]] is not
"lexical"
If
envRec
.[[ThisBindingStatus]] is
"initialized"
, throw a
ReferenceError
exception.
Set
envRec
.[[ThisValue]] to
Set
envRec
.[[ThisBindingStatus]] to
"initialized"
Return
8.1.1.3.2
HasThisBinding ( )
Let
envRec
be the
function Environment Record
for which the method was invoked.
If
envRec
.[[ThisBindingStatus]] is
"lexical"
, return
false
; otherwise, return
true
8.1.1.3.3
HasSuperBinding ( )
Let
envRec
be the
function Environment Record
for which the method was invoked.
If
envRec
.[[ThisBindingStatus]] is
"lexical"
, return
false
If
envRec
.[[HomeObject]] has the value
undefined
, return
false
; otherwise, return
true
8.1.1.3.4
GetThisBinding ( )
Let
envRec
be the
function Environment Record
for which the method was invoked.
Assert
envRec
.[[ThisBindingStatus]] is not
"lexical"
If
envRec
.[[ThisBindingStatus]] is
"uninitialized"
, throw a
ReferenceError
exception.
Return
envRec
.[[ThisValue]].
8.1.1.3.5
GetSuperBase ( )
Let
envRec
be the
function Environment Record
for which the method was invoked.
Let
be
envRec
.[[HomeObject]].
If
has the value
undefined
, return
undefined
Assert
Type
) is Object.
Return ?
.[[GetPrototypeOf]]().
8.1.1.4
Global Environment Records
A global
Environment Record
is used to represent the outer most scope that is shared by all of the ECMAScript
Script
elements that are processed in a common
realm
. A global
Environment Record
provides the bindings for built-in globals (clause
18
), properties of the
global object
, and for all top-level declarations (
13.2.8
13.2.10
) that occur within a
Script
A global
Environment Record
is logically a single record but it is specified as a composite encapsulating an object
Environment Record
and a declarative
Environment Record
. The object
Environment Record
has as its base object the
global object
of the associated
Realm Record
. This
global object
is the value returned by the global
Environment Record
's GetThisBinding concrete method. The object
Environment Record
component of a global
Environment Record
contains the bindings for all built-in globals (clause
18
) and all bindings introduced by a
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
, or
VariableStatement
contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the declarative
Environment Record
component of the global
Environment Record
Properties may be created directly on a
global object
. Hence, the object
Environment Record
component of a global
Environment Record
may contain both bindings created explicitly by
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
, or
VariableDeclaration
declarations and bindings created implicitly as properties of the
global object
. In order to identify which bindings were explicitly created using declarations, a global
Environment Record
maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.
Global Environment Records have the additional fields listed in
Table 17
and the additional methods listed in
Table 18
Table 17: Additional Fields of Global Environment Records
Field Name
Value
Meaning
[[ObjectRecord]]
Object
Environment Record
Binding object is the
global object
. It contains global built-in bindings as well as
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
, and
VariableDeclaration
bindings in global code for the associated
realm
[[GlobalThisValue]]
Object
The value returned by
this
in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]]
Declarative
Environment Record
Contains bindings for all declarations in global code for the associated
realm
code except for
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
, and
VariableDeclaration
bindings
[[VarNames]]
List
of String
The string names bound by
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
, and
VariableDeclaration
declarations in global code for the associated
realm
Table 18: Additional Methods of Global Environment Records
Method
Purpose
GetThisBinding()
Return the value of this
Environment Record
's
this
binding.
HasVarDeclaration (N)
Determines if the argument identifier has a binding in this
Environment Record
that was created using a
VariableDeclaration
FunctionDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
, or
AsyncGeneratorDeclaration
HasLexicalDeclaration (N)
Determines if the argument identifier has a binding in this
Environment Record
that was created using a lexical declaration such as a
LexicalDeclaration
or a
ClassDeclaration
HasRestrictedGlobalProperty (N)
Determines if the argument is the name of a
global object
property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument
CanDeclareGlobalFunction (N)
Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument
CreateGlobalVarBinding(N, D)
Used to create and initialize to
undefined
a global
var
binding in the [[ObjectRecord]] component of a global
Environment Record
. The binding will be a mutable binding. The corresponding
global object
property will have attribute values appropriate for a
var
. The String value
is the bound name. If
is
true
the binding may be deleted. Logically equivalent to
CreateMutableBinding followed by a SetMutableBinding but it allows var
declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global
function
binding in the [[ObjectRecord]] component of a global
Environment Record
. The binding will be a mutable binding. The corresponding
global object
property will have attribute values appropriate for a
function
. The String value
is the bound name.
is the initialization value. If the Boolean argument
is
true
the binding may be deleted. Logically equivalent to
CreateMutableBinding followed by a SetMutableBinding but it allows
function declarations to receive special treatment.
The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.
8.1.1.4.1
HasBinding (
The concrete
Environment Record
method HasBinding for global Environment Records simply determines if
the argument identifier is one of the identifiers bound by the record:
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, return
true
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Return ?
ObjRec
.HasBinding(
).
8.1.1.4.2
CreateMutableBinding (
The concrete
Environment Record
method CreateMutableBinding for global Environment Records creates a new mutable binding for the name
that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for
must not already exist in the DeclarativeRecord. If Boolean argument
has the value
true
the new binding is marked as being subject to deletion.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, throw a
TypeError
exception.
Return
DclRec
.CreateMutableBinding(
).
8.1.1.4.3
CreateImmutableBinding (
The concrete
Environment Record
method CreateImmutableBinding for global Environment Records creates a new immutable binding for the name
that is uninitialized. A binding must not already exist in this
Environment Record
for
. If the Boolean argument
has the value
true
the new binding is marked as a strict binding.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, throw a
TypeError
exception.
Return
DclRec
.CreateImmutableBinding(
).
8.1.1.4.4
InitializeBinding (
The concrete
Environment Record
method InitializeBinding for global Environment Records is used to set
the bound value of the current binding of the identifier whose name is
the value of the argument
to the value of argument
. An uninitialized binding for
must already exist.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, then
Return
DclRec
.InitializeBinding(
).
Assert
: If the binding exists, it must be in the object
Environment Record
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Return ?
ObjRec
.InitializeBinding(
).
8.1.1.4.5
SetMutableBinding (
The concrete
Environment Record
method SetMutableBinding for global Environment Records attempts to
change the bound value of the current binding of the identifier whose
name is the value of the argument
to the value of argument
. If the binding is an immutable binding, a
TypeError
is thrown if
is
true
. A property named
normally already exists but if it does not or is not currently
writable, error handling is determined by the value of the Boolean
argument
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, then
Return
DclRec
.SetMutableBinding(
).
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Return ?
ObjRec
.SetMutableBinding(
).
8.1.1.4.6
GetBindingValue (
The concrete
Environment Record
method GetBindingValue for global Environment Records returns the value
of its bound identifier whose name is the value of the argument
. If the binding is an uninitialized binding throw a
ReferenceError
exception. A property named
normally already exists but if it does not or is not currently
writable, error handling is determined by the value of the Boolean
argument
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, then
Return
DclRec
.GetBindingValue(
).
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Return ?
ObjRec
.GetBindingValue(
).
8.1.1.4.7
DeleteBinding (
The concrete
Environment Record
method DeleteBinding for global Environment Records can only delete
bindings that have been explicitly designated as being subject to
deletion.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
If
DclRec
.HasBinding(
) is
true
, then
Return
DclRec
.DeleteBinding(
).
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
existingProp
be ?
HasOwnProperty
globalObject
).
If
existingProp
is
true
, then
Let
status
be ?
ObjRec
.DeleteBinding(
).
If
status
is
true
, then
Let
varNames
be
envRec
.[[VarNames]].
If
is an element of
varNames
, remove that element from the
varNames
Return
status
Return
true
8.1.1.4.8
HasThisBinding ( )
Return
true
8.1.1.4.9
HasSuperBinding ( )
Return
false
8.1.1.4.10
WithBaseObject ( )
Global Environment Records always return
undefined
as their WithBaseObject.
Return
undefined
8.1.1.4.11
GetThisBinding ( )
Let
envRec
be the global
Environment Record
for which the method was invoked.
Return
envRec
.[[GlobalThisValue]].
8.1.1.4.12
HasVarDeclaration (
The concrete
Environment Record
method HasVarDeclaration for global Environment Records determines if
the argument identifier has a binding in this record that was created
using a
VariableStatement
or a
FunctionDeclaration
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
varDeclaredNames
be
envRec
.[[VarNames]].
If
varDeclaredNames
contains
, return
true
Return
false
8.1.1.4.13
HasLexicalDeclaration (
The concrete
Environment Record
method HasLexicalDeclaration for global Environment Records determines
if the argument identifier has a binding in this record that was created
using a lexical declaration such as a
LexicalDeclaration
or a
ClassDeclaration
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
DclRec
be
envRec
.[[DeclarativeRecord]].
Return
DclRec
.HasBinding(
).
8.1.1.4.14
HasRestrictedGlobalProperty (
The concrete
Environment Record
method HasRestrictedGlobalProperty for global Environment Records
determines if the argument identifier is the name of a property of the
global object
that must not be shadowed by a global lexical binding:
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
existingProp
be ?
globalObject
.[[GetOwnProperty]](
).
If
existingProp
is
undefined
, return
false
If
existingProp
.[[Configurable]] is
true
, return
false
Return
true
Note
Properties may exist upon a
global object
that were directly created rather than being declared using a var or
function declaration. A global lexical binding may not be created that
has the same name as a non-configurable property of the
global object
. The global property
undefined
is an example of such a property.
8.1.1.4.15
CanDeclareGlobalVar (
The concrete
Environment Record
method CanDeclareGlobalVar for global Environment Records determines if
a corresponding CreateGlobalVarBinding call would succeed if called for
the same argument
. Redundant var declarations and var declarations for pre-existing
global object
properties are allowed.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
hasProperty
be ?
HasOwnProperty
globalObject
).
If
hasProperty
is
true
, return
true
Return ?
IsExtensible
globalObject
).
8.1.1.4.16
CanDeclareGlobalFunction (
The concrete
Environment Record
method CanDeclareGlobalFunction for global Environment Records
determines if a corresponding CreateGlobalFunctionBinding call would
succeed if called for the same argument
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
existingProp
be ?
globalObject
.[[GetOwnProperty]](
).
If
existingProp
is
undefined
, return ?
IsExtensible
globalObject
).
If
existingProp
.[[Configurable]] is
true
, return
true
If
IsDataDescriptor
existingProp
) is
true
and
existingProp
has attribute values { [[Writable]]:
true
, [[Enumerable]]:
true
}, return
true
Return
false
8.1.1.4.17
CreateGlobalVarBinding (
The concrete
Environment Record
method CreateGlobalVarBinding for global Environment Records creates
and initializes a mutable binding in the associated object
Environment Record
and records the bound name in the associated [[VarNames]]
List
. If a binding already exists, it is reused and assumed to be initialized.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
hasProperty
be ?
HasOwnProperty
globalObject
).
Let
extensible
be ?
IsExtensible
globalObject
).
If
hasProperty
is
false
and
extensible
is
true
, then
Perform ?
ObjRec
.CreateMutableBinding(
).
Perform ?
ObjRec
.InitializeBinding(
undefined
).
Let
varDeclaredNames
be
envRec
.[[VarNames]].
If
varDeclaredNames
does not contain
, then
Append
to
varDeclaredNames
Return
NormalCompletion
empty
).
8.1.1.4.18
CreateGlobalFunctionBinding (
The concrete
Environment Record
method CreateGlobalFunctionBinding for global Environment Records
creates and initializes a mutable binding in the associated object
Environment Record
and records the bound name in the associated [[VarNames]]
List
. If a binding already exists, it is replaced.
Let
envRec
be the global
Environment Record
for which the method was invoked.
Let
ObjRec
be
envRec
.[[ObjectRecord]].
Let
globalObject
be the binding object for
ObjRec
Let
existingProp
be ?
globalObject
.[[GetOwnProperty]](
).
If
existingProp
is
undefined
or
existingProp
.[[Configurable]] is
true
, then
Let
desc
be the PropertyDescriptor { [[Value]]:
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
}.
Else,
Let
desc
be the PropertyDescriptor { [[Value]]:
}.
Perform ?
DefinePropertyOrThrow
globalObject
desc
).
Record
that the binding for
in
ObjRec
has been initialized.
Perform ?
Set
globalObject
false
).
Let
varDeclaredNames
be
envRec
.[[VarNames]].
If
varDeclaredNames
does not contain
, then
Append
to
varDeclaredNames
Return
NormalCompletion
empty
).
Note
Global function declarations are always represented as own properties of the
global object
If possible, an existing own property is reconfigured to have a
standard set of attribute values. Steps 8-9 are equivalent to what
calling the InitializeBinding concrete method would do and if
globalObject
is a Proxy will produce the same sequence of Proxy trap calls.
8.1.1.5
Module Environment Records
A module
Environment Record
is a declarative
Environment Record
that is used to represent the outer scope of an ECMAScript
Module
In additional to normal mutable and immutable bindings, module
Environment Records also provide immutable import bindings which are
bindings that provide indirect access to a target binding that exists in
another
Environment Record
Module Environment Records support all of the declarative
Environment Record
methods listed in
Table 14
and share the same specifications for all of those methods except for
GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In
addition, module Environment Records support the methods listed in
Table 19
Table 19: Additional Methods of Module Environment Records
Method
Purpose
CreateImportBinding(N, M, N2)
Create an immutable indirect binding in a module
Environment Record
. The String value
is the text of the bound name.
is a
Module Record
, and
N2
is a binding that exists in M's module
Environment Record
GetThisBinding()
Return the value of this
Environment Record
's
this
binding.
The behaviour of the additional concrete specification
methods for module Environment Records are defined by the following
algorithms:
8.1.1.5.1
GetBindingValue (
The concrete
Environment Record
method GetBindingValue for module Environment Records returns the value
of its bound identifier whose name is the value of the argument
However, if the binding is an indirect binding the value of the target
binding is returned. If the binding exists but is uninitialized a
ReferenceError
is thrown.
Assert
is
true
Let
envRec
be the module
Environment Record
for which the method was invoked.
Assert
envRec
has a binding for
If the binding for
is an indirect binding, then
Let
and
N2
be the indirection values provided when this binding for
was created.
Let
targetEnv
be
.[[Environment]].
If
targetEnv
is
undefined
, throw a
ReferenceError
exception.
Let
targetER
be
targetEnv
's
EnvironmentRecord
Return ?
targetER
.GetBindingValue(
N2
true
).
If the binding for
in
envRec
is an uninitialized binding, throw a
ReferenceError
exception.
Return the value currently bound to
in
envRec
Note
will always be
true
because a
Module
is always
strict mode code
8.1.1.5.2
DeleteBinding (
The concrete
Environment Record
method DeleteBinding for module Environment Records refuses to delete bindings.
Assert
: This method is never invoked. See
12.5.3.1
Note
Module Environment Records are only used within strict code and an
early error
rule prevents the delete operator, in strict code, from being applied to a
Reference
that would resolve to a module
Environment Record
binding. See
12.5.3.1
8.1.1.5.3
HasThisBinding ( )
Module Environment Records provide a
this
binding.
Return
true
8.1.1.5.4
GetThisBinding ( )
Return
undefined
8.1.1.5.5
CreateImportBinding (
N2
The concrete
Environment Record
method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name
. A binding must not already exist in this
Environment Record
for
is a
Module Record
, and
N2
is the name of a binding that exists in M's module
Environment Record
. Accesses to the value of the new binding will indirectly access the bound value of the target binding.
Let
envRec
be the module
Environment Record
for which the method was invoked.
Assert
envRec
does not already have a binding for
Assert
is a
Module Record
Assert
: When
.[[Environment]] is instantiated it will have a direct binding for
N2
Create an immutable indirect binding in
envRec
for
that references
and
N2
as its target binding and record that the binding is initialized.
Return
NormalCompletion
empty
).
8.1.2
Lexical Environment Operations
The following
abstract operations
are used in this specification to operate upon lexical environments:
8.1.2.1
GetIdentifierReference (
lex
name
strict
The abstract operation GetIdentifierReference is called with a
Lexical Environment
lex
, a String
name
, and a Boolean flag
strict
. The value of
lex
may be
null
. When called, the following steps are performed:
If
lex
is the value
null
, then
Return a value of type
Reference
whose base value component is
undefined
, whose referenced name component is
name
, and whose strict reference flag is
strict
Let
envRec
be
lex
's
EnvironmentRecord
Let
exists
be ?
envRec
.HasBinding(
name
).
If
exists
is
true
, then
Return a value of type
Reference
whose base value component is
envRec
, whose referenced name component is
name
, and whose strict reference flag is
strict
Else,
Let
outer
be the value of
lex
's outer environment reference.
Return ?
GetIdentifierReference
outer
name
strict
).
8.1.2.2
NewDeclarativeEnvironment (
When the abstract operation NewDeclarativeEnvironment is called with a
Lexical Environment
as argument
the following steps are performed:
Let
env
be a new
Lexical Environment
Let
envRec
be a new declarative
Environment Record
containing no bindings.
Set
env
's
EnvironmentRecord
to
envRec
Set the outer lexical environment reference of
env
to
Return
env
8.1.2.3
NewObjectEnvironment (
When the abstract operation NewObjectEnvironment is called with an Object
and a
Lexical Environment
as arguments, the following steps are performed:
Let
env
be a new
Lexical Environment
Let
envRec
be a new object
Environment Record
containing
as the binding object.
Set
env
's
EnvironmentRecord
to
envRec
Set the outer lexical environment reference of
env
to
Return
env
8.1.2.4
NewFunctionEnvironment (
newTarget
When the abstract operation NewFunctionEnvironment is called with arguments
and
newTarget
the following steps are performed:
Assert
is an ECMAScript function.
Assert
Type
newTarget
) is Undefined or Object.
Let
env
be a new
Lexical Environment
Let
envRec
be a new
function Environment Record
containing no bindings.
Set
envRec
.[[FunctionObject]] to
If
.[[ThisMode]] is
lexical
, set
envRec
.[[ThisBindingStatus]] to
"lexical"
Else, set
envRec
.[[ThisBindingStatus]] to
"uninitialized"
Let
be
.[[HomeObject]].
Set
envRec
.[[HomeObject]] to
Set
envRec
.[[NewTarget]] to
newTarget
Set
env
's
EnvironmentRecord
to
envRec
Set the outer lexical environment reference of
env
to
.[[Environment]].
Return
env
8.1.2.5
NewGlobalEnvironment (
thisValue
When the abstract operation NewGlobalEnvironment is called with arguments
and
thisValue
, the following steps are performed:
Let
env
be a new
Lexical Environment
Let
objRec
be a new object
Environment Record
containing
as the binding object.
Let
dclRec
be a new declarative
Environment Record
containing no bindings.
Let
globalRec
be a new global
Environment Record
Set
globalRec
.[[ObjectRecord]] to
objRec
Set
globalRec
.[[GlobalThisValue]] to
thisValue
Set
globalRec
.[[DeclarativeRecord]] to
dclRec
Set
globalRec
.[[VarNames]] to a new empty
List
Set
env
's
EnvironmentRecord
to
globalRec
Set the outer lexical environment reference of
env
to
null
Return
env
8.1.2.6
NewModuleEnvironment (
When the abstract operation NewModuleEnvironment is called with a
Lexical Environment
argument
the following steps are performed:
Let
env
be a new
Lexical Environment
Let
envRec
be a new module
Environment Record
containing no bindings.
Set
env
's
EnvironmentRecord
to
envRec
Set the outer lexical environment reference of
env
to
Return
env
8.2
Realms
Before it is evaluated, all ECMAScript code must be associated with a
realm
. Conceptually, a
realm
consists of a set of intrinsic objects, an ECMAScript
global environment
, all of the ECMAScript code that is loaded within the scope of that
global environment
, and other associated state and resources.
realm
is represented in this specification as a
Realm Record
with the fields specified in
Table 20
Table 20:
Realm Record
Fields
Field Name
Value
Meaning
[[Intrinsics]]
Record
whose field names are intrinsic keys and whose values are objects
The intrinsic values used by code associated with this
realm
[[GlobalObject]]
Object
The
global object
for this
realm
[[GlobalEnv]]
Lexical Environment
The
global environment
for this
realm
[[TemplateMap]]
List
of
Record
{ [[Site]]:
Parse Node
, [[Array]]: Object }.
Template objects are canonicalized separately for each
realm
using its
Realm Record
's [[TemplateMap]]. Each [[Site]] value is a
Parse Node
that is a
TemplateLiteral
. The associated [[Array]] value is the corresponding template object that is passed to a tag function.
Note
Once a
Parse Node
becomes unreachable, the corresponding [[Array]] is also unreachable,
and it would be unobservable if an implementation removed the pair from
the [[TemplateMap]] list.
[[HostDefined]]
Any, default value is
undefined
Field reserved for use by host environments that need to associate additional information with a
Realm Record
8.2.1
CreateRealm ( )
The abstract operation CreateRealm with no arguments performs the following steps:
Let
realmRec
be a new
Realm Record
Perform
CreateIntrinsics
realmRec
).
Set
realmRec
.[[GlobalObject]] to
undefined
Set
realmRec
.[[GlobalEnv]] to
undefined
Set
realmRec
.[[TemplateMap]] to a new empty
List
Return
realmRec
8.2.2
CreateIntrinsics (
realmRec
The abstract operation CreateIntrinsics with argument
realmRec
performs the following steps:
Let
intrinsics
be a new
Record
Set
realmRec
.[[Intrinsics]] to
intrinsics
Let
objProto
be
ObjectCreate
null
).
Set
intrinsics
.[[
%ObjectPrototype%
]] to
objProto
Let
throwerSteps
be the algorithm steps specified in
9.2.9.1
for the
%ThrowTypeError%
function.
Let
thrower
be
CreateBuiltinFunction
throwerSteps
, « »,
realmRec
null
).
Set
intrinsics
.[[
%ThrowTypeError%
]] to
thrower
Let
noSteps
be an empty sequence of algorithm steps.
Let
funcProto
be
CreateBuiltinFunction
noSteps
, « »,
realmRec
objProto
).
Set
intrinsics
.[[
%FunctionPrototype%
]] to
funcProto
Call
thrower
.[[SetPrototypeOf]](
funcProto
).
Perform
AddRestrictedFunctionProperties
funcProto
realmRec
).
Set fields of
intrinsics
with the values listed in
Table 7
that have not already been handled above. The field names are the names
listed in column one of the table. The value of each field is a new
object value fully and recursively populated with property values as
defined by the specification of each object in clauses 18-26. All object
property values are newly created object values. All values that are
built-in function objects are created by performing
CreateBuiltinFunction
(, ,
realmRec
) where is the definition of that
function provided by this specification, is a list of the
names, if any, of the function's specified internal slots, and
is the specified value of the function's [[Prototype]]
internal slot. The creation of the intrinsics and their properties must
be ordered to avoid any dependencies upon objects that have not yet
been created.
Return
intrinsics
8.2.3
SetRealmGlobalObject (
realmRec
globalObj
thisValue
The abstract operation SetRealmGlobalObject with arguments
realmRec
globalObj
, and
thisValue
performs the following steps:
If
globalObj
is
undefined
, then
Let
intrinsics
be
realmRec
.[[Intrinsics]].
Set
globalObj
to
ObjectCreate
intrinsics
.[[
%ObjectPrototype%
]]).
Assert
Type
globalObj
) is Object.
If
thisValue
is
undefined
, set
thisValue
to
globalObj
Set
realmRec
.[[GlobalObject]] to
globalObj
Let
newGlobalEnv
be
NewGlobalEnvironment
globalObj
thisValue
).
Set
realmRec
.[[GlobalEnv]] to
newGlobalEnv
Return
realmRec
8.2.4
SetDefaultGlobalBindings (
realmRec
The abstract operation SetDefaultGlobalBindings with argument
realmRec
performs the following steps:
Let
global
be
realmRec
.[[GlobalObject]].
For each property of the Global Object specified in clause
18
, do
Let
name
be the String value of the
property name
Let
desc
be the fully populated
data property
descriptor for the property containing the specified attributes for the property. For properties listed in
18.2
18.3
, or
18.4
the value of the [[Value]] attribute is the corresponding intrinsic object from
realmRec
Perform ?
DefinePropertyOrThrow
global
name
desc
).
Return
global
8.3
Execution Contexts
An
execution context
is a specification device that
is used to track the runtime evaluation of code by an ECMAScript
implementation. At any point in time, there is at most one execution
context per
agent
that is actually executing code. This is known as the
agent
's
running execution context
. All references to the
running execution context
in this specification denote the
running execution context
of the
surrounding agent
The
execution context stack
is used to track execution contexts. The
running execution context
is always the top element of this stack. A new execution context is
created whenever control is transferred from the executable code
associated with the currently
running execution context
to executable code that is not associated with that execution context.
The newly created execution context is pushed onto the stack and becomes
the
running execution context
An execution context contains whatever implementation specific
state is necessary to track the execution progress of its associated
code. Each execution context has at least the state components listed in
Table 21
Table 21: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with this
execution context
Function
If this
execution context
is evaluating the code of a
function object
, then the value of this component is that
function object
. If the context is evaluating the code of a
Script
or
Module
, the value is
null
Realm
The
Realm Record
from which associated code accesses ECMAScript resources.
ScriptOrModule
The
Module Record
or
Script Record
from which associated code originates. If there is no originating script or module, as is the case for the original
execution context
created in
InitializeHostDefinedRealm
, the value is
null
Evaluation of code by the
running execution context
may be suspended at various points defined within this specification. Once the
running execution context
has been suspended a different execution context may become the
running execution context
and commence evaluating its code. At some later time a suspended execution context may again become the
running execution context
and continue evaluating its code at the point where it had previously been suspended. Transition of the
running execution context
status among execution contexts usually occurs in stack-like
last-in/first-out manner. However, some ECMAScript features require
non-LIFO transitions of the
running execution context
The value of the
Realm
component of the
running execution context
is also called
the current Realm Record
. The value of the Function component of the
running execution context
is also called the
active function object
Execution contexts for ECMAScript code have the additional state components listed in
Table 22
Table 22: Additional State Components for ECMAScript Code Execution Contexts
Component
Purpose
LexicalEnvironment
Identifies the
Lexical Environment
used to resolve identifier references made by code within this
execution context
VariableEnvironment
Identifies the
Lexical Environment
whose
EnvironmentRecord
holds bindings created by
VariableStatement
s within this
execution context
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments.
Execution contexts representing the evaluation of generator objects have the additional state components listed in
Table 23
Table 23: Additional State Components for Generator Execution Contexts
Component
Purpose
Generator
The GeneratorObject that this
execution context
is evaluating.
In most situations only the
running execution context
(the top of the
execution context stack
is directly manipulated by algorithms within this specification. Hence
when the terms “LexicalEnvironment”, and “VariableEnvironment” are used
without qualification they are in reference to those components of the
running execution context
An execution context is purely a specification mechanism and need
not correspond to any particular artefact of an ECMAScript
implementation. It is impossible for ECMAScript code to directly access
or observe an execution context.
8.3.1
GetActiveScriptOrModule ( )
The GetActiveScriptOrModule abstract operation is used to determine the running script or module, based on the
running execution context
. GetActiveScriptOrModule performs the following steps:
If the
execution context stack
is empty, return
null
Let
ec
be the topmost
execution context
on the
execution context stack
whose ScriptOrModule component is not
null
If no such
execution context
exists, return
null
. Otherwise, return
ec
's ScriptOrModule component.
8.3.2
ResolveBinding (
name
[ ,
env
] )
The ResolveBinding abstract operation is used to determine the binding of
name
passed as a String value. The optional argument
env
can be used to explicitly provide the
Lexical Environment
that is to be searched for the binding. During execution of ECMAScript
code, ResolveBinding is performed using the following algorithm:
If
env
is not present or if
env
is
undefined
, then
Set
env
to the
running execution context
's LexicalEnvironment.
Assert
env
is a
Lexical Environment
If the code matching the syntactic production that is being evaluated is contained in
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return ?
GetIdentifierReference
env
name
strict
).
Note
The result of ResolveBinding is always a
Reference
value with its referenced name component equal to the
name
argument.
8.3.3
GetThisEnvironment ( )
The abstract operation GetThisEnvironment finds the
Environment Record
that currently supplies the binding of the keyword
this
. GetThisEnvironment performs the following steps:
Let
lex
be the
running execution context
's LexicalEnvironment.
Repeat,
Let
envRec
be
lex
's
EnvironmentRecord
Let
exists
be
envRec
.HasThisBinding().
If
exists
is
true
, return
envRec
Let
outer
be the value of
lex
's outer environment reference.
Assert
outer
is not
null
Set
lex
to
outer
Note
The loop in step 2 will always terminate because the list of environments always ends with the
global environment
which has a
this
binding.
8.3.4
ResolveThisBinding ( )
The abstract operation ResolveThisBinding determines the binding of the keyword
this
using the LexicalEnvironment of the
running execution context
. ResolveThisBinding performs the following steps:
Let
envRec
be
GetThisEnvironment
().
Return ?
envRec
.GetThisBinding().
8.3.5
GetNewTarget ( )
The abstract operation GetNewTarget determines the NewTarget value using the LexicalEnvironment of the
running execution context
. GetNewTarget performs the following steps:
Let
envRec
be
GetThisEnvironment
().
Assert
envRec
has a [[NewTarget]] field.
Return
envRec
.[[NewTarget]].
8.3.6
GetGlobalObject ( )
The abstract operation GetGlobalObject returns the
global object
used by the currently
running execution context
. GetGlobalObject performs the following steps:
Let
ctx
be the
running execution context
Let
currentRealm
be
ctx
's
Realm
Return
currentRealm
.[[GlobalObject]].
8.4
Jobs and Job Queues
A Job is an abstract operation that initiates an ECMAScript
computation when no other ECMAScript computation is currently in
progress. A Job abstract operation may be defined to accept an arbitrary
set of job parameters.
Execution of a Job can be initiated only when there is no
running execution context
and the
execution context stack
is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal
Record
whose fields are specified in
Table 24
Once execution of a Job is initiated, the Job always executes to
completion. No other Job may be initiated until the currently running
Job completes. However, the currently running Job or external events may
cause the enqueuing of additional PendingJobs that may be initiated
sometime after completion of the currently running Job.
Table 24: PendingJob
Record
Fields
Field Name
Value
Meaning
[[Job]]
The name of a Job abstract operation
This is the abstract operation that is performed when execution of this PendingJob is initiated.
[[Arguments]]
List
The
List
of argument values that are to be passed to [[Job]] when it is activated.
[[Realm]]
Realm Record
The
Realm Record
for the initial
execution context
when this PendingJob is initiated.
[[ScriptOrModule]]
Script Record
or
Module Record
The script or module for the initial
execution context
when this PendingJob is initiated.
[[HostDefined]]
Any, default value is
undefined
Field reserved for use by host environments that need to associate additional information with a pending Job.
A Job Queue is a FIFO queue of PendingJob records. Each Job Queue
has a name and the full set of available Job Queues are defined by an
ECMAScript implementation. Every ECMAScript implementation has at least
the Job Queues defined in
Table 25
Each
agent
has its own set of named Job Queues. All references to a named job
queue in this specification denote the named job queue of the
surrounding agent
Table 25: Required Job Queues
Name
Purpose
ScriptJobs
Jobs that validate and evaluate ECMAScript
Script
and
Module
source text. See clauses 10 and 15.
PromiseJobs
Jobs that are responses to the settlement of a Promise (see
25.6
).
A request for the future execution of a Job is made by
enqueueing, on a Job Queue, a PendingJob record that includes a Job
abstract operation name and any necessary argument values. When there is
no
running execution context
and the
execution context stack
is empty, the ECMAScript implementation removes the first PendingJob
from a Job Queue and uses the information contained in it to create an
execution context
and starts execution of the associated Job abstract operation.
The PendingJob records from a single Job Queue are always
initiated in FIFO order. This specification does not define the order in
which multiple Job Queues are serviced. An ECMAScript implementation
may interweave the FIFO evaluation of the PendingJob records of a Job
Queue with the evaluation of the PendingJob records of one or more other
Job Queues. An implementation must define what occurs when there are no
running execution context
and all Job Queues are empty.
Note
Typically an ECMAScript implementation will have its Job Queues
pre-initialized with at least one PendingJob and one of those Jobs will
be the first to be executed. An implementation might choose to free all
resources and terminate if the current Job completes and all Job Queues
are empty. Alternatively, it might choose to wait for a some
implementation specific
agent
or mechanism to enqueue new PendingJob requests.
The following
abstract operations
are used to create and manage Jobs and Job Queues:
8.4.1
EnqueueJob (
queueName
job
arguments
The EnqueueJob abstract operation requires three arguments:
queueName
job
, and
arguments
. It performs the following steps:
Assert
Type
queueName
) is String and its value is the name of a Job Queue recognized by this implementation.
Assert
job
is the name of a Job.
Assert
arguments
is a
List
that has the same number of elements as the number of parameters required by
job
Let
callerContext
be the
running execution context
Let
callerRealm
be
callerContext
's
Realm
Let
callerScriptOrModule
be
callerContext
's ScriptOrModule.
Let
pending
be PendingJob { [[Job]]:
job
, [[Arguments]]:
arguments
, [[Realm]]:
callerRealm
, [[ScriptOrModule]]:
callerScriptOrModule
, [[HostDefined]]:
undefined
}.
Perform any implementation or host environment defined processing of
pending
. This may include modifying the [[HostDefined]] field or any other field of
pending
Add
pending
at the back of the Job Queue named by
queueName
Return
NormalCompletion
empty
).
8.5
InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm performs the following steps:
Let
realm
be
CreateRealm
().
Let
newContext
be a new
execution context
Set the Function of
newContext
to
null
Set the
Realm
of
newContext
to
realm
Set the ScriptOrModule of
newContext
to
null
Push
newContext
onto the
execution context stack
newContext
is now the
running execution context
If the host requires use of an
exotic object
to serve as
realm
's
global object
, let
global
be such an object created in an implementation-defined manner. Otherwise, let
global
be
undefined
, indicating that an ordinary object should be created as the
global object
If the host requires that the
this
binding in
realm
's global scope return an object other than the
global object
, let
thisValue
be such an object created in an implementation-defined manner. Otherwise, let
thisValue
be
undefined
, indicating that
realm
's global
this
binding should be the
global object
Perform
SetRealmGlobalObject
realm
global
thisValue
).
Let
globalObj
be ?
SetDefaultGlobalBindings
realm
).
Create any implementation-defined
global object
properties on
globalObj
Return
NormalCompletion
empty
).
8.6
RunJobs ( )
The abstract operation RunJobs performs the following steps:
Perform ?
InitializeHostDefinedRealm
().
In an implementation-dependent manner, obtain the ECMAScript source texts (see clause
10
) and any associated host-defined values for zero or more ECMAScript scripts and/or ECMAScript modules. For each such
sourceText
and
hostDefined
, do
If
sourceText
is the source code of a script, then
Perform
EnqueueJob
"ScriptJobs"
ScriptEvaluationJob
, «
sourceText
hostDefined
»).
Else
sourceText
is the source code of a module,
Perform
EnqueueJob
"ScriptJobs"
TopLevelModuleEvaluationJob
, «
sourceText
hostDefined
»).
Repeat,
Suspend
the
running execution context
and remove it from the
execution context stack
Assert
: The
execution context stack
is now empty.
Let
nextQueue
be a non-empty Job Queue chosen in an implementation-defined manner. If
all Job Queues are empty, the result is implementation-defined.
Let
nextPending
be the PendingJob record at the front of
nextQueue
. Remove that record from
nextQueue
Let
newContext
be a new
execution context
Set
newContext
's Function to
null
Set
newContext
's
Realm
to
nextPending
.[[Realm]].
Set
newContext
's ScriptOrModule to
nextPending
.[[ScriptOrModule]].
Push
newContext
onto the
execution context stack
newContext
is now the
running execution context
Perform any implementation or host environment defined job initialization using
nextPending
Let
result
be the result of performing the abstract operation named by
nextPending
.[[Job]] using the elements of
nextPending
.[[Arguments]] as its arguments.
If
result
is an
abrupt completion
, perform
HostReportErrors
(«
result
.[[Value]] »).
8.7
Agents
An
agent
comprises a set of ECMAScript execution contexts, an
execution context stack
, a
running execution context
, a set of named job queues, an
Agent Record
, and an
executing thread
. Except for the
executing thread
, the constituents of an
agent
belong exclusively to that
agent
An
agent
's
executing thread
executes the jobs in the
agent
's job queues on the
agent
's execution contexts independently of other agents, except that an
executing thread
may be used as the
executing thread
by multiple agents, provided none of the agents sharing the thread have an
Agent Record
whose [[CanBlock]] property is
true
Note 1
Some web browsers share a single
executing thread
across multiple unrelated tabs of a browser window, for example.
While an
agent
's
executing thread
executes the jobs in the
agent
's job queues, the
agent
is the
surrounding agent
for the code in those jobs. The code uses the
surrounding agent
to access the specification level execution objects held within the
agent
: the
running execution context
, the
execution context stack
, the named job queues, and the
Agent Record
's fields.
Table 26:
Agent Record
Fields
Field Name
Value
Meaning
[[LittleEndian]]
Boolean
The default value computed for the
isLittleEndian
parameter when it is needed by the algorithms
GetValueFromBuffer
and
SetValueInBuffer
The choice is implementation-dependent and should be the alternative
that is most efficient for the implementation. Once the value has been
observed it cannot change.
[[CanBlock]]
Boolean
Determines whether the
agent
can block or not.
[[Signifier]]
Any globally-unique value
Uniquely identifies the
agent
within its
agent cluster
[[IsLockFree1]]
Boolean
true
if atomic operations on one-byte values are lock-free,
false
otherwise.
[[IsLockFree2]]
Boolean
true
if atomic operations on two-byte values are lock-free,
false
otherwise.
[[CandidateExecution]]
candidate execution
Record
See the
memory model
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any
agent
in the
agent cluster
they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not
necessarily determined by the hardware, but may also reflect
implementation choices that can vary over time and between ECMAScript
implementations.
There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.
In practice, if an atomic operation is implemented with any
type of lock the operation is not lock-free. Lock-free does not imply
wait-free: there is no upper bound on how many machine steps may be
required to complete a lock-free atomic operation.
That an atomic access of size
is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size
, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See
ReadSharedMemory
and
WriteSharedMemory
for details.
Note 3
An
agent
is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
8.7.1
AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments. It performs the following steps:
Let
AR
be the
Agent Record
of the
surrounding agent
Return
AR
.[[Signifier]].
8.7.2
AgentCanSuspend ( )
The abstract operation AgentCanSuspend takes no arguments. It performs the following steps:
Let
AR
be the
Agent Record
of the
surrounding agent
Return
AR
.[[CanBlock]].
Note
In some environments it may not be reasonable for a given
agent
to suspend. For example, in a web browser environment, it may be
reasonable to disallow suspending a document's main event handling
thread, while still allowing workers' event handling threads to suspend.
8.8
Agent Clusters
An
agent cluster
is a maximal set of agents that can communicate by operating on shared memory.
Note 1
Programs within different agents may share memory by
unspecified means. At a minimum, the backing memory for
SharedArrayBuffer objects can be shared among the agents in the cluster.
There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.
Every
agent
belongs to exactly one agent cluster.
Note 2
The agents in a cluster need not all be alive at some particular point in time. If
agent
creates another
agent
, after which
terminates and
creates
agent
, the three agents are in the same cluster if
could share some memory with
and
could share some memory with
All agents within a cluster must have the same value for the [[LittleEndian]] property in their respective
Agent
Records.
Note 3
If different agents within an agent cluster have different
values of [[LittleEndian]] it becomes hard to use shared memory for
multi-byte data.
All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respective
Agent
Records; similarly for the [[IsLockFree2]] property.
All agents within a cluster must have different values for the [[Signifier]] property in their respective
Agent
Records.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an
agent
without the
agent
's
knowledge or cooperation. If the embedding does so, it must not leave
some agents in the cluster active while other agents in the cluster are
deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an
agent
deadlocks or starves because another
agent
has been deactivated. For example, if an HTML shared worker that has a
lifetime independent of documents in any windows were allowed to share
memory with the dedicated worker of such an independent document, and
the document and its dedicated worker were to be deactivated while the
dedicated worker holds a lock (say, the document is pushed into its
window's history), and the shared worker then tries to acquire the lock,
then the shared worker will be blocked until the dedicated worker is
activated again, if ever. Meanwhile other workers trying to access the
shared worker from other windows will starve.
The implication of the restriction is that it will not be
possible to share memory between agents that don't belong to the same
suspend/wake collective within the embedding.
An embedding may terminate an
agent
without any of the
agent
's cluster's other agents' prior knowledge or cooperation. If an
agent
is terminated not by programmatic action of its own or of another
agent
in the cluster but by forces external to the cluster, then the
embedding must choose one of two strategies: Either terminate all the
agents in the cluster, or provide reliable APIs that allow the agents in
the cluster to coordinate so that at least one remaining member of the
cluster will be able to detect the termination, with the termination
data containing enough information to identify the
agent
that was terminated.
Note 5
Examples of that type of termination are: operating systems or
users terminating agents that are running in separate processes; the
embedding itself terminating an
agent
that is running in-process with the other agents when per-
agent
resource accounting indicates that the
agent
is runaway.
Prior to any evaluation of any ECMAScript code by any
agent
in a cluster, the [[CandidateExecution]] field of the
Agent Record
for all agents in the cluster is set to the initial
candidate execution
. The initial
candidate execution
is an
empty candidate execution
whose [[EventsRecords]] field is a
List
containing, for each
agent
, an
Agent Events Record
whose [[AgentSignifier]] field is that
agent
's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.
Note 6
All agents in an agent cluster share the same
candidate execution
in its
Agent Record
's [[CandidateExecution]] field. The
candidate execution
is a specification mechanism used by the
memory model
Note 7
An agent cluster is a specification mechanism and need not
correspond to any particular artefact of an ECMAScript implementation.
8.9
Forward Progress
For an
agent
to
make forward progress
is for it to perform an evaluation step according to this specification.
An
agent
becomes
blocked
when its
running execution context
waits synchronously and indefinitely for an external event. Only agents whose
Agent Record
's [[CanBlock]] property is
true
can become blocked in this sense. An
unblocked
agent
is one that is not blocked.
Implementations must ensure that:
every unblocked
agent
with a dedicated
executing thread
eventually makes forward progress
in a set of agents that share an
executing thread
, one
agent
eventually makes forward progress
an
agent
does not cause another
agent
to become blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the
memory model
, ensures that all
"SeqCst"
writes eventually become observable to all agents.
Ordinary and Exotic Objects Behaviours
9.1
Ordinary Object Internal Methods and Internal Slots
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either
null
or an object and is used for implementing inheritance. Data properties
of the [[Prototype]] object are inherited (and visible as properties of
the child object) for the purposes of get access, but not for set
access. Accessor properties are inherited for both get access and set
access.
Every ordinary object has a Boolean-valued [[Extensible]]
internal slot which is used to fulfill the extensibility-related
internal method invariants specified in
6.1.7.3
. Namely, once the value of an object's [[Extensible]] internal slot has been set to
false
it is no longer possible to add properties to the object, to modify the
value of the object's [[Prototype]] internal slot, or to subsequently
change the value of [[Extensible]] to
true
In the following algorithm descriptions, assume
is an ordinary object,
is a property key value,
is any
ECMAScript language value
, and
Desc
is a
Property Descriptor
record.
Each ordinary object internal method delegates to a
similarly-named abstract operation. If such an abstract operation
depends on another internal method, then the internal method is invoked
on
rather than calling the similarly-named abstract
operation directly. These semantics ensure that exotic objects have
their overridden internal methods invoked when ordinary object internal
methods are applied to them.
9.1.1
[[GetPrototypeOf]] ( )
When the [[GetPrototypeOf]] internal method of
is called, the following steps are taken:
Return !
OrdinaryGetPrototypeOf
).
9.1.1.1
OrdinaryGetPrototypeOf (
When the abstract operation OrdinaryGetPrototypeOf is called with Object
, the following steps are taken:
Return
.[[Prototype]].
9.1.2
[[SetPrototypeOf]] (
When the [[SetPrototypeOf]] internal method of
is called with argument
, the following steps are taken:
Return !
OrdinarySetPrototypeOf
).
9.1.2.1
OrdinarySetPrototypeOf (
When the abstract operation OrdinarySetPrototypeOf is called with Object
and value
, the following steps are taken:
Assert
: Either
Type
) is Object or
Type
) is Null.
Let
extensible
be
.[[Extensible]].
Let
current
be
.[[Prototype]].
If
SameValue
current
) is
true
, return
true
If
extensible
is
false
, return
false
Let
be
Let
done
be
false
Repeat, while
done
is
false
If
is
null
, set
done
to
true
Else if
SameValue
) is
true
, return
false
Else,
If
.[[GetPrototypeOf]] is not the ordinary object internal method defined in
9.1.1
, set
done
to
true
Else, set
to
.[[Prototype]].
Set
.[[Prototype]] to
Return
true
Note
The loop in step 8 guarantees that there will be no
circularities in any prototype chain that only includes objects that use
the ordinary object definitions for [[GetPrototypeOf]] and
[[SetPrototypeOf]].
9.1.3
[[IsExtensible]] ( )
When the [[IsExtensible]] internal method of
is called, the following steps are taken:
Return !
OrdinaryIsExtensible
).
9.1.3.1
OrdinaryIsExtensible (
When the abstract operation OrdinaryIsExtensible is called with Object
, the following steps are taken:
Return
.[[Extensible]].
9.1.4
[[PreventExtensions]] ( )
When the [[PreventExtensions]] internal method of
is called, the following steps are taken:
Return !
OrdinaryPreventExtensions
).
9.1.4.1
OrdinaryPreventExtensions (
When the abstract operation OrdinaryPreventExtensions is called with Object
, the following steps are taken:
Set
.[[Extensible]] to
false
Return
true
9.1.5
[[GetOwnProperty]] (
When the [[GetOwnProperty]] internal method of
is called with property key
, the following steps are taken:
Return !
OrdinaryGetOwnProperty
).
9.1.5.1
OrdinaryGetOwnProperty (
When the abstract operation OrdinaryGetOwnProperty is called with Object
and with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
does not have an own property with key
, return
undefined
Let
be a newly created
Property Descriptor
with no fields.
Let
be
's own property whose key is
If
is a
data property
, then
Set
.[[Value]] to the value of
's [[Value]] attribute.
Set
.[[Writable]] to the value of
's [[Writable]] attribute.
Else
is an
accessor property
Set
.[[Get]] to the value of
's [[Get]] attribute.
Set
.[[Set]] to the value of
's [[Set]] attribute.
Set
.[[Enumerable]] to the value of
's [[Enumerable]] attribute.
Set
.[[Configurable]] to the value of
's [[Configurable]] attribute.
Return
9.1.6
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of
is called with property key
and
Property Descriptor
Desc
, the following steps are taken:
Return ?
OrdinaryDefineOwnProperty
Desc
).
9.1.6.1
OrdinaryDefineOwnProperty (
Desc
When the abstract operation OrdinaryDefineOwnProperty is called with Object
, property key
, and
Property Descriptor
Desc
, the following steps are taken:
Let
current
be ?
.[[GetOwnProperty]](
).
Let
extensible
be ?
IsExtensible
).
Return
ValidateAndApplyPropertyDescriptor
extensible
Desc
current
).
9.1.6.2
IsCompatiblePropertyDescriptor (
Extensible
Desc
Current
When the abstract operation IsCompatiblePropertyDescriptor is called with Boolean value
Extensible
, and Property Descriptors
Desc
, and
Current
, the following steps are taken:
Return
ValidateAndApplyPropertyDescriptor
undefined
undefined
Extensible
Desc
Current
).
9.1.6.3
ValidateAndApplyPropertyDescriptor (
extensible
Desc
current
When the abstract operation ValidateAndApplyPropertyDescriptor is called with Object
, property key
, Boolean value
extensible
, and Property Descriptors
Desc
, and
current
, the following steps are taken:
Note
If
undefined
is passed as
, only validation is performed and no object updates are performed.
Assert
: If
is not
undefined
, then
IsPropertyKey
) is
true
If
current
is
undefined
, then
If
extensible
is
false
, return
false
Assert
extensible
is
true
If
IsGenericDescriptor
Desc
) is
true
or
IsDataDescriptor
Desc
) is
true
, then
If
is not
undefined
, create an own
data property
named
of object
whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by
Desc
. If the value of an attribute field of
Desc
is absent, the attribute of the newly created property is set to its default value.
Else
Desc
must be an accessor
Property Descriptor
If
is not
undefined
, create an own
accessor property
named
of object
whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by
Desc
. If the value of an attribute field of
Desc
is absent, the attribute of the newly created property is set to its default value.
Return
true
If every field in
Desc
is absent, return
true
If
current
.[[Configurable]] is
false
, then
If
Desc
.[[Configurable]] is present and its value is
true
, return
false
If
Desc
.[[Enumerable]] is present and the [[Enumerable]] fields of
current
and
Desc
are the Boolean negation of each other, return
false
If
IsGenericDescriptor
Desc
) is
true
, no further validation is required.
Else if
IsDataDescriptor
current
) and
IsDataDescriptor
Desc
) have different results, then
If
current
.[[Configurable]] is
false
, return
false
If
IsDataDescriptor
current
) is
true
, then
If
is not
undefined
, convert the property named
of object
from a
data property
to an
accessor property
Preserve the existing values of the converted property's
[[Configurable]] and [[Enumerable]] attributes and set the rest of the
property's attributes to their default values.
Else,
If
is not
undefined
, convert the property named
of object
from an
accessor property
to a
data property
Preserve the existing values of the converted property's
[[Configurable]] and [[Enumerable]] attributes and set the rest of the
property's attributes to their default values.
Else if
IsDataDescriptor
current
) and
IsDataDescriptor
Desc
) are both
true
, then
If
current
.[[Configurable]] is
false
and
current
.[[Writable]] is
false
, then
If
Desc
.[[Writable]] is present and
Desc
.[[Writable]] is
true
, return
false
If
Desc
.[[Value]] is present and
SameValue
Desc
.[[Value]],
current
.[[Value]]) is
false
, return
false
Return
true
Else
IsAccessorDescriptor
current
) and
IsAccessorDescriptor
Desc
) are both
true
If
current
.[[Configurable]] is
false
, then
If
Desc
.[[Set]] is present and
SameValue
Desc
.[[Set]],
current
.[[Set]]) is
false
, return
false
If
Desc
.[[Get]] is present and
SameValue
Desc
.[[Get]],
current
.[[Get]]) is
false
, return
false
Return
true
If
is not
undefined
, then
For each field of
Desc
that is present, set the corresponding attribute of the property named
of object
to the value of the field.
Return
true
9.1.7
[[HasProperty]] (
When the [[HasProperty]] internal method of
is called with property key
, the following steps are taken:
Return ?
OrdinaryHasProperty
).
9.1.7.1
OrdinaryHasProperty (
When the abstract operation OrdinaryHasProperty is called with Object
and with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
hasOwn
be ?
.[[GetOwnProperty]](
).
If
hasOwn
is not
undefined
, return
true
Let
parent
be ?
.[[GetPrototypeOf]]().
If
parent
is not
null
, then
Return ?
parent
.[[HasProperty]](
).
Return
false
9.1.8
[[Get]] (
Receiver
When the [[Get]] internal method of
is called with property key
and
ECMAScript language value
Receiver
, the following steps are taken:
Return ?
OrdinaryGet
Receiver
).
9.1.8.1
OrdinaryGet (
Receiver
When the abstract operation OrdinaryGet is called with Object
, property key
, and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
desc
be ?
.[[GetOwnProperty]](
).
If
desc
is
undefined
, then
Let
parent
be ?
.[[GetPrototypeOf]]().
If
parent
is
null
, return
undefined
Return ?
parent
.[[Get]](
Receiver
).
If
IsDataDescriptor
desc
) is
true
, return
desc
.[[Value]].
Assert
IsAccessorDescriptor
desc
) is
true
Let
getter
be
desc
.[[Get]].
If
getter
is
undefined
, return
undefined
Return ?
Call
getter
Receiver
).
9.1.9
[[Set]] (
Receiver
When the [[Set]] internal method of
is called with property key
, value
, and
ECMAScript language value
Receiver
, the following steps are taken:
Return ?
OrdinarySet
Receiver
).
9.1.9.1
OrdinarySet (
Receiver
When the abstract operation OrdinarySet is called with Object
, property key
, value
, and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
ownDesc
be ?
.[[GetOwnProperty]](
).
Return
OrdinarySetWithOwnDescriptor
Receiver
ownDesc
).
9.1.9.2
OrdinarySetWithOwnDescriptor (
Receiver
ownDesc
When the abstract operation OrdinarySetWithOwnDescriptor is called with Object
, property key
, value
ECMAScript language value
Receiver
, and
Property Descriptor
(or
undefined
ownDesc
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
ownDesc
is
undefined
, then
Let
parent
be ?
.[[GetPrototypeOf]]().
If
parent
is not
null
, then
Return ?
parent
.[[Set]](
Receiver
).
Else,
Set
ownDesc
to the PropertyDescriptor { [[Value]]:
undefined
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
If
IsDataDescriptor
ownDesc
) is
true
, then
If
ownDesc
.[[Writable]] is
false
, return
false
If
Type
Receiver
) is not Object, return
false
Let
existingDescriptor
be ?
Receiver
.[[GetOwnProperty]](
).
If
existingDescriptor
is not
undefined
, then
If
IsAccessorDescriptor
existingDescriptor
) is
true
, return
false
If
existingDescriptor
.[[Writable]] is
false
, return
false
Let
valueDesc
be the PropertyDescriptor { [[Value]]:
}.
Return ?
Receiver
.[[DefineOwnProperty]](
valueDesc
).
Else
Receiver
does not currently have a property
Return ?
CreateDataProperty
Receiver
).
Assert
IsAccessorDescriptor
ownDesc
) is
true
Let
setter
be
ownDesc
.[[Set]].
If
setter
is
undefined
, return
false
Perform ?
Call
setter
Receiver
, «
»).
Return
true
9.1.10
[[Delete]] (
When the [[Delete]] internal method of
is called with property key
, the following steps are taken:
Return ?
OrdinaryDelete
).
9.1.10.1
OrdinaryDelete (
When the abstract operation OrdinaryDelete is called with Object
and property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
desc
be ?
.[[GetOwnProperty]](
).
If
desc
is
undefined
, return
true
If
desc
.[[Configurable]] is
true
, then
Remove the own property with name
from
Return
true
Return
false
9.1.11
[[OwnPropertyKeys]] ( )
When the [[OwnPropertyKeys]] internal method of
is called, the following steps are taken:
Return !
OrdinaryOwnPropertyKeys
).
9.1.11.1
OrdinaryOwnPropertyKeys (
When the abstract operation OrdinaryOwnPropertyKeys is called with Object
, the following steps are taken:
Let
keys
be a new empty
List
For each own property key
of
that is an
array index
, in ascending numeric index order, do
Add
as the last element of
keys
For each own property key
of
that is a String but is not an
array index
, in ascending chronological order of property creation, do
Add
as the last element of
keys
For each own property key
of
that is a Symbol, in ascending chronological order of property creation, do
Add
as the last element of
keys
Return
keys
9.1.12
ObjectCreate (
proto
[ ,
internalSlotsList
] )
The abstract operation ObjectCreate with argument
proto
(an object or null) is used to specify the runtime creation of new ordinary objects. The optional argument
internalSlotsList
is a
List
of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty
List
is used. This abstract operation performs the following steps:
If
internalSlotsList
is not present, set
internalSlotsList
to a new empty
List
Let
obj
be a newly created object with an internal slot for each name in
internalSlotsList
Set
obj
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
obj
.[[Prototype]] to
proto
Set
obj
.[[Extensible]] to
true
Return
obj
9.1.13
OrdinaryCreateFromConstructor (
constructor
intrinsicDefaultProto
[ ,
internalSlotsList
] )
The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]] value is retrieved from a
constructor
's
prototype
property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto
is used for [[Prototype]]. The optional
internalSlotsList
is a
List
of the names of additional internal slots that must be defined as part of the object. If the list is not provided, a new empty
List
is used. This abstract operation performs the following steps:
Assert
intrinsicDefaultProto
is a String value that is this specification's name of an intrinsic
object. The corresponding object must be an intrinsic that is intended
to be used as the [[Prototype]] value of an object.
Let
proto
be ?
GetPrototypeFromConstructor
constructor
intrinsicDefaultProto
).
Return
ObjectCreate
proto
internalSlotsList
).
9.1.14
GetPrototypeFromConstructor (
constructor
intrinsicDefaultProto
The abstract operation GetPrototypeFromConstructor determines
the [[Prototype]] value that should be used to create an object
corresponding to a specific
constructor
. The value is retrieved from the
constructor
's
prototype
property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto
is used for [[Prototype]]. This abstract operation performs the following steps:
Assert
intrinsicDefaultProto
is a String value that is this specification's name of an intrinsic
object. The corresponding object must be an intrinsic that is intended
to be used as the [[Prototype]] value of an object.
Assert
IsCallable
constructor
) is
true
Let
proto
be ?
Get
constructor
"prototype"
).
If
Type
proto
) is not Object, then
Let
realm
be ?
GetFunctionRealm
constructor
).
Set
proto
to
realm
's intrinsic object named
intrinsicDefaultProto
Return
proto
Note
If
constructor
does not supply a [[Prototype]] value, the default value that is used is obtained from the
realm
of the
constructor
function rather than from the
running execution context
9.2
ECMAScript Function Objects
ECMAScript function objects encapsulate parameterized ECMAScript
code closed over a lexical environment and support the dynamic
evaluation of that code. An ECMAScript
function object
is an ordinary object and has the same internal slots and the same
internal methods as other ordinary objects. The code of an ECMAScript
function object
may be either
strict mode code
10.2.1
) or
non-strict code
. An ECMAScript
function object
whose code is
strict mode code
is called a
strict function
. One whose code is not
strict mode code
is called a
non-strict function
ECMAScript function objects have the additional internal slots listed in
Table 27
Table 27: Internal Slots of ECMAScript Function Objects
Internal Slot
Type
Description
[[Environment]]
Lexical Environment
The
Lexical Environment
that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[FormalParameters]]
Parse Node
The root parse node of the source text that defines the function's formal parameter list.
[[FunctionKind]]
String
Either
"normal"
"classConstructor"
"generator"
"async"
, or
"async generator"
[[ECMAScriptCode]]
Parse Node
The root parse node of the source text that defines the function's body.
[[ConstructorKind]]
String
Either
"base"
or
"derived"
[[Realm]]
Realm Record
The
realm
in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]]
Script Record
or
Module Record
The script or module in which the function was created.
[[ThisMode]]
(lexical, strict, global)
Defines how
this
references are interpreted within the formal parameters and code body of the function.
lexical
means that
this
refers to the
this
value of a lexically enclosing function.
strict
means that the
this
value is used exactly as provided by an invocation of the function.
global
means that a
this
value of
undefined
is interpreted as a reference to the
global object
[[Strict]]
Boolean
true
if this is a
strict function
false
if this is a
non-strict function
[[HomeObject]]
Object
If the function uses
super
, this is the object whose [[GetPrototypeOf]] provides the object where
super
property lookups begin.
[[SourceText]]
String
The
source text
that defines the function.
All ECMAScript function objects have the [[Call]] internal method
defined here. ECMAScript functions that are also constructors in
addition have the [[Construct]] internal method.
9.2.1
[[Call]] (
thisArgument
argumentsList
The [[Call]] internal method for an ECMAScript
function object
is called with parameters
thisArgument
and
argumentsList
, a
List
of ECMAScript language values. The following steps are taken:
Assert
is an ECMAScript
function object
If
.[[FunctionKind]] is
"classConstructor"
, throw a
TypeError
exception.
Let
callerContext
be the
running execution context
Let
calleeContext
be
PrepareForOrdinaryCall
undefined
).
Assert
calleeContext
is now the
running execution context
Perform
OrdinaryCallBindThis
calleeContext
thisArgument
).
Let
result
be
OrdinaryCallEvaluateBody
argumentsList
).
Remove
calleeContext
from the
execution context stack
and restore
callerContext
as the
running execution context
If
result
.[[Type]] is
return
, return
NormalCompletion
result
.[[Value]]).
ReturnIfAbrupt
result
).
Return
NormalCompletion
undefined
).
Note
When
calleeContext
is removed from the
execution context stack
in step 8 it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.
9.2.1.1
PrepareForOrdinaryCall (
newTarget
When the abstract operation PrepareForOrdinaryCall is called with
function object
and
ECMAScript language value
newTarget
, the following steps are taken:
Assert
Type
newTarget
) is Undefined or Object.
Let
callerContext
be the
running execution context
Let
calleeContext
be a new ECMAScript code
execution context
Set the Function of
calleeContext
to
Let
calleeRealm
be
.[[Realm]].
Set the
Realm
of
calleeContext
to
calleeRealm
Set the ScriptOrModule of
calleeContext
to
.[[ScriptOrModule]].
Let
localEnv
be
NewFunctionEnvironment
newTarget
).
Set the LexicalEnvironment of
calleeContext
to
localEnv
Set the VariableEnvironment of
calleeContext
to
localEnv
If
callerContext
is not already suspended, suspend
callerContext
Push
calleeContext
onto the
execution context stack
calleeContext
is now the
running execution context
NOTE: Any exception objects produced after this point are associated with
calleeRealm
Return
calleeContext
9.2.1.2
OrdinaryCallBindThis (
calleeContext
thisArgument
When the abstract operation OrdinaryCallBindThis is called with
function object
execution context
calleeContext
, and ECMAScript value
thisArgument
, the following steps are taken:
Let
thisMode
be
.[[ThisMode]].
If
thisMode
is
lexical
, return
NormalCompletion
undefined
).
Let
calleeRealm
be
.[[Realm]].
Let
localEnv
be the LexicalEnvironment of
calleeContext
If
thisMode
is
strict
, let
thisValue
be
thisArgument
Else,
If
thisArgument
is
undefined
or
null
, then
Let
globalEnv
be
calleeRealm
.[[GlobalEnv]].
Let
globalEnvRec
be
globalEnv
's
EnvironmentRecord
Assert
globalEnvRec
is a global
Environment Record
Let
thisValue
be
globalEnvRec
.[[GlobalThisValue]].
Else,
Let
thisValue
be !
ToObject
thisArgument
).
NOTE:
ToObject
produces wrapper objects using
calleeRealm
Let
envRec
be
localEnv
's
EnvironmentRecord
Assert
envRec
is a
function Environment Record
Assert
: The next step never returns an
abrupt completion
because
envRec
.[[ThisBindingStatus]] is not
"initialized"
Return
envRec
.BindThisValue(
thisValue
).
9.2.1.3
OrdinaryCallEvaluateBody (
argumentsList
When the abstract operation OrdinaryCallEvaluateBody is called with
function object
and
List
argumentsList
, the following steps are taken:
Return the result of EvaluateBody of the parsed code that is
.[[ECMAScriptCode]] passing
and
argumentsList
as the arguments.
9.2.2
[[Construct]] (
argumentsList
newTarget
The [[Construct]] internal method for an ECMAScript
function object
is called with parameters
argumentsList
and
newTarget
argumentsList
is a possibly empty
List
of ECMAScript language values. The following steps are taken:
Assert
is an ECMAScript
function object
Assert
Type
newTarget
) is Object.
Let
callerContext
be the
running execution context
Let
kind
be
.[[ConstructorKind]].
If
kind
is
"base"
, then
Let
thisArgument
be ?
OrdinaryCreateFromConstructor
newTarget
"%ObjectPrototype%"
).
Let
calleeContext
be
PrepareForOrdinaryCall
newTarget
).
Assert
calleeContext
is now the
running execution context
If
kind
is
"base"
, perform
OrdinaryCallBindThis
calleeContext
thisArgument
).
Let
constructorEnv
be the LexicalEnvironment of
calleeContext
Let
envRec
be
constructorEnv
's
EnvironmentRecord
Let
result
be
OrdinaryCallEvaluateBody
argumentsList
).
Remove
calleeContext
from the
execution context stack
and restore
callerContext
as the
running execution context
If
result
.[[Type]] is
return
, then
If
Type
result
.[[Value]]) is Object, return
NormalCompletion
result
.[[Value]]).
If
kind
is
"base"
, return
NormalCompletion
thisArgument
).
If
result
.[[Value]] is not
undefined
, throw a
TypeError
exception.
Else,
ReturnIfAbrupt
result
).
Return ?
envRec
.GetThisBinding().
9.2.3
FunctionAllocate (
functionPrototype
strict
functionKind
The abstract operation FunctionAllocate requires the three arguments
functionPrototype
strict
and
functionKind
. FunctionAllocate performs the following steps:
Assert
Type
functionPrototype
) is Object.
Assert
functionKind
is either
"normal"
"non-constructor"
"generator"
"async"
, or
"async generator"
If
functionKind
is
"normal"
, let
needsConstruct
be
true
Else, let
needsConstruct
be
false
If
functionKind
is
"non-constructor"
, set
functionKind
to
"normal"
Let
be a newly created ECMAScript
function object
with the internal slots listed in
Table 27
. All of those internal slots are initialized to
undefined
Set
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
.[[Call]] to the definition specified in
9.2.1
If
needsConstruct
is
true
, then
Set
.[[Construct]] to the definition specified in
9.2.2
Set
.[[ConstructorKind]] to
"base"
Set
.[[Strict]] to
strict
Set
.[[FunctionKind]] to
functionKind
Set
.[[Prototype]] to
functionPrototype
Set
.[[Extensible]] to
true
Set
.[[Realm]] to
the current Realm Record
Return
9.2.4
FunctionInitialize (
kind
ParameterList
Body
Scope
The abstract operation FunctionInitialize requires the arguments: a
function object
kind
which is one of (Normal, Method, Arrow), a parameter list
Parse Node
specified by
ParameterList
, a body
Parse Node
specified by
Body
, a
Lexical Environment
specified by
Scope
. FunctionInitialize performs the following steps:
Let
len
be the ExpectedArgumentCount of
ParameterList
Perform !
SetFunctionLength
len
).
Let
Strict
be
.[[Strict]].
Set
.[[Environment]] to
Scope
Set
.[[FormalParameters]] to
ParameterList
Set
.[[ECMAScriptCode]] to
Body
Set
.[[ScriptOrModule]] to
GetActiveScriptOrModule
().
If
kind
is
Arrow
, set
.[[ThisMode]] to
lexical
Else if
Strict
is
true
, set
.[[ThisMode]] to
strict
Else, set
.[[ThisMode]] to
global
Return
9.2.5
FunctionCreate (
kind
ParameterList
Body
Scope
Strict
[ ,
prototype
] )
The abstract operation FunctionCreate requires the arguments:
kind
which is one of (Normal, Method, Arrow), a parameter list
Parse Node
specified by
ParameterList
, a body
Parse Node
specified by
Body
, a
Lexical Environment
specified by
Scope
, a Boolean flag
Strict
, and optionally, an object
prototype
. FunctionCreate performs the following steps:
If
prototype
is not present, then
Set
prototype
to the intrinsic object
%FunctionPrototype%
If
kind
is not
Normal
, let
allocKind
be
"non-constructor"
Else, let
allocKind
be
"normal"
Let
be
FunctionAllocate
prototype
Strict
allocKind
).
Return
FunctionInitialize
kind
ParameterList
Body
Scope
).
9.2.6
GeneratorFunctionCreate (
kind
ParameterList
Body
Scope
Strict
The abstract operation GeneratorFunctionCreate requires the arguments:
kind
which is one of (Normal, Method), a parameter list
Parse Node
specified by
ParameterList
, a body
Parse Node
specified by
Body
, a
Lexical Environment
specified by
Scope
, and a Boolean flag
Strict
. GeneratorFunctionCreate performs the following steps:
Let
functionPrototype
be the intrinsic object
%Generator%
Let
be
FunctionAllocate
functionPrototype
Strict
"generator"
).
Return
FunctionInitialize
kind
ParameterList
Body
Scope
).
9.2.7
AsyncGeneratorFunctionCreate (
kind
ParameterList
Body
Scope
Strict
The abstract operation AsyncGeneratorFunctionCreate requires the arguments:
kind
which is one of (
Normal
Method
), a parameter list
Parse Node
specified by
ParameterList
, a body
Parse Node
specified by
Body
, a
Lexical Environment
specified by
Scope
, and a Boolean flag
Strict
. AsyncGeneratorFunctionCreate performs the following steps:
Let
functionPrototype
be the intrinsic object
%AsyncGenerator%
Let
be !
FunctionAllocate
functionPrototype
Strict
"generator"
).
Return !
FunctionInitialize
kind
ParameterList
Body
Scope
).
9.2.8
AsyncFunctionCreate (
kind
parameters
body
Scope
Strict
The abstract operation AsyncFunctionCreate requires the arguments:
kind
which is one of (
Normal
Method
Arrow
), a parameter list
Parse Node
specified by
parameters
, a body
Parse Node
specified by
body
, a
Lexical Environment
specified by
Scope
, and a Boolean flag
Strict
. AsyncFunctionCreate performs the following steps:
Let
functionPrototype
be the intrinsic object
%AsyncFunctionPrototype%
Let
be !
FunctionAllocate
functionPrototype
Strict
"async"
).
Return !
FunctionInitialize
kind
parameters
body
Scope
).
9.2.9
AddRestrictedFunctionProperties (
realm
The abstract operation AddRestrictedFunctionProperties is called with a
function object
and
Realm Record
realm
as its argument. It performs the following steps:
Assert
realm
.[[Intrinsics]].[[
%ThrowTypeError%
]] exists and has been initialized.
Let
thrower
be
realm
.[[Intrinsics]].[[
%ThrowTypeError%
]].
Perform !
DefinePropertyOrThrow
"caller"
, PropertyDescriptor { [[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Return !
DefinePropertyOrThrow
"arguments"
, PropertyDescriptor { [[Get]]:
thrower
, [[Set]]:
thrower
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
9.2.9.1
%ThrowTypeError% ( )
The
%ThrowTypeError%
intrinsic is an anonymous built-in
function object
that is defined once for each
realm
. When %ThrowTypeError% is called it performs the following steps:
Throw a
TypeError
exception.
The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is
false
The
"length"
property of a %ThrowTypeError% function has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
9.2.10
MakeConstructor (
[ ,
writablePrototype
[ ,
prototype
] ] )
The abstract operation MakeConstructor requires a Function argument
and optionally, a Boolean
writablePrototype
and an object
prototype
. If
prototype
is provided it is assumed to already contain, if needed, a
"constructor"
property whose value is
. This operation converts
into a
constructor
by performing the following steps:
Assert
is an ECMAScript
function object
Assert
IsConstructor
) is
true
Assert
is an extensible object that does not have a
prototype
own property.
If
writablePrototype
is not present, set
writablePrototype
to
true
If
prototype
is not present, then
Set
prototype
to
ObjectCreate
%ObjectPrototype%
).
Perform !
DefinePropertyOrThrow
prototype
"constructor"
, PropertyDescriptor { [[Value]]:
, [[Writable]]:
writablePrototype
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Perform !
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
writablePrototype
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Return
NormalCompletion
undefined
).
9.2.11
MakeClassConstructor (
The abstract operation MakeClassConstructor with argument
performs the following steps:
Assert
is an ECMAScript
function object
Assert
.[[FunctionKind]] is
"normal"
Set
.[[FunctionKind]] to
"classConstructor"
Return
NormalCompletion
undefined
).
9.2.12
MakeMethod (
homeObject
The abstract operation MakeMethod with arguments
and
homeObject
configures
as a method by performing the following steps:
Assert
is an ECMAScript
function object
Assert
Type
homeObject
) is Object.
Set
.[[HomeObject]] to
homeObject
Return
NormalCompletion
undefined
).
9.2.13
SetFunctionName (
name
[ ,
prefix
] )
The abstract operation SetFunctionName requires a Function argument
, a String or Symbol argument
name
and optionally a String argument
prefix
. This operation adds a
name
property to
by performing the following steps:
Assert
is an extensible object that does not have a
name
own property.
Assert
Type
name
) is either Symbol or String.
Assert
: If
prefix
is present, then
Type
prefix
) is String.
If
Type
name
) is Symbol, then
Let
description
be
name
's [[Description]] value.
If
description
is
undefined
, set
name
to the empty String.
Else, set
name
to the
string-concatenation
of
"["
description
, and
"]"
If
prefix
is present, then
Set
name
to the
string-concatenation
of
prefix
, the code unit 0x0020 (SPACE), and
name
Return !
DefinePropertyOrThrow
"name"
, PropertyDescriptor { [[Value]]:
name
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
9.2.14
SetFunctionLength (
length
The abstract operation SetFunctionLength requires a Function argument
and a Number argument
length
. This operation adds a
"length"
property to
by performing the following steps:
Assert
is an extensible object that does not have a
"length"
own property.
Assert
Type
length
) is Number.
Assert
length
≥ 0 and !
ToInteger
length
) is equal to
length
Return !
DefinePropertyOrThrow
"length"
, PropertyDescriptor { [[Value]]:
length
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
9.2.15
FunctionDeclarationInstantiation (
func
argumentsList
Note 1
When an
execution context
is established for evaluating an ECMAScript function a new
function Environment Record
is created and bindings for each formal parameter are instantiated in that
Environment Record
Each declaration in the function body is also instantiated. If the
function's formal parameters do not include any default value
initializers then the body declarations are instantiated in the same
Environment Record
as the parameters. If default value parameter initializers exist, a second
Environment Record
is created for the body declarations. Formal parameters and functions
are initialized as part of FunctionDeclarationInstantiation. All other
bindings are initialized during evaluation of the function body.
FunctionDeclarationInstantiation is performed as follows using arguments
func
and
argumentsList
func
is the
function object
for which the
execution context
is being established.
Let
calleeContext
be the
running execution context
Let
env
be the LexicalEnvironment of
calleeContext
Let
envRec
be
env
's
EnvironmentRecord
Let
code
be
func
.[[ECMAScriptCode]].
Let
strict
be
func
.[[Strict]].
Let
formals
be
func
.[[FormalParameters]].
Let
parameterNames
be the BoundNames of
formals
If
parameterNames
has any duplicate entries, let
hasDuplicates
be
true
. Otherwise, let
hasDuplicates
be
false
Let
simpleParameterList
be IsSimpleParameterList of
formals
Let
hasParameterExpressions
be ContainsExpression of
formals
Let
varNames
be the VarDeclaredNames of
code
Let
varDeclarations
be the VarScopedDeclarations of
code
Let
lexicalNames
be the LexicallyDeclaredNames of
code
Let
functionNames
be a new empty
List
Let
functionsToInitialize
be a new empty
List
For each
in
varDeclarations
, in reverse list order, do
If
is neither a
VariableDeclaration
nor a
ForBinding
nor a
BindingIdentifier
, then
Assert
is either a
FunctionDeclaration
, a
GeneratorDeclaration
, an
AsyncFunctionDeclaration
, or an
AsyncGeneratorDeclaration
Let
fn
be the sole element of the BoundNames of
If
fn
is not an element of
functionNames
, then
Insert
fn
as the first element of
functionNames
NOTE: If there are multiple function declarations for the same name, the last declaration is used.
Insert
as the first element of
functionsToInitialize
Let
argumentsObjectNeeded
be
true
If
func
.[[ThisMode]] is
lexical
, then
NOTE: Arrow functions never have an arguments objects.
Set
argumentsObjectNeeded
to
false
Else if
"arguments"
is an element of
parameterNames
, then
Set
argumentsObjectNeeded
to
false
Else if
hasParameterExpressions
is
false
, then
If
"arguments"
is an element of
functionNames
or if
"arguments"
is an element of
lexicalNames
, then
Set
argumentsObjectNeeded
to
false
For each String
paramName
in
parameterNames
, do
Let
alreadyDeclared
be
envRec
.HasBinding(
paramName
).
NOTE:
Early errors ensure that duplicate parameter names can only occur in
non-strict functions that do not have parameter default values or rest
parameters.
If
alreadyDeclared
is
false
, then
Perform !
envRec
.CreateMutableBinding(
paramName
false
).
If
hasDuplicates
is
true
, then
Perform !
envRec
.InitializeBinding(
paramName
undefined
).
If
argumentsObjectNeeded
is
true
, then
If
strict
is
true
or if
simpleParameterList
is
false
, then
Let
ao
be
CreateUnmappedArgumentsObject
argumentsList
).
Else,
NOTE:
mapped argument object is only provided for non-strict functions that
don't have a rest parameter, any parameter default value initializers,
or any destructured parameters.
Let
ao
be
CreateMappedArgumentsObject
func
formals
argumentsList
envRec
).
If
strict
is
true
, then
Perform !
envRec
.CreateImmutableBinding(
"arguments"
false
).
Else,
Perform !
envRec
.CreateMutableBinding(
"arguments"
false
).
Call
envRec
.InitializeBinding(
"arguments"
ao
).
Let
parameterBindings
be a new
List
of
parameterNames
with
"arguments"
appended.
Else,
Let
parameterBindings
be
parameterNames
Let
iteratorRecord
be
CreateListIteratorRecord
argumentsList
).
If
hasDuplicates
is
true
, then
Perform ? IteratorBindingInitialization for
formals
with
iteratorRecord
and
undefined
as arguments.
Else,
Perform ? IteratorBindingInitialization for
formals
with
iteratorRecord
and
env
as arguments.
If
hasParameterExpressions
is
false
, then
NOTE: Only a single lexical environment is needed for the parameters and top-level vars.
Let
instantiatedVarNames
be a copy of the
List
parameterBindings
For each
in
varNames
, do
If
is not an element of
instantiatedVarNames
, then
Append
to
instantiatedVarNames
Perform !
envRec
.CreateMutableBinding(
false
).
Call
envRec
.InitializeBinding(
undefined
).
Let
varEnv
be
env
Let
varEnvRec
be
envRec
Else,
NOTE: A separate
Environment Record
is needed to ensure that closures created by expressions in the formal
parameter list do not have visibility of declarations in the function
body.
Let
varEnv
be
NewDeclarativeEnvironment
env
).
Let
varEnvRec
be
varEnv
's
EnvironmentRecord
Set the VariableEnvironment of
calleeContext
to
varEnv
Let
instantiatedVarNames
be a new empty
List
For each
in
varNames
, do
If
is not an element of
instantiatedVarNames
, then
Append
to
instantiatedVarNames
Perform !
varEnvRec
.CreateMutableBinding(
false
).
If
is not an element of
parameterBindings
or if
is an element of
functionNames
, let
initialValue
be
undefined
Else,
Let
initialValue
be !
envRec
.GetBindingValue(
false
).
Call
varEnvRec
.InitializeBinding(
initialValue
).
NOTE:
vars whose names are the same as a formal parameter, initially have the
same value as the corresponding initialized parameter.
NOTE: Annex
B.3.3.1
adds additional steps at this point.
If
strict
is
false
, then
Let
lexEnv
be
NewDeclarativeEnvironment
varEnv
).
NOTE: Non-strict functions use a separate lexical
Environment Record
for top-level lexical declarations so that a
direct eval
can determine whether any var scoped declarations introduced by the
eval code conflict with pre-existing top-level lexically scoped
declarations. This is not needed for strict functions because a strict
direct eval
always places all declarations into a new
Environment Record
Else, let
lexEnv
be
varEnv
Let
lexEnvRec
be
lexEnv
's
EnvironmentRecord
Set the LexicalEnvironment of
calleeContext
to
lexEnv
Let
lexDeclarations
be the LexicallyScopedDeclarations of
code
For each element
in
lexDeclarations
, do
NOTE:
A lexically declared name cannot be the same as a function/generator
declaration, formal parameter, or a var name. Lexically declared names
are only instantiated here but not initialized.
For each element
dn
of the BoundNames of
, do
If IsConstantDeclaration of
is
true
, then
Perform !
lexEnvRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform !
lexEnvRec
.CreateMutableBinding(
dn
false
).
For each
Parse Node
in
functionsToInitialize
, do
Let
fn
be the sole element of the BoundNames of
Let
fo
be the result of performing InstantiateFunctionObject for
with argument
lexEnv
Perform !
varEnvRec
.SetMutableBinding(
fn
fo
false
).
Return
NormalCompletion
empty
).
Note 2
B.3.3
provides an extension to the above algorithm that is necessary for
backwards compatibility with web browser implementations of ECMAScript
that predate ECMAScript 2015.
Note 3
Parameter
Initializer
s may contain
direct eval
expressions. Any top level declarations of such evals are only visible to the eval code (
10.2
). The creation of the environment for such declarations is described in
14.1.19
9.3
Built-in Function Objects
The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (
9.2
whose behaviour is provided using ECMAScript code or as implementation
provided function exotic objects whose behaviour is provided in some
other manner. In either case, the effect of calling such functions must
conform to their specifications. An implementation may also provide
additional built-in function objects that are not defined in this
specification.
If a built-in
function object
is implemented as an
exotic object
it must have the ordinary object behaviour specified in
9.1
. All such function exotic objects also have [[Prototype]], [[Extensible]], [[Realm]], and [[ScriptOrModule]] internal slots.
Unless otherwise specified every built-in
function object
has the
%FunctionPrototype%
object as the initial value of its [[Prototype]] internal slot.
The behaviour specified for each built-in function via algorithm
steps or other means is the specification of the function body behaviour
for both [[Call]] and [[Construct]] invocations of the function.
However, [[Construct]] invocation is not supported by all built-in
functions. For each built-in function, when invoked with [[Call]], the
[[Call]]
thisArgument
provides the
this
value, the [[Call]]
argumentsList
provides the named parameters, and the NewTarget value is
undefined
. When invoked with [[Construct]], the
this
value is uninitialized, the [[Construct]]
argumentsList
provides the named parameters, and the [[Construct]]
newTarget
parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript
function object
then this specified behaviour must be implemented by the ECMAScript
code that is the body of the function. Built-in functions that are
ECMAScript function objects must be strict functions. If a built-in
constructor
has any [[Call]] behaviour other than throwing a
TypeError
exception, an ECMAScript implementation of the function must be done in
a manner that does not cause the function's [[FunctionKind]] internal
slot to have the value
"classConstructor"
Built-in function objects that are not identified as constructors
do not implement the [[Construct]] internal method unless otherwise
specified in the description of a particular function. When a built-in
constructor
is called as part of a
new
expression the
argumentsList
parameter of the invoked [[Construct]] internal method provides the values for the built-in
constructor
's named parameters.
Built-in functions that are not constructors do not have a
prototype
property unless otherwise specified in the description of a particular function.
If a built-in
function object
is not implemented as an ECMAScript function it must provide [[Call]]
and [[Construct]] internal methods that conform to the following
definitions:
9.3.1
[[Call]] (
thisArgument
argumentsList
The [[Call]] internal method for a built-in
function object
is called with parameters
thisArgument
and
argumentsList
, a
List
of ECMAScript language values. The following steps are taken:
Let
callerContext
be the
running execution context
If
callerContext
is not already suspended, suspend
callerContext
Let
calleeContext
be a new ECMAScript code
execution context
Set the Function of
calleeContext
to
Let
calleeRealm
be
.[[Realm]].
Set the
Realm
of
calleeContext
to
calleeRealm
Set the ScriptOrModule of
calleeContext
to
.[[ScriptOrModule]].
Perform any necessary implementation-defined initialization of
calleeContext
Push
calleeContext
onto the
execution context stack
calleeContext
is now the
running execution context
Let
result
be the
Completion Record
that is the result of evaluating
in an implementation-defined manner that conforms to the specification of
thisArgument
is the
this
value,
argumentsList
provides the named parameters, and the NewTarget value is
undefined
Remove
calleeContext
from the
execution context stack
and restore
callerContext
as the
running execution context
Return
result
Note
When
calleeContext
is removed from the
execution context stack
it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.
9.3.2
[[Construct]] (
argumentsList
newTarget
The [[Construct]] internal method for built-in
function object
is called with parameters
argumentsList
and
newTarget
. The steps performed are the same as [[Call]] (see
9.3.1
) except that step 10 is replaced by:
Let
result
be the
Completion Record
that is the result of evaluating
in an implementation-defined manner that conforms to the specification of
. The
this
value is uninitialized,
argumentsList
provides the named parameters, and
newTarget
provides the NewTarget value.
9.3.3
CreateBuiltinFunction (
steps
internalSlotsList
[ ,
realm
[ ,
prototype
] ] )
The abstract operation CreateBuiltinFunction takes arguments
steps
internalSlotsList
realm
, and
prototype
. The argument
internalSlotsList
is a
List
of the names of additional internal slots that must be defined as part of the object. CreateBuiltinFunction returns a built-in
function object
created by the following steps:
Assert
steps
is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
If
realm
is not present, set
realm
to
the current Realm Record
Assert
realm
is a
Realm Record
If
prototype
is not present, set
prototype
to
realm
.[[Intrinsics]].[[
%FunctionPrototype%
]].
Let
func
be a new built-in
function object
that when called performs the action described by
steps
. The new
function object
has internal slots whose names are the elements of
internalSlotsList
. The initial value of each of those internal slots is
undefined
Set
func
.[[Realm]] to
realm
Set
func
.[[Prototype]] to
prototype
Set
func
.[[Extensible]] to
true
Set
func
.[[ScriptOrModule]] to
null
Return
func
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.
9.4
Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic
objects. These objects generally behave similar to ordinary objects
except for a few specific situations. The following exotic objects use
the ordinary object internal methods except where it is explicitly
specified otherwise below:
9.4.1
Bound Function Exotic Objects
bound function
is an
exotic object
that wraps another
function object
A bound function is callable (it has a [[Call]] internal method and may
have a [[Construct]] internal method). Calling a bound function
generally results in a call of its wrapped function.
Bound function objects do not have the internal slots of ECMAScript function objects defined in
Table 27
. Instead they have the internal slots defined in
Table 28
Table 28: Internal Slots of Bound Function Exotic Objects
Internal Slot
Type
Description
[[BoundTargetFunction]]
Callable Object
The wrapped
function object
[[BoundThis]]
Any
The value that is always passed as the
this
value when calling the wrapped function.
[[BoundArguments]]
List
of Any
A list of values whose elements are used as the first arguments to any call to the wrapped function.
Bound function objects provide all of the essential internal methods as specified in
9.1
. However, they use the following definitions for the essential internal methods of function objects.
9.4.1.1
[[Call]] (
thisArgument
argumentsList
When the [[Call]] internal method of a
bound function
exotic object
, which was created using the bind function is called with parameters
thisArgument
and
argumentsList
, a
List
of ECMAScript language values, the following steps are taken:
Let
target
be
.[[BoundTargetFunction]].
Let
boundThis
be
.[[BoundThis]].
Let
boundArgs
be
.[[BoundArguments]].
Let
args
be a new list containing the same values as the list
boundArgs
in the same order followed by the same values as the list
argumentsList
in the same order.
Return ?
Call
target
boundThis
args
).
9.4.1.2
[[Construct]] (
argumentsList
newTarget
When the [[Construct]] internal method of a
bound function
exotic object
that was created using the bind function is called with a list of arguments
argumentsList
and
newTarget
, the following steps are taken:
Let
target
be
.[[BoundTargetFunction]].
Assert
IsConstructor
target
) is
true
Let
boundArgs
be
.[[BoundArguments]].
Let
args
be a new list containing the same values as the list
boundArgs
in the same order followed by the same values as the list
argumentsList
in the same order.
If
SameValue
newTarget
) is
true
, set
newTarget
to
target
Return ?
Construct
target
args
newTarget
).
9.4.1.3
BoundFunctionCreate (
targetFunction
boundThis
boundArgs
The abstract operation BoundFunctionCreate with arguments
targetFunction
boundThis
and
boundArgs
is used to specify the creation of new Bound Function exotic objects. It performs the following steps:
Assert
Type
targetFunction
) is Object.
Let
proto
be ?
targetFunction
.[[GetPrototypeOf]]().
Let
obj
be a newly created object.
Set
obj
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
obj
.[[Call]] as described in
9.4.1.1
If
IsConstructor
targetFunction
) is
true
, then
Set
obj
.[[Construct]] as described in
9.4.1.2
Set
obj
.[[Prototype]] to
proto
Set
obj
.[[Extensible]] to
true
Set
obj
.[[BoundTargetFunction]] to
targetFunction
Set
obj
.[[BoundThis]] to
boundThis
Set
obj
.[[BoundArguments]] to
boundArgs
Return
obj
9.4.2
Array Exotic Objects
An
Array object
is an
exotic object
that gives special treatment to
array index
property keys (see
6.1.7
). A property whose
property name
is an
array index
is also called an
element
. Every Array object has a non-configurable
"length"
property whose value is always a nonnegative integer less than 2
32
. The value of the
"length"
property is numerically greater than the name of every own property whose name is an
array index
whenever an own property of an Array object is created or changed,
other properties are adjusted as necessary to maintain this invariant.
Specifically, whenever an own property is added whose name is an
array index
, the value of the
"length"
property is changed, if necessary, to be one more than the numeric value of that
array index
; and whenever the value of the
"length"
property is changed, every own property whose name is an
array index
whose value is not smaller than the new length is deleted. This
constraint applies only to own properties of an Array object and is
unaffected by
"length"
or
array index
properties that may be inherited from its prototypes.
Note
A String
property name
is an
array index
if and only if
ToString
ToUint32
)) is equal to
and
ToUint32
) is not equal to
32
- 1
Array exotic objects provide an alternative definition for the
[[DefineOwnProperty]] internal method. Except for that internal method,
Array exotic objects provide all of the other essential internal methods
as specified in
9.1
9.4.2.1
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of an Array
exotic object
is called with property key
, and
Property Descriptor
Desc
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
is
"length"
, then
Return ?
ArraySetLength
Desc
).
Else if
is an
array index
, then
Let
oldLenDesc
be
OrdinaryGetOwnProperty
"length"
).
Assert
oldLenDesc
will never be
undefined
or an accessor descriptor because Array objects are created with a length
data property
that cannot be deleted or reconfigured.
Let
oldLen
be
oldLenDesc
.[[Value]].
Let
index
be !
ToUint32
).
If
index
oldLen
and
oldLenDesc
.[[Writable]] is
false
, return
false
Let
succeeded
be !
OrdinaryDefineOwnProperty
Desc
).
If
succeeded
is
false
, return
false
If
index
oldLen
, then
Set
oldLenDesc
.[[Value]] to
index
+ 1.
Let
succeeded
be
OrdinaryDefineOwnProperty
"length"
oldLenDesc
).
Assert
succeeded
is
true
Return
true
Return
OrdinaryDefineOwnProperty
Desc
).
9.4.2.2
ArrayCreate (
length
[ ,
proto
] )
The abstract operation ArrayCreate with argument
length
(either 0 or a positive integer) and optional argument
proto
is used to specify the creation of new Array exotic objects. It performs the following steps:
Assert
length
is an integer Number ≥ 0.
If
length
is
-0
, set
length
to
+0
If
length
> 2
32
- 1, throw a
RangeError
exception.
If
proto
is not present, set
proto
to the intrinsic object
%ArrayPrototype%
Let
be a newly created Array
exotic object
Set
's essential internal methods except for [[DefineOwnProperty]] to the default ordinary object definitions specified in
9.1
Set
.[[DefineOwnProperty]] as specified in
9.4.2.1
Set
.[[Prototype]] to
proto
Set
.[[Extensible]] to
true
Perform !
OrdinaryDefineOwnProperty
"length"
, PropertyDescriptor { [[Value]]:
length
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Return
9.4.2.3
ArraySpeciesCreate (
originalArray
length
The abstract operation ArraySpeciesCreate with arguments
originalArray
and
length
is used to specify the creation of a new Array object using a
constructor
function that is derived from
originalArray
. It performs the following steps:
Assert
length
is an integer Number ≥ 0.
If
length
is
-0
, set
length
to
+0
Let
isArray
be ?
IsArray
originalArray
).
If
isArray
is
false
, return ?
ArrayCreate
length
).
Let
be ?
Get
originalArray
"constructor"
).
If
IsConstructor
) is
true
, then
Let
thisRealm
be
the current Realm Record
Let
realmC
be ?
GetFunctionRealm
).
If
thisRealm
and
realmC
are not the same
Realm Record
, then
If
SameValue
realmC
.[[Intrinsics]].[[
%Array%
]]) is
true
, set
to
undefined
If
Type
) is Object, then
Set
to ?
Get
, @@species).
If
is
null
, set
to
undefined
If
is
undefined
, return ?
ArrayCreate
length
).
If
IsConstructor
) is
false
, throw a
TypeError
exception.
Return ?
Construct
, «
length
»).
Note
If
originalArray
was created using the standard built-in Array
constructor
for a
realm
that is not the
realm
of the
running execution context
, then a new Array is created using the
realm
of the
running execution context
This maintains compatibility with Web browsers that have historically
had that behaviour for the Array.prototype methods that now are defined
using ArraySpeciesCreate.
9.4.2.4
ArraySetLength (
Desc
When the abstract operation ArraySetLength is called with an Array
exotic object
, and
Property Descriptor
Desc
, the following steps are taken:
If
Desc
.[[Value]] is absent, then
Return
OrdinaryDefineOwnProperty
"length"
Desc
).
Let
newLenDesc
be a copy of
Desc
Let
newLen
be ?
ToUint32
Desc
.[[Value]]).
Let
numberLen
be ?
ToNumber
Desc
.[[Value]]).
If
newLen
numberLen
, throw a
RangeError
exception.
Set
newLenDesc
.[[Value]] to
newLen
Let
oldLenDesc
be
OrdinaryGetOwnProperty
"length"
).
Assert
oldLenDesc
will never be
undefined
or an accessor descriptor because Array objects are created with a length
data property
that cannot be deleted or reconfigured.
Let
oldLen
be
oldLenDesc
.[[Value]].
If
newLen
oldLen
, then
Return
OrdinaryDefineOwnProperty
"length"
newLenDesc
).
If
oldLenDesc
.[[Writable]] is
false
, return
false
If
newLenDesc
.[[Writable]] is absent or has the value
true
, let
newWritable
be
true
Else,
Need to defer setting the [[Writable]] attribute to
false
in case any elements cannot be deleted.
Let
newWritable
be
false
Set
newLenDesc
.[[Writable]] to
true
Let
succeeded
be !
OrdinaryDefineOwnProperty
"length"
newLenDesc
).
If
succeeded
is
false
, return
false
Repeat, while
newLen
oldLen
Decrease
oldLen
by 1.
Let
deleteSucceeded
be !
.[[Delete]](!
ToString
oldLen
)).
If
deleteSucceeded
is
false
, then
Set
newLenDesc
.[[Value]] to
oldLen
+ 1.
If
newWritable
is
false
, set
newLenDesc
.[[Writable]] to
false
Perform !
OrdinaryDefineOwnProperty
"length"
newLenDesc
).
Return
false
If
newWritable
is
false
, then
Return
OrdinaryDefineOwnProperty
"length"
, PropertyDescriptor { [[Writable]]:
false
}). This call will always return
true
Return
true
Note
In steps 3 and 4, if
Desc
.[[Value]] is an object then its
valueOf
method is called twice. This is legacy behaviour that was specified with this effect starting with the 2
nd
Edition of this specification.
9.4.3
String Exotic Objects
String object
is an
exotic object
that encapsulates a String value and exposes virtual integer-indexed
data properties corresponding to the individual code unit elements of
the String value. String exotic objects always have a
data property
named
"length"
whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the
"length"
property are non-writable and non-configurable.
String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.
String exotic objects provide alternative definitions for the following internal methods. All of the other String
exotic object
essential internal methods that are not defined below are as specified in
9.1
9.4.3.1
[[GetOwnProperty]] (
When the [[GetOwnProperty]] internal method of a String
exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
desc
be
OrdinaryGetOwnProperty
).
If
desc
is not
undefined
, return
desc
Return !
StringGetOwnProperty
).
9.4.3.2
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of a String
exotic object
is called with property key
, and
Property Descriptor
Desc
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
stringDesc
be !
StringGetOwnProperty
).
If
stringDesc
is not
undefined
, then
Let
extensible
be
.[[Extensible]].
Return !
IsCompatiblePropertyDescriptor
extensible
Desc
stringDesc
).
Return !
OrdinaryDefineOwnProperty
Desc
).
9.4.3.3
[[OwnPropertyKeys]] ( )
When the [[OwnPropertyKeys]] internal method of a String
exotic object
is called, the following steps are taken:
Let
keys
be a new empty
List
Let
str
be
.[[StringData]].
Assert
Type
str
) is String.
Let
len
be the length of
str
For each integer
starting with 0 such that
len
, in ascending order, do
Add !
ToString
) as the last element of
keys
For each own property key
of
such that
is an
array index
and
ToInteger
) ≥
len
, in ascending numeric index order, do
Add
as the last element of
keys
For each own property key
of
such that
Type
) is String and
is not an
array index
, in ascending chronological order of property creation, do
Add
as the last element of
keys
For each own property key
of
such that
Type
) is Symbol, in ascending chronological order of property creation, do
Add
as the last element of
keys
Return
keys
9.4.3.4
StringCreate (
value
prototype
The abstract operation StringCreate with arguments
value
and
prototype
is used to specify the creation of new String exotic objects. It performs the following steps:
Assert
Type
value
) is String.
Let
be a newly created String
exotic object
Set
.[[StringData]] to
value
Set
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
.[[GetOwnProperty]] as specified in
9.4.3.1
Set
.[[DefineOwnProperty]] as specified in
9.4.3.2
Set
.[[OwnPropertyKeys]] as specified in
9.4.3.3
Set
.[[Prototype]] to
prototype
Set
.[[Extensible]] to
true
Let
length
be the number of code unit elements in
value
Perform !
DefinePropertyOrThrow
"length"
, PropertyDescriptor { [[Value]]:
length
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Return
9.4.3.5
StringGetOwnProperty (
The abstract operation StringGetOwnProperty called with arguments
and
performs the following steps:
Assert
is an Object that has a [[StringData]] internal slot.
Assert
IsPropertyKey
) is
true
If
Type
) is not String, return
undefined
Let
index
be !
CanonicalNumericIndexString
).
If
index
is
undefined
, return
undefined
If
IsInteger
index
) is
false
, return
undefined
If
index
-0
, return
undefined
Let
str
be
.[[StringData]].
Assert
Type
str
) is String.
Let
len
be the length of
str
If
index
< 0 or
len
index
, return
undefined
Let
resultStr
be the String value of length 1, containing one code unit from
str
, specifically the code unit at index
index
Return a PropertyDescriptor { [[Value]]:
resultStr
, [[Writable]]:
false
, [[Enumerable]]:
true
, [[Configurable]]:
false
}.
9.4.4
Arguments Exotic Objects
Most ECMAScript functions make an arguments object available to
their code. Depending upon the characteristics of the function
definition, its arguments object is either an ordinary object or an
arguments
exotic object
. An arguments
exotic object
is an
exotic object
whose
array index
properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.
Arguments exotic objects have the same internal slots as
ordinary objects. They also have a [[ParameterMap]] internal slot.
Ordinary arguments objects also have a [[ParameterMap]] internal slot
whose value is always undefined. For ordinary argument objects the
[[ParameterMap]] internal slot is only used by
Object.prototype.toString
19.1.3.6
) to identify them as such.
Arguments exotic objects provide alternative definitions for the following internal methods. All of the other arguments
exotic object
essential internal methods that are not defined below are as specified in
9.1
Note 1
The integer-indexed data properties of an arguments
exotic object
whose numeric name values are less than the number of formal parameters of the corresponding
function object
initially share their values with the corresponding argument bindings in the function's
execution context
This means that changing the property changes the corresponding value
of the argument binding and vice-versa. This correspondence is broken if
such a property is deleted and then redefined or if the property is
changed into an
accessor property
If the arguments object is an ordinary object, the values of its
properties are simply a copy of the arguments passed to the function and
there is no dynamic linkage between the property values and the formal
parameter values.
Note 2
The ParameterMap object and its property values are used as a
device for specifying the arguments object correspondence to argument
bindings. The ParameterMap object and the objects that are the values of
its properties are not directly observable from ECMAScript code. An
ECMAScript implementation does not need to actually create or use such
objects to implement the specified semantics.
Note 3
Ordinary arguments objects define a non-configurable
accessor property
named
"callee"
which throws a
TypeError
exception on access. The
"callee"
property has a more specific meaning for arguments exotic objects,
which are created only for some class of non-strict functions. The
definition of this property in the ordinary variant exists to ensure
that it is not defined in any other manner by conforming ECMAScript
implementations.
Note 4
ECMAScript implementations of arguments exotic objects have historically contained an
accessor property
named
"caller"
. Prior to ECMAScript 2017, this specification included the definition of a throwing
"caller"
property on ordinary arguments objects. Since implementations do not
contain this extension any longer, ECMAScript 2017 dropped the
requirement for a throwing
"caller"
accessor.
9.4.4.1
[[GetOwnProperty]] (
The [[GetOwnProperty]] internal method of an arguments
exotic object
when called with a property key
performs the following steps:
Let
args
be the arguments object.
Let
desc
be
OrdinaryGetOwnProperty
args
).
If
desc
is
undefined
, return
desc
Let
map
be
args
.[[ParameterMap]].
Let
isMapped
be !
HasOwnProperty
map
).
If
isMapped
is
true
, then
Set
desc
.[[Value]] to
Get
map
).
Return
desc
9.4.4.2
[[DefineOwnProperty]] (
Desc
The [[DefineOwnProperty]] internal method of an arguments
exotic object
when called with a property key
and
Property Descriptor
Desc
performs the following steps:
Let
args
be the arguments object.
Let
map
be
args
.[[ParameterMap]].
Let
isMapped
be
HasOwnProperty
map
).
Let
newArgDesc
be
Desc
If
isMapped
is
true
and
IsDataDescriptor
Desc
) is
true
, then
If
Desc
.[[Value]] is not present and
Desc
.[[Writable]] is present and its value is
false
, then
Set
newArgDesc
to a copy of
Desc
Set
newArgDesc
.[[Value]] to
Get
map
).
Let
allowed
be ?
OrdinaryDefineOwnProperty
args
newArgDesc
).
If
allowed
is
false
, return
false
If
isMapped
is
true
, then
If
IsAccessorDescriptor
Desc
) is
true
, then
Call
map
.[[Delete]](
).
Else,
If
Desc
.[[Value]] is present, then
Let
setStatus
be
Set
map
Desc
.[[Value]],
false
).
Assert
setStatus
is
true
because formal parameters mapped by argument objects are always writable.
If
Desc
.[[Writable]] is present and its value is
false
, then
Call
map
.[[Delete]](
).
Return
true
9.4.4.3
[[Get]] (
Receiver
The [[Get]] internal method of an arguments
exotic object
when called with a property key
and
ECMAScript language value
Receiver
performs the following steps:
Let
args
be the arguments object.
Let
map
be
args
.[[ParameterMap]].
Let
isMapped
be !
HasOwnProperty
map
).
If
isMapped
is
false
, then
Return ?
OrdinaryGet
args
Receiver
).
Else
map
contains a formal parameter mapping for
Return
Get
map
).
9.4.4.4
[[Set]] (
Receiver
The [[Set]] internal method of an arguments
exotic object
when called with property key
, value
, and
ECMAScript language value
Receiver
performs the following steps:
Let
args
be the arguments object.
If
SameValue
args
Receiver
) is
false
, then
Let
isMapped
be
false
Else,
Let
map
be
args
.[[ParameterMap]].
Let
isMapped
be !
HasOwnProperty
map
).
If
isMapped
is
true
, then
Let
setStatus
be
Set
map
false
).
Assert
setStatus
is
true
because formal parameters mapped by argument objects are always writable.
Return ?
OrdinarySet
args
Receiver
).
9.4.4.5
[[Delete]] (
The [[Delete]] internal method of an arguments
exotic object
when called with a property key
performs the following steps:
Let
args
be the arguments object.
Let
map
be
args
.[[ParameterMap]].
Let
isMapped
be !
HasOwnProperty
map
).
Let
result
be ?
OrdinaryDelete
args
).
If
result
is
true
and
isMapped
is
true
, then
Call
map
.[[Delete]](
).
Return
result
9.4.4.6
CreateUnmappedArgumentsObject (
argumentsList
The abstract operation CreateUnmappedArgumentsObject called with an argument
argumentsList
performs the following steps:
Let
len
be the number of elements in
argumentsList
Let
obj
be
ObjectCreate
%ObjectPrototype%
, « [[ParameterMap]] »).
Set
obj
.[[ParameterMap]] to
undefined
Perform
DefinePropertyOrThrow
obj
"length"
, PropertyDescriptor { [[Value]]:
len
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Let
index
be 0.
Repeat, while
index
len
Let
val
be
argumentsList
index
].
Perform
CreateDataProperty
obj
, !
ToString
index
),
val
).
Increase
index
by 1.
Perform !
DefinePropertyOrThrow
obj
, @@iterator, PropertyDescriptor { [[Value]]:
%ArrayProto_values%
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Perform !
DefinePropertyOrThrow
obj
"callee"
, PropertyDescriptor { [[Get]]:
%ThrowTypeError%
, [[Set]]:
%ThrowTypeError%
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Return
obj
9.4.4.7
CreateMappedArgumentsObject (
func
formals
argumentsList
env
The abstract operation CreateMappedArgumentsObject is called with object
func
Parse Node
formals
List
argumentsList
, and
Environment Record
env
. The following steps are performed:
Assert
formals
does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let
len
be the number of elements in
argumentsList
Let
obj
be a newly created arguments
exotic object
with a [[ParameterMap]] internal slot.
Set
obj
.[[GetOwnProperty]] as specified in
9.4.4.1
Set
obj
.[[DefineOwnProperty]] as specified in
9.4.4.2
Set
obj
.[[Get]] as specified in
9.4.4.3
Set
obj
.[[Set]] as specified in
9.4.4.4
Set
obj
.[[Delete]] as specified in
9.4.4.5
Set the remainder of
obj
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
obj
.[[Prototype]] to
%ObjectPrototype%
Set
obj
.[[Extensible]] to
true
Let
map
be
ObjectCreate
null
).
Set
obj
.[[ParameterMap]] to
map
Let
parameterNames
be the BoundNames of
formals
Let
numberOfParameters
be the number of elements in
parameterNames
Let
index
be 0.
Repeat, while
index
len
Let
val
be
argumentsList
index
].
Perform
CreateDataProperty
obj
, !
ToString
index
),
val
).
Increase
index
by 1.
Perform
DefinePropertyOrThrow
obj
"length"
, PropertyDescriptor { [[Value]]:
len
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Let
mappedNames
be a new empty
List
Let
index
be
numberOfParameters
- 1.
Repeat, while
index
≥ 0,
Let
name
be
parameterNames
index
].
If
name
is not an element of
mappedNames
, then
Add
name
as an element of the list
mappedNames
If
index
len
, then
Let
be
MakeArgGetter
name
env
).
Let
be
MakeArgSetter
name
env
).
Perform
map
.[[DefineOwnProperty]](!
ToString
index
), PropertyDescriptor { [[Set]]:
, [[Get]]:
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Decrease
index
by 1.
Perform !
DefinePropertyOrThrow
obj
, @@iterator, PropertyDescriptor { [[Value]]:
%ArrayProto_values%
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Perform !
DefinePropertyOrThrow
obj
"callee"
, PropertyDescriptor { [[Value]]:
func
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}).
Return
obj
9.4.4.7.1
MakeArgGetter (
name
env
The abstract operation MakeArgGetter called with String
name
and
Environment Record
env
creates a built-in
function object
that when executed returns the value bound for
name
in
env
. It performs the following steps:
Let
steps
be the steps of an ArgGetter function as specified below.
Let
getter
be
CreateBuiltinFunction
steps
, « [[Name]], [[Env]] »).
Set
getter
.[[Name]] to
name
Set
getter
.[[Env]] to
env
Return
getter
An ArgGetter function is an anonymous built-in function
with [[Name]] and [[Env]] internal slots. When an ArgGetter function
that expects no arguments is called it performs the following steps:
Let
be the
active function object
Let
name
be
.[[Name]].
Let
env
be
.[[Env]].
Return
env
.GetBindingValue(
name
false
).
Note
ArgGetter functions are never directly accessible to ECMAScript code.
9.4.4.7.2
MakeArgSetter (
name
env
The abstract operation MakeArgSetter called with String
name
and
Environment Record
env
creates a built-in
function object
that when executed sets the value bound for
name
in
env
. It performs the following steps:
Let
steps
be the steps of an ArgSetter function as specified below.
Let
setter
be
CreateBuiltinFunction
steps
, « [[Name]], [[Env]] »).
Set
setter
.[[Name]] to
name
Set
setter
.[[Env]] to
env
Return
setter
An ArgSetter function is an anonymous built-in function
with [[Name]] and [[Env]] internal slots. When an ArgSetter function is
called with argument
value
it performs the following steps:
Let
be the
active function object
Let
name
be
.[[Name]].
Let
env
be
.[[Env]].
Return
env
.SetMutableBinding(
name
value
false
).
Note
ArgSetter functions are never directly accessible to ECMAScript code.
9.4.5
Integer-Indexed Exotic Objects
An
Integer-Indexed exotic object
is an
exotic object
that performs special handling of
integer index
property keys.
Integer-Indexed exotic objects
have the same internal slots as ordinary objects and additionally
[[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and
[[TypedArrayName]] internal slots.
Integer-Indexed exotic objects
provide alternative definitions for the following internal methods. All of the other
Integer-Indexed exotic object
essential internal methods that are not defined below are as specified in
9.1
9.4.5.1
[[GetOwnProperty]] (
When the [[GetOwnProperty]] internal method of an
Integer-Indexed exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Assert
is an Object that has a [[ViewedArrayBuffer]] internal slot.
If
Type
) is String, then
Let
numericIndex
be !
CanonicalNumericIndexString
).
If
numericIndex
is not
undefined
, then
Let
value
be ?
IntegerIndexedElementGet
numericIndex
).
If
value
is
undefined
, return
undefined
Return a PropertyDescriptor { [[Value]]:
value
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
false
}.
Return
OrdinaryGetOwnProperty
).
9.4.5.2
[[HasProperty]] (
When the [[HasProperty]] internal method of an
Integer-Indexed exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Assert
is an Object that has a [[ViewedArrayBuffer]] internal slot.
If
Type
) is String, then
Let
numericIndex
be !
CanonicalNumericIndexString
).
If
numericIndex
is not
undefined
, then
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
If
IsInteger
numericIndex
) is
false
, return
false
If
numericIndex
-0
, return
false
If
numericIndex
< 0, return
false
If
numericIndex
.[[ArrayLength]], return
false
Return
true
Return ?
OrdinaryHasProperty
).
9.4.5.3
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of an
Integer-Indexed exotic object
is called with property key
, and
Property Descriptor
Desc
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Assert
is an Object that has a [[ViewedArrayBuffer]] internal slot.
If
Type
) is String, then
Let
numericIndex
be !
CanonicalNumericIndexString
).
If
numericIndex
is not
undefined
, then
If
IsInteger
numericIndex
) is
false
, return
false
If
numericIndex
-0
, return
false
If
numericIndex
< 0, return
false
Let
length
be
.[[ArrayLength]].
If
numericIndex
length
, return
false
If
IsAccessorDescriptor
Desc
) is
true
, return
false
If
Desc
has a [[Configurable]] field and if
Desc
.[[Configurable]] is
true
, return
false
If
Desc
has an [[Enumerable]] field and if
Desc
.[[Enumerable]] is
false
, return
false
If
Desc
has a [[Writable]] field and if
Desc
.[[Writable]] is
false
, return
false
If
Desc
has a [[Value]] field, then
Let
value
be
Desc
.[[Value]].
Return ?
IntegerIndexedElementSet
numericIndex
value
).
Return
true
Return !
OrdinaryDefineOwnProperty
Desc
).
9.4.5.4
[[Get]] (
Receiver
When the [[Get]] internal method of an
Integer-Indexed exotic object
is called with property key
and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
Type
) is String, then
Let
numericIndex
be !
CanonicalNumericIndexString
).
If
numericIndex
is not
undefined
, then
Return ?
IntegerIndexedElementGet
numericIndex
).
Return ?
OrdinaryGet
Receiver
).
9.4.5.5
[[Set]] (
Receiver
When the [[Set]] internal method of an
Integer-Indexed exotic object
is called with property key
, value
, and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
Type
) is String, then
Let
numericIndex
be !
CanonicalNumericIndexString
).
If
numericIndex
is not
undefined
, then
Return ?
IntegerIndexedElementSet
numericIndex
).
Return ?
OrdinarySet
Receiver
).
9.4.5.6
[[OwnPropertyKeys]] ( )
When the [[OwnPropertyKeys]] internal method of an
Integer-Indexed exotic object
is called, the following steps are taken:
Let
keys
be a new empty
List
Assert
is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let
len
be
.[[ArrayLength]].
For each integer
starting with 0 such that
len
, in ascending order, do
Add !
ToString
) as the last element of
keys
For each own property key
of
such that
Type
) is String and
is not an
integer index
, in ascending chronological order of property creation, do
Add
as the last element of
keys
For each own property key
of
such that
Type
) is Symbol, in ascending chronological order of property creation, do
Add
as the last element of
keys
Return
keys
9.4.5.7
IntegerIndexedObjectCreate (
prototype
internalSlotsList
The abstract operation IntegerIndexedObjectCreate with arguments
prototype
and
internalSlotsList
is used to specify the creation of new
Integer-Indexed exotic objects
. The argument
internalSlotsList
is a
List
of the names of additional internal slots that must be defined as part
of the object. IntegerIndexedObjectCreate performs the following steps:
Assert
internalSlotsList
contains the names [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]].
Let
be a newly created object with an internal slot for each name in
internalSlotsList
Set
's essential internal methods to the default ordinary object definitions specified in
9.1
Set
.[[GetOwnProperty]] as specified in
9.4.5.1
Set
.[[HasProperty]] as specified in
9.4.5.2
Set
.[[DefineOwnProperty]] as specified in
9.4.5.3
Set
.[[Get]] as specified in
9.4.5.4
Set
.[[Set]] as specified in
9.4.5.5
Set
.[[OwnPropertyKeys]] as specified in
9.4.5.6
Set
.[[Prototype]] to
prototype
Set
.[[Extensible]] to
true
Return
9.4.5.8
IntegerIndexedElementGet (
index
The abstract operation IntegerIndexedElementGet with arguments
and
index
performs the following steps:
Assert
Type
index
) is Number.
Assert
is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
If
IsInteger
index
) is
false
, return
undefined
If
index
-0
, return
undefined
Let
length
be
.[[ArrayLength]].
If
index
< 0 or
index
length
, return
undefined
Let
offset
be
.[[ByteOffset]].
Let
arrayTypeName
be the String value of
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
indexedPosition
be (
index
elementSize
) +
offset
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Return
GetValueFromBuffer
buffer
indexedPosition
elementType
true
"Unordered"
).
9.4.5.9
IntegerIndexedElementSet (
index
value
The abstract operation IntegerIndexedElementSet with arguments
index
, and
value
performs the following steps:
Assert
Type
index
) is Number.
Assert
is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Let
numValue
be ?
ToNumber
value
).
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
If
IsInteger
index
) is
false
, return
false
If
index
-0
, return
false
Let
length
be
.[[ArrayLength]].
If
index
< 0 or
index
length
, return
false
Let
offset
be
.[[ByteOffset]].
Let
arrayTypeName
be the String value of
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
indexedPosition
be (
index
elementSize
) +
offset
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Perform
SetValueInBuffer
buffer
indexedPosition
elementType
numValue
true
"Unordered"
).
Return
true
9.4.6
Module Namespace Exotic Objects
module namespace object
is an
exotic object
that exposes the bindings exported from an ECMAScript
Module
(See
15.2.3
). There is a one-to-one correspondence between the String-keyed own properties of a module namespace
exotic object
and the binding names exported by the
Module
. The exported bindings include any bindings that are indirectly exported using
export *
export items. Each String-valued own property key is the StringValue of
the corresponding exported binding name. These are the only
String-keyed properties of a module namespace
exotic object
. Each such property has the attributes { [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
false
}. Module namespace objects are not extensible.
Module namespace objects have the internal slots defined in
Table 29
Table 29: Internal Slots of Module Namespace Exotic Objects
Internal Slot
Type
Description
[[Module]]
Module Record
The
Module Record
whose exports this namespace exposes.
[[Exports]]
List
of String
List
containing the String values of the exported names exposed as own
properties of this object. The list is ordered as if an Array of those
String values had been sorted using
Array.prototype.sort
using
undefined
as
comparefn
[[Prototype]]
Null
This slot always contains the value
null
(see
9.4.6.1
).
Module namespace exotic objects provide alternative definitions
for all of the internal methods except [[GetPrototypeOf]], which
behaves as defined in
9.1.1
9.4.6.1
[[SetPrototypeOf]] (
When the [[SetPrototypeOf]] internal method of a module namespace
exotic object
is called with argument
, the following steps are taken:
Return ?
SetImmutablePrototype
).
9.4.6.2
[[IsExtensible]] ( )
When the [[IsExtensible]] internal method of a module namespace
exotic object
is called, the following steps are taken:
Return
false
9.4.6.3
[[PreventExtensions]] ( )
When the [[PreventExtensions]] internal method of a module namespace
exotic object
is called, the following steps are taken:
Return
true
9.4.6.4
[[GetOwnProperty]] (
When the [[GetOwnProperty]] internal method of a module namespace
exotic object
is called with property key
, the following steps are taken:
If
Type
) is Symbol, return
OrdinaryGetOwnProperty
).
Let
exports
be
.[[Exports]].
If
is not an element of
exports
, return
undefined
Let
value
be ?
.[[Get]](
).
Return PropertyDescriptor { [[Value]]:
value
, [[Writable]]:
true
, [[Enumerable]]:
true
, [[Configurable]]:
false
}.
9.4.6.5
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of a module namespace
exotic object
is called with property key
and
Property Descriptor
Desc
, the following steps are taken:
If
Type
) is Symbol, return
OrdinaryDefineOwnProperty
Desc
).
Let
current
be ?
.[[GetOwnProperty]](
).
If
current
is
undefined
, return
false
If
IsAccessorDescriptor
Desc
) is
true
, return
false
If
Desc
.[[Writable]] is present and has value
false
, return
false
If
Desc
.[[Enumerable]] is present and has value
false
, return
false
If
Desc
.[[Configurable]] is present and has value
true
, return
false
If
Desc
.[[Value]] is present, return
SameValue
Desc
.[[Value]],
current
.[[Value]]).
Return
true
9.4.6.6
[[HasProperty]] (
When the [[HasProperty]] internal method of a module namespace
exotic object
is called with property key
, the following steps are taken:
If
Type
) is Symbol, return
OrdinaryHasProperty
).
Let
exports
be
.[[Exports]].
If
is an element of
exports
, return
true
Return
false
9.4.6.7
[[Get]] (
Receiver
When the [[Get]] internal method of a module namespace
exotic object
is called with property key
and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
Type
) is Symbol, then
Return ?
OrdinaryGet
Receiver
).
Let
exports
be
.[[Exports]].
If
is not an element of
exports
, return
undefined
Let
be
.[[Module]].
Let
binding
be !
.ResolveExport(
, « »).
Assert
binding
is a
ResolvedBinding Record
Let
targetModule
be
binding
.[[Module]].
Assert
targetModule
is not
undefined
Let
targetEnv
be
targetModule
.[[Environment]].
If
targetEnv
is
undefined
, throw a
ReferenceError
exception.
Let
targetEnvRec
be
targetEnv
's
EnvironmentRecord
Return ?
targetEnvRec
.GetBindingValue(
binding
.[[BindingName]],
true
).
Note
ResolveExport is idempotent and side-effect free. An
implementation might choose to pre-compute or cache the ResolveExport
results for the [[Exports]] of each module namespace
exotic object
9.4.6.8
[[Set]] (
Receiver
When the [[Set]] internal method of a module namespace
exotic object
is called with property key
, value
, and
ECMAScript language value
Receiver
, the following steps are taken:
Return
false
9.4.6.9
[[Delete]] (
When the [[Delete]] internal method of a module namespace
exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
If
Type
) is Symbol, then
Return ?
OrdinaryDelete
).
Let
exports
be
.[[Exports]].
If
is an element of
exports
, return
false
Return
true
9.4.6.10
[[OwnPropertyKeys]] ( )
When the [[OwnPropertyKeys]] internal method of a module namespace
exotic object
is called, the following steps are taken:
Let
exports
be a copy of
.[[Exports]].
Let
symbolKeys
be !
OrdinaryOwnPropertyKeys
).
Append all the entries of
symbolKeys
to the end of
exports
Return
exports
9.4.6.11
ModuleNamespaceCreate (
module
exports
The abstract operation ModuleNamespaceCreate with arguments
module
, and
exports
is used to specify the creation of new module namespace exotic objects. It performs the following steps:
Assert
module
is a
Module Record
Assert
module
.[[Namespace]] is
undefined
Assert
exports
is a
List
of String values.
Let
be a newly created object.
Set
's essential internal methods to the definitions specified in
9.4.6
Set
.[[Module]] to
module
Let
sortedExports
be a new
List
containing the same values as the list
exports
where the values are ordered as if an Array of the same values had been sorted using
Array.prototype.sort
using
undefined
as
comparefn
Set
.[[Exports]] to
sortedExports
Create own properties of
corresponding to the definitions in
26.3
Set
module
.[[Namespace]] to
Return
9.4.7
Immutable Prototype Exotic Objects
An
immutable prototype exotic object
is an
exotic object
that has a [[Prototype]] internal slot that will not change once it is initialized.
Immutable prototype exotic objects have the same internal slots
as ordinary objects. They are exotic only in the following internal
methods. All other internal methods of immutable prototype exotic
objects that are not explicitly defined below are instead defined as in
ordinary objects.
9.4.7.1
[[SetPrototypeOf]] (
When the [[SetPrototypeOf]] internal method of an
immutable prototype exotic object
is called with argument
, the following steps are taken:
Return ?
SetImmutablePrototype
).
9.4.7.2
SetImmutablePrototype (
When the SetImmutablePrototype abstract operation is called with arguments
and
, the following steps are taken:
Assert
: Either
Type
) is Object or
Type
) is Null.
Let
current
be ?
.[[GetPrototypeOf]]().
If
SameValue
current
) is
true
, return
true
Return
false
9.5
Proxy Object Internal Methods and Internal Slots
A proxy object is an
exotic object
whose essential internal methods are partially implemented using
ECMAScript code. Every proxy object has an internal slot called
[[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the
proxy's
handler object
, or
null
. Methods (see
Table 30
of a handler object may be used to augment the implementation for one
or more of the proxy object's internal methods. Every proxy object also
has an internal slot called [[ProxyTarget]] whose value is either an
object or the
null
value. This object is called the proxy's
target object
Table 30: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a
proxy object internal method, the handler method is passed the proxy's
target object as a parameter. A proxy's handler object does not
necessarily have a method corresponding to every essential internal
method. Invoking an internal method on the proxy results in the
invocation of the corresponding internal method on the proxy's target
object if the handler object does not have a method corresponding to the
internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a
proxy object are always initialized when the object is created and
typically may not be modified. Some proxy objects are created in a
manner that permits them to be subsequently
revoked
. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to
null
causing subsequent invocations of internal methods on that proxy object to throw a
TypeError
exception.
Because proxy objects permit the implementation of internal
methods to be provided by arbitrary ECMAScript code, it is possible to
define a proxy object whose handler methods violates the invariants
defined in
6.1.7.3
. Some of the internal method invariants defined in
6.1.7.3
are essential integrity invariants. These invariants are explicitly
enforced by the proxy object internal methods specified in this section.
An ECMAScript implementation must be robust in the presence of all
possible invariant violations.
In the following algorithm descriptions, assume
is an ECMAScript proxy object,
is a property key value,
is any
ECMAScript language value
and
Desc
is a
Property Descriptor
record.
9.5.1
[[GetPrototypeOf]] ( )
When the [[GetPrototypeOf]] internal method of a Proxy
exotic object
is called, the following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"getPrototypeOf"
).
If
trap
is
undefined
, then
Return ?
target
.[[GetPrototypeOf]]().
Let
handlerProto
be ?
Call
trap
handler
, «
target
»).
If
Type
handlerProto
) is neither Object nor Null, throw a
TypeError
exception.
Let
extensibleTarget
be ?
IsExtensible
target
).
If
extensibleTarget
is
true
, return
handlerProto
Let
targetProto
be ?
target
.[[GetPrototypeOf]]().
If
SameValue
handlerProto
targetProto
) is
false
, throw a
TypeError
exception.
Return
handlerProto
Note
[[GetPrototypeOf]] for proxy objects enforces the following invariants:
The result of [[GetPrototypeOf]] must be either an Object or
null
If the target object is not extensible, [[GetPrototypeOf]]
applied to the proxy object must return the same value as
[[GetPrototypeOf]] applied to the proxy object's target object.
9.5.2
[[SetPrototypeOf]] (
When the [[SetPrototypeOf]] internal method of a Proxy
exotic object
is called with argument
, the following steps are taken:
Assert
: Either
Type
) is Object or
Type
) is Null.
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"setPrototypeOf"
).
If
trap
is
undefined
, then
Return ?
target
.[[SetPrototypeOf]](
).
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
»)).
If
booleanTrapResult
is
false
, return
false
Let
extensibleTarget
be ?
IsExtensible
target
).
If
extensibleTarget
is
true
, return
true
Let
targetProto
be ?
target
.[[GetPrototypeOf]]().
If
SameValue
targetProto
) is
false
, throw a
TypeError
exception.
Return
true
Note
[[SetPrototypeOf]] for proxy objects enforces the following invariants:
The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value
must be the same as the result of [[GetPrototypeOf]] applied to target
object.
9.5.3
[[IsExtensible]] ( )
When the [[IsExtensible]] internal method of a Proxy
exotic object
is called, the following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"isExtensible"
).
If
trap
is
undefined
, then
Return ?
target
.[[IsExtensible]]().
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
»)).
Let
targetResult
be ?
target
.[[IsExtensible]]().
If
SameValue
booleanTrapResult
targetResult
) is
false
, throw a
TypeError
exception.
Return
booleanTrapResult
Note
[[IsExtensible]] for proxy objects enforces the following invariants:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the
same value as [[IsExtensible]] applied to the proxy object's target
object with the same argument.
9.5.4
[[PreventExtensions]] ( )
When the [[PreventExtensions]] internal method of a Proxy
exotic object
is called, the following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"preventExtensions"
).
If
trap
is
undefined
, then
Return ?
target
.[[PreventExtensions]]().
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
»)).
If
booleanTrapResult
is
true
, then
Let
targetIsExtensible
be ?
target
.[[IsExtensible]]().
If
targetIsExtensible
is
true
, throw a
TypeError
exception.
Return
booleanTrapResult
Note
[[PreventExtensions]] for proxy objects enforces the following invariants:
The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the proxy object only returns
true
if [[IsExtensible]] applied to the proxy object's target object is
false
9.5.5
[[GetOwnProperty]] (
When the [[GetOwnProperty]] internal method of a Proxy
exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"getOwnPropertyDescriptor"
).
If
trap
is
undefined
, then
Return ?
target
.[[GetOwnProperty]](
).
Let
trapResultObj
be ?
Call
trap
handler
, «
target
»).
If
Type
trapResultObj
) is neither Object nor Undefined, throw a
TypeError
exception.
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
If
trapResultObj
is
undefined
, then
If
targetDesc
is
undefined
, return
undefined
If
targetDesc
.[[Configurable]] is
false
, throw a
TypeError
exception.
Let
extensibleTarget
be ?
IsExtensible
target
).
If
extensibleTarget
is
false
, throw a
TypeError
exception.
Return
undefined
Let
extensibleTarget
be ?
IsExtensible
target
).
Let
resultDesc
be ?
ToPropertyDescriptor
trapResultObj
).
Call
CompletePropertyDescriptor
resultDesc
).
Let
valid
be
IsCompatiblePropertyDescriptor
extensibleTarget
resultDesc
targetDesc
).
If
valid
is
false
, throw a
TypeError
exception.
If
resultDesc
.[[Configurable]] is
false
, then
If
targetDesc
is
undefined
or
targetDesc
.[[Configurable]] is
true
, then
Throw a
TypeError
exception.
Return
resultDesc
Note
[[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or
undefined
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists
as an own property of the target object and the target object is not
extensible.
A property cannot be reported as existent, if it does not
exist as an own property of the target object and the target object is
not extensible.
A property cannot be reported as non-configurable, if it
does not exist as an own property of the target object or if it exists
as a configurable own property of the target object.
9.5.6
[[DefineOwnProperty]] (
Desc
When the [[DefineOwnProperty]] internal method of a Proxy
exotic object
is called with property key
and
Property Descriptor
Desc
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"defineProperty"
).
If
trap
is
undefined
, then
Return ?
target
.[[DefineOwnProperty]](
Desc
).
Let
descObj
be
FromPropertyDescriptor
Desc
).
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
descObj
»)).
If
booleanTrapResult
is
false
, return
false
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
Let
extensibleTarget
be ?
IsExtensible
target
).
If
Desc
has a [[Configurable]] field and if
Desc
.[[Configurable]] is
false
, then
Let
settingConfigFalse
be
true
Else, let
settingConfigFalse
be
false
If
targetDesc
is
undefined
, then
If
extensibleTarget
is
false
, throw a
TypeError
exception.
If
settingConfigFalse
is
true
, throw a
TypeError
exception.
Else
targetDesc
is not
undefined
If
IsCompatiblePropertyDescriptor
extensibleTarget
Desc
targetDesc
) is
false
, throw a
TypeError
exception.
If
settingConfigFalse
is
true
and
targetDesc
.[[Configurable]] is
true
, throw a
TypeError
exception.
Return
true
Note
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a
corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the
Property Descriptor
of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
9.5.7
[[HasProperty]] (
When the [[HasProperty]] internal method of a Proxy
exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"has"
).
If
trap
is
undefined
, then
Return ?
target
.[[HasProperty]](
).
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
»)).
If
booleanTrapResult
is
false
, then
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
If
targetDesc
is not
undefined
, then
If
targetDesc
.[[Configurable]] is
false
, throw a
TypeError
exception.
Let
extensibleTarget
be ?
IsExtensible
target
).
If
extensibleTarget
is
false
, throw a
TypeError
exception.
Return
booleanTrapResult
Note
[[HasProperty]] for proxy objects enforces the following invariants:
The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists
as an own property of the target object and the target object is not
extensible.
9.5.8
[[Get]] (
Receiver
When the [[Get]] internal method of a Proxy
exotic object
is called with property key
and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"get"
).
If
trap
is
undefined
, then
Return ?
target
.[[Get]](
Receiver
).
Let
trapResult
be ?
Call
trap
handler
, «
target
Receiver
»).
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
If
targetDesc
is not
undefined
and
targetDesc
.[[Configurable]] is
false
, then
If
IsDataDescriptor
targetDesc
) is
true
and
targetDesc
.[[Writable]] is
false
, then
If
SameValue
trapResult
targetDesc
.[[Value]]) is
false
, throw a
TypeError
exception.
If
IsAccessorDescriptor
targetDesc
) is
true
and
targetDesc
.[[Get]] is
undefined
, then
If
trapResult
is not
undefined
, throw a
TypeError
exception.
Return
trapResult
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the
value of the corresponding target object property if the target object
property is a non-writable, non-configurable own
data property
The value reported for a property must be
undefined
if the corresponding target object property is a non-configurable own
accessor property
that has
undefined
as its [[Get]] attribute.
9.5.9
[[Set]] (
Receiver
When the [[Set]] internal method of a Proxy
exotic object
is called with property key
, value
, and
ECMAScript language value
Receiver
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"set"
).
If
trap
is
undefined
, then
Return ?
target
.[[Set]](
Receiver
).
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
Receiver
»)).
If
booleanTrapResult
is
false
, return
false
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
If
targetDesc
is not
undefined
and
targetDesc
.[[Configurable]] is
false
, then
If
IsDataDescriptor
targetDesc
) is
true
and
targetDesc
.[[Writable]] is
false
, then
If
SameValue
targetDesc
.[[Value]]) is
false
, throw a
TypeError
exception.
If
IsAccessorDescriptor
targetDesc
) is
true
, then
If
targetDesc
.[[Set]] is
undefined
, throw a
TypeError
exception.
Return
true
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from
the value of the corresponding target object property if the
corresponding target object property is a non-writable, non-configurable
own
data property
Cannot set the value of a property if the corresponding target object property is a non-configurable own
accessor property
that has
undefined
as its [[Set]] attribute.
9.5.10
[[Delete]] (
When the [[Delete]] internal method of a Proxy
exotic object
is called with property key
, the following steps are taken:
Assert
IsPropertyKey
) is
true
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"deleteProperty"
).
If
trap
is
undefined
, then
Return ?
target
.[[Delete]](
).
Let
booleanTrapResult
be
ToBoolean
(?
Call
trap
handler
, «
target
»)).
If
booleanTrapResult
is
false
, return
false
Let
targetDesc
be ?
target
.[[GetOwnProperty]](
).
If
targetDesc
is
undefined
, return
true
If
targetDesc
.[[Configurable]] is
false
, throw a
TypeError
exception.
Return
true
Note
[[Delete]] for proxy objects enforces the following invariants:
The result of [[Delete]] is a Boolean value.
A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
9.5.11
[[OwnPropertyKeys]] ( )
When the [[OwnPropertyKeys]] internal method of a Proxy
exotic object
is called, the following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"ownKeys"
).
If
trap
is
undefined
, then
Return ?
target
.[[OwnPropertyKeys]]().
Let
trapResultArray
be ?
Call
trap
handler
, «
target
»).
Let
trapResult
be ?
CreateListFromArrayLike
trapResultArray
, « String, Symbol »).
If
trapResult
contains any duplicate entries, throw a
TypeError
exception.
Let
extensibleTarget
be ?
IsExtensible
target
).
Let
targetKeys
be ?
target
.[[OwnPropertyKeys]]().
Assert
targetKeys
is a
List
containing only String and Symbol values.
Assert
targetKeys
contains no duplicate entries.
Let
targetConfigurableKeys
be a new empty
List
Let
targetNonconfigurableKeys
be a new empty
List
For each element
key
of
targetKeys
, do
Let
desc
be ?
target
.[[GetOwnProperty]](
key
).
If
desc
is not
undefined
and
desc
.[[Configurable]] is
false
, then
Append
key
as an element of
targetNonconfigurableKeys
Else,
Append
key
as an element of
targetConfigurableKeys
If
extensibleTarget
is
true
and
targetNonconfigurableKeys
is empty, then
Return
trapResult
Let
uncheckedResultKeys
be a new
List
which is a copy of
trapResult
For each
key
that is an element of
targetNonconfigurableKeys
, do
If
key
is not an element of
uncheckedResultKeys
, throw a
TypeError
exception.
Remove
key
from
uncheckedResultKeys
If
extensibleTarget
is
true
, return
trapResult
For each
key
that is an element of
targetConfigurableKeys
, do
If
key
is not an element of
uncheckedResultKeys
, throw a
TypeError
exception.
Remove
key
from
uncheckedResultKeys
If
uncheckedResultKeys
is not empty, throw a
TypeError
exception.
Return
trapResult
Note
[[OwnPropertyKeys]] for proxy objects enforces the following invariants:
The result of [[OwnPropertyKeys]] is a
List
The returned
List
contains no duplicate entries.
The Type of each result
List
element is either String or Symbol.
The result
List
must contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the result
List
must contain all the keys of the own properties of the target object and no other values.
9.5.12
[[Call]] (
thisArgument
argumentsList
The [[Call]] internal method of a Proxy
exotic object
is called with parameters
thisArgument
and
argumentsList
, a
List
of ECMAScript language values. The following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Let
trap
be ?
GetMethod
handler
"apply"
).
If
trap
is
undefined
, then
Return ?
Call
target
thisArgument
argumentsList
).
Let
argArray
be
CreateArrayFromList
argumentsList
).
Return ?
Call
trap
handler
, «
target
thisArgument
argArray
»).
Note
A Proxy
exotic object
only has a [[Call]] internal method if the initial value of its
[[ProxyTarget]] internal slot is an object that has a [[Call]] internal
method.
9.5.13
[[Construct]] (
argumentsList
newTarget
The [[Construct]] internal method of a Proxy
exotic object
is called with parameters
argumentsList
which is a possibly empty
List
of ECMAScript language values and
newTarget
. The following steps are taken:
Let
handler
be
.[[ProxyHandler]].
If
handler
is
null
, throw a
TypeError
exception.
Assert
Type
handler
) is Object.
Let
target
be
.[[ProxyTarget]].
Assert
IsConstructor
target
) is
true
Let
trap
be ?
GetMethod
handler
"construct"
).
If
trap
is
undefined
, then
Return ?
Construct
target
argumentsList
newTarget
).
Let
argArray
be
CreateArrayFromList
argumentsList
).
Let
newObj
be ?
Call
trap
handler
, «
target
argArray
newTarget
»).
If
Type
newObj
) is not Object, throw a
TypeError
exception.
Return
newObj
Note 1
A Proxy
exotic object
only has a [[Construct]] internal method if the initial value of its
[[ProxyTarget]] internal slot is an object that has a [[Construct]]
internal method.
Note 2
[[Construct]] for proxy objects enforces the following invariants:
The result of [[Construct]] must be an Object.
9.5.14
ProxyCreate (
target
handler
The abstract operation ProxyCreate with arguments
target
and
handler
is used to specify the creation of new Proxy exotic objects. It performs the following steps:
If
Type
target
) is not Object, throw a
TypeError
exception.
If
target
is a Proxy
exotic object
and
target
.[[ProxyHandler]] is
null
, throw a
TypeError
exception.
If
Type
handler
) is not Object, throw a
TypeError
exception.
If
handler
is a Proxy
exotic object
and
handler
.[[ProxyHandler]] is
null
, throw a
TypeError
exception.
Let
be a newly created object.
Set
's essential internal methods (except for [[Call]] and [[Construct]]) to the definitions specified in
9.5
If
IsCallable
target
) is
true
, then
Set
.[[Call]] as specified in
9.5.12
If
IsConstructor
target
) is
true
, then
Set
.[[Construct]] as specified in
9.5.13
Set
.[[ProxyTarget]] to
target
Set
.[[ProxyHandler]] to
handler
Return
10
ECMAScript Language: Source Code
10.1
Source Text
Syntax
SourceCharacter
::
any Unicode code point
ECMAScript code is expressed using Unicode. ECMAScript source
text is a sequence of code points. All Unicode code point values from
U+0000 to U+10FFFF, including surrogate code points, may occur in source
text where permitted by the ECMAScript grammars. The actual encodings
used to store and interchange ECMAScript source text is not relevant to
this specification. Regardless of the external source text encoding, a
conforming ECMAScript implementation processes the source text as if it
was an equivalent sequence of
SourceCharacter
values, each
SourceCharacter
being a Unicode code point. Conforming ECMAScript implementations are
not required to perform any normalization of source text, or behave as
though they were performing normalization of source text.
The components of a combining character sequence are treated as
individual Unicode code points even though a user might think of the
whole sequence as a single character.
Note
In string literals, regular expression literals, template
literals and identifiers, any Unicode code point may also be expressed
using Unicode escape sequences that explicitly express a code point's
numeric value. Within a comment, such an escape sequence is effectively
ignored as part of the comment.
ECMAScript differs from the Java programming language in the
behaviour of Unicode escape sequences. In a Java program, if the Unicode
escape sequence
\u000A
, for example, occurs within a
single-line comment, it is interpreted as a line terminator (Unicode
code point U+000A is LINE FEED (LF)) and therefore the next code point
is not part of the comment. Similarly, if the Unicode escape sequence
\u000A
occurs within a string literal in a Java program, it is likewise
interpreted as a line terminator, which is not allowed within a string
literal—one must write
\n
instead of
\u000A
to
cause a LINE FEED (LF) to be part of the String value of a string
literal. In an ECMAScript program, a Unicode escape sequence occurring
within a comment is never interpreted and therefore cannot contribute to
termination of the comment. Similarly, a Unicode escape sequence
occurring within a string literal in an ECMAScript program always
contributes to the literal and is never interpreted as a line terminator
or as a code point that might terminate the string literal.
10.1.1
Static Semantics: UTF16Encoding (
cp
The UTF16Encoding of a numeric code point value,
cp
, is determined as follows:
Assert
: 0 ≤
cp
≤ 0x10FFFF.
If
cp
≤ 0xFFFF, return
cp
Let
cu1
be
floor
((
cp
- 0x10000) / 0x400) + 0xD800.
Let
cu2
be ((
cp
- 0x10000)
modulo
0x400) + 0xDC00.
Return the code unit sequence consisting of
cu1
followed by
cu2
10.1.2
Static Semantics: UTF16Decode (
lead
trail
Two code units,
lead
and
trail
, that form a UTF-16
surrogate pair
are converted to a code point by performing the following steps:
Assert
lead
is a
leading surrogate
and
trail
is a
trailing surrogate
Let
cp
be (
lead
- 0xD800) × 0x400 + (
trail
- 0xDC00) + 0x10000.
Return the code point
cp
10.2
Types of Source Code
There are four types of ECMAScript code:
Global code
is source text that is treated as an ECMAScript
Script
. The global code of a particular
Script
does not include any source text that is parsed as part of a
FunctionDeclaration
FunctionExpression
GeneratorDeclaration
GeneratorExpression
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
AsyncGeneratorExpression
MethodDefinition
ArrowFunction
AsyncArrowFunction
ClassDeclaration
, or
ClassExpression
Eval code
is the source text supplied to the built-in
eval
function. More precisely, if the parameter to the built-in
eval
function is a String, it is treated as an ECMAScript
Script
. The eval code for a particular invocation of
eval
is the global code portion of that
Script
Function code
is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see
9.2
) of an ECMAScript
function object
The function code of a particular ECMAScript function does not include
any source text that is parsed as the function code of a nested
FunctionDeclaration
FunctionExpression
GeneratorDeclaration
GeneratorExpression
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
AsyncGeneratorExpression
MethodDefinition
ArrowFunction
AsyncArrowFunction
ClassDeclaration
, or
ClassExpression
Module code
is source text that is code that is provided as a
ModuleBody
It is the code that is directly evaluated when a module is initialized.
The module code of a particular module does not include any source text
that is parsed as part of a nested
FunctionDeclaration
FunctionExpression
GeneratorDeclaration
GeneratorExpression
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
AsyncGeneratorExpression
MethodDefinition
ArrowFunction
AsyncArrowFunction
ClassDeclaration
, or
ClassExpression
Note
Function code is generally provided as the bodies of Function Definitions (
14.1
), Arrow Function Definitions (
14.2
), Method Definitions (
14.3
), Generator Function Definitions (
14.4
), Async Function Definitions (
14.7
), Async Generator Function Definitions (
14.5
), and Async Arrow Functions (
14.8
). Function code is also derived from the arguments to the
Function
constructor
19.2.1.1
), the
GeneratorFunction
constructor
25.2.1.1
), and the
AsyncFunction
constructor
25.7.1.1
).
10.2.1
Strict Mode Code
An ECMAScript
Script
syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as
strict mode code
in the following situations:
Global code is strict mode code if it begins with a
Directive Prologue
that contains a
Use Strict Directive
Module code is always strict mode code.
All parts of a
ClassDeclaration
or a
ClassExpression
are strict mode code.
Eval code is strict mode code if it begins with a
Directive Prologue
that contains a
Use Strict Directive
or if the call to
eval
is a
direct eval
that is contained in strict mode code.
Function code is strict mode code if the associated
FunctionDeclaration
FunctionExpression
GeneratorDeclaration
GeneratorExpression
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
AsyncGeneratorExpression
MethodDefinition
ArrowFunction
, or
AsyncArrowFunction
is contained in strict mode code or if the code that produces the value
of the function's [[ECMAScriptCode]] internal slot begins with a
Directive Prologue
that contains a
Use Strict Directive
Function code that is supplied as the arguments to the built-in
Function
Generator
AsyncFunction
, and
AsyncGenerator
constructors is strict mode code if the last argument is a String that when processed is a
FunctionBody
that begins with a
Directive Prologue
that contains a
Use Strict Directive
ECMAScript code that is not strict mode code is called
non-strict code
10.2.2
Non-ECMAScript Functions
An ECMAScript implementation may support the evaluation of
function exotic objects whose evaluative behaviour is expressed in some
implementation-defined form of executable code other than via ECMAScript
code. Whether a
function object
is an ECMAScript code function or a non-ECMAScript function is not
semantically observable from the perspective of an ECMAScript code
function that calls or is called by such a non-ECMAScript function.
11
ECMAScript Language: Lexical Grammar
The source text of an ECMAScript
Script
or
Module
is first converted into a sequence of input elements, which are tokens,
line terminators, comments, or white space. The source text is scanned
from left to right, repeatedly taking the longest possible sequence of
code points as the next input element.
There are several situations where the identification of lexical
input elements is sensitive to the syntactic grammar context that is
consuming the input elements. This requires multiple goal symbols for
the lexical grammar. The
InputElementRegExpOrTemplateTail
goal is used in syntactic grammar contexts where a
RegularExpressionLiteral
, a
TemplateMiddle
, or a
TemplateTail
is permitted. The
InputElementRegExp
goal symbol
is used in all syntactic grammar contexts where a
RegularExpressionLiteral
is permitted but neither a
TemplateMiddle
, nor a
TemplateTail
is permitted. The
InputElementTemplateTail
goal is used in all syntactic grammar contexts where a
TemplateMiddle
or a
TemplateTail
is permitted but a
RegularExpressionLiteral
is not permitted. In all other contexts,
InputElementDiv
is used as the lexical
goal symbol
Note
The use of multiple lexical goals ensures that there are no
lexical ambiguities that would affect automatic semicolon insertion. For
example, there are no syntactic grammar contexts where both a leading
division or division-assignment, and a leading
RegularExpressionLiteral
are permitted. This is not affected by semicolon insertion (see
11.9
); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a
LineTerminator
is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the
LineTerminator
. That is, the above example is interpreted in the same way as:
a = b / hi / g.exec(c).map(d);
Syntax
InputElementDiv
::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
RightBracePunctuator
InputElementRegExp
::
WhiteSpace
LineTerminator
Comment
CommonToken
RightBracePunctuator
RegularExpressionLiteral
InputElementRegExpOrTemplateTail
::
WhiteSpace
LineTerminator
Comment
CommonToken
RegularExpressionLiteral
TemplateSubstitutionTail
InputElementTemplateTail
::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
TemplateSubstitutionTail
11.1
Unicode Format-Control Characters
The Unicode format-control characters (i.e., the characters in
category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT
MARK or RIGHT-TO-LEFT MARK) are control codes used to control the
formatting of a range of text in the absence of higher-level protocols
for this (such as mark-up languages).
It is useful to allow format-control characters in source text to
facilitate editing and display. All format control characters may be
used within comments, and within string literals, template literals, and
regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are
format-control characters that are used to make necessary distinctions
when forming words or phrases in certain languages. In ECMAScript source
text these code points may also be used in an
IdentifierName
after the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character
used primarily at the start of a text to mark it as Unicode and to allow
detection of the text's encoding and byte order.
characters intended for this purpose can sometimes also appear after the
start of a text, for example as a result of concatenating files. In
ECMAScript source text code points are treated as white
space characters (see
11.2
).
The special treatment of certain format-control characters
outside of comments, string literals, and regular expression literals is
summarized in
Table 31
Table 31: Format-Control Code Point Usage
Code Point
Name
Abbreviation
Usage
U+200C
ZERO WIDTH NON-JOINER

IdentifierPart
U+200D
ZERO WIDTH JOINER

IdentifierPart
U+FEFF
ZERO WIDTH NO-BREAK SPACE

WhiteSpace
11.2
White Space
White space code points are used to improve source text
readability and to separate tokens (indivisible lexical units) from each
other, but are otherwise insignificant. White space code points may
occur between any two tokens and at the start or end of input. White
space code points may occur within a
StringLiteral
, a
RegularExpressionLiteral
, a
Template
, or a
TemplateSubstitutionTail
where they are considered significant code points forming part of a literal value. They may also occur within a
Comment
, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed in
Table 32
Table 32: White Space Code Points
Code Point
Name
Abbreviation
U+0009
CHARACTER TABULATION

U+000B
LINE TABULATION

U+000C
FORM FEED (FF)

U+0020
SPACE

U+00A0
NO-BREAK SPACE

U+FEFF
ZERO WIDTH NO-BREAK SPACE

Other category “Zs”
Any other Unicode “Space_Separator” code point

ECMAScript implementations must recognize as
WhiteSpace
code points listed in the “Space_Separator” (“Zs”) category.
Note
Other than for the code points listed in
Table 32
, ECMAScript
WhiteSpace
intentionally excludes all code points that have the Unicode
“White_Space” property but which are not classified in category
“Space_Separator” (“Zs”).
Syntax
WhiteSpace
::







11.3
Line Terminators
Like white space code points, line terminator code points are
used to improve source text readability and to separate tokens
(indivisible lexical units) from each other. However, unlike white space
code points, line terminators have some influence over the behaviour of
the syntactic grammar. In general, line terminators may occur between
any two tokens, but there are a few places where they are forbidden by
the syntactic grammar. Line terminators also affect the process of
automatic semicolon insertion (
11.9
). A line terminator cannot occur within any token except a
StringLiteral
Template
, or
TemplateSubstitutionTail
. and line terminators cannot occur within a
StringLiteral
token except as part of a
LineContinuation
A line terminator can occur within a
MultiLineComment
but cannot occur within a
SingleLineComment
Line terminators are included in the set of white space code points that are matched by the
\s
class in regular expressions.
The ECMAScript line terminator code points are listed in
Table 33
Table 33: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)

U+000D
CARRIAGE RETURN (CR)

U+2028
LINE SEPARATOR

U+2029
PARAGRAPH SEPARATOR

Only the Unicode code points in
Table 33
are treated as line terminators. Other new line or line breaking
Unicode code points are not treated as line terminators but are treated
as white space if they meet the requirements listed in
Table 32
. The sequence is commonly used as a line terminator. It should be considered a single
SourceCharacter
for the purpose of reporting line numbers.
Syntax
LineTerminator
::




LineTerminatorSequence
::


[lookahead ≠





11.4
Comments
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a
LineTerminator
code point, and because of the general rule that a token is always as
long as possible, a single-line comment always consists of all code
points from the
//
marker to the end of the line. However, the
LineTerminator
at the end of the line is not considered to be part of the single-line
comment; it is recognized separately by the lexical grammar and becomes
part of the stream of input elements for the syntactic grammar. This
point is very important, because it implies that the presence or absence
of single-line comments does not affect the process of automatic
semicolon insertion (see
11.9
).
Comments behave like white space and are discarded except that, if a
MultiLineComment
contains a line terminator code point, then the entire comment is considered to be a
LineTerminator
for purposes of parsing by the syntactic grammar.
Syntax
Comment
::
MultiLineComment
SingleLineComment
MultiLineComment
::
/*
MultiLineCommentChars
opt
*/
MultiLineCommentChars
::
MultiLineNotAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
PostAsteriskCommentChars
::
MultiLineNotForwardSlashOrAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
MultiLineNotAsteriskChar
::
SourceCharacter
but not
MultiLineNotForwardSlashOrAsteriskChar
::
SourceCharacter
but not one of
or
SingleLineComment
::
//
SingleLineCommentChars
opt
SingleLineCommentChars
::
SingleLineCommentChar
SingleLineCommentChars
opt
SingleLineCommentChar
::
SourceCharacter
but not
LineTerminator
11.5
Tokens
Syntax
CommonToken
::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
Template
Note
The
DivPunctuator
RegularExpressionLiteral
RightBracePunctuator
, and
TemplateSubstitutionTail
productions derive additional tokens that are not included in the
CommonToken
production.
11.6
Names and Keywords
IdentifierName
and
ReservedWord
are tokens that are interpreted according to the Default Identifier
Syntax given in Unicode Standard Annex #31, Identifier and Pattern
Syntax, with some small modifications.
ReservedWord
is an enumerated subset of
IdentifierName
. The syntactic grammar defines
Identifier
as an
IdentifierName
that is not a
ReservedWord
The Unicode identifier grammar is based on character properties
specified by the Unicode Standard. The Unicode code points in the
specified categories in the latest version of the Unicode standard must
be treated as in those categories by all conforming ECMAScript
implementations. ECMAScript implementations may recognize identifier
code points defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an
IdentifierName
and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO
WIDTH JOINER) are permitted anywhere after the first code point of an
IdentifierName
Unicode escape sequences are permitted in an
IdentifierName
, where they contribute a single Unicode code point to the
IdentifierName
. The code point is expressed by the
CodePoint
of the
UnicodeEscapeSequence
(see
11.8.4
). The
preceding the
UnicodeEscapeSequence
and the
and
{ }
code units, if they appear, do not contribute code points to the
IdentifierName
. A
UnicodeEscapeSequence
cannot be used to put a code point into an
IdentifierName
that would otherwise be illegal. In other words, if a
UnicodeEscapeSequence
sequence were replaced by the
SourceCharacter
it contributes, the result must still be a valid
IdentifierName
that has the exact same sequence of
SourceCharacter
elements as the original
IdentifierName
. All interpretations of
IdentifierName
within this specification are based upon their actual code points
regardless of whether or not an escape sequence was used to contribute
any particular code point.
Two
IdentifierName
s that are canonically equivalent according to the Unicode standard are
not
equal unless, after replacement of each
UnicodeEscapeSequence
, they are represented by the exact same sequence of code points.
Syntax
IdentifierName
::
IdentifierStart
IdentifierName
IdentifierPart
IdentifierStart
::
UnicodeIDStart
UnicodeEscapeSequence
IdentifierPart
::
UnicodeIDContinue
UnicodeEscapeSequence


UnicodeIDStart
::
any Unicode code point with the Unicode property “ID_Start”
UnicodeIDContinue
::
any Unicode code point with the Unicode property “ID_Continue”
The definitions of the nonterminal
UnicodeEscapeSequence
is given in
11.8.4
Note 2
The nonterminal
IdentifierPart
derives
via
UnicodeIDContinue
Note 3
The sets of code points with Unicode properties “ID_Start” and
“ID_Continue” include, respectively, the code points with Unicode
properties “Other_ID_Start” and “Other_ID_Continue”.
11.6.1
Identifier Names
11.6.1.1
Static Semantics: Early Errors
IdentifierStart
::
UnicodeEscapeSequence
It is a Syntax Error if SV(
UnicodeEscapeSequence
) is none of
"$"
, or
"_"
, or the
UTF16Encoding
of a code point matched by the
UnicodeIDStart
lexical grammar production.
IdentifierPart
::
UnicodeEscapeSequence
It is a Syntax Error if SV(
UnicodeEscapeSequence
) is none of
"$"
, or
"_"
, or the
UTF16Encoding
of either or , or the
UTF16Encoding
of a Unicode code point that would be matched by the
UnicodeIDContinue
lexical grammar production.
11.6.1.2
Static Semantics: StringValue
IdentifierName
::
IdentifierStart
IdentifierName
IdentifierPart
Return the String value consisting of the sequence of code units corresponding to
IdentifierName
. In determining the sequence any occurrences of
UnicodeEscapeSequence
are first replaced with the code point represented by the
UnicodeEscapeSequence
and then the code points of the entire
IdentifierName
are converted to code units by
UTF16Encoding
each code point.
11.6.2
Reserved Words
A reserved word is an
IdentifierName
that cannot be used as an
Identifier
Syntax
ReservedWord
::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
Note
The
ReservedWord
definitions are specified as literal sequences of specific
SourceCharacter
elements. A code point in a
ReservedWord
cannot be expressed by a
UnicodeEscapeSequence
11.6.2.1
Keywords
The following tokens are ECMAScript keywords and may not be used as
Identifier
s in ECMAScript programs.
Syntax
Keyword
::
one of
await
break
case
catch
class
const
continue
debugger
default
delete
do
else
export
extends
finally
for
function
if
import
in
instanceof
new
return
super
switch
this
throw
try
typeof
var
void
while
with
yield
Note
In some contexts
yield
and
await
are given the semantics of an
Identifier
. See
12.1.1
. In
strict mode code
let
and
static
are treated as reserved words through static semantic restrictions (see
12.1.1
13.3.1.1
13.7.5.1
, and
14.6.1
) rather than the lexical grammar.
11.6.2.2
Future Reserved Words
The following tokens are reserved for use as keywords in future language extensions.
Syntax
FutureReservedWord
::
enum
Note
Use of the following tokens within
strict mode code
is also reserved. That usage is restricted using static semantic restrictions (see
12.1.1
) rather than the lexical grammar:
implements
package
protected
interface
private
public
11.7
Punctuators
Syntax
Punctuator
::
one of
...
<=
>=
==
!=
===
!==
**
++
--
<<
>>
>>>
&&
||
+=
-=
*=
%=
**=
<<=
>>=
>>>=
&=
|=
^=
=>
DivPunctuator
::
/=
RightBracePunctuator
::
11.8
Literals
11.8.1
Null Literals
Syntax
NullLiteral
::
null
11.8.2
Boolean Literals
Syntax
BooleanLiteral
::
true
false
11.8.3
Numeric Literals
Syntax
NumericLiteral
::
DecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalIntegerLiteral
ExponentPart
opt
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
opt
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit
::
one of
NonZeroDigit
::
one of
ExponentPart
::
ExponentIndicator
SignedInteger
ExponentIndicator
::
one of
SignedInteger
::
DecimalDigits
DecimalDigits
DecimalDigits
BinaryIntegerLiteral
::
0b
BinaryDigits
0B
BinaryDigits
BinaryDigits
::
BinaryDigit
BinaryDigits
BinaryDigit
BinaryDigit
::
one of
OctalIntegerLiteral
::
0o
OctalDigits
0O
OctalDigits
OctalDigits
::
OctalDigit
OctalDigits
OctalDigit
OctalDigit
::
one of
HexIntegerLiteral
::
0x
HexDigits
0X
HexDigits
HexDigits
::
HexDigit
HexDigits
HexDigit
HexDigit
::
one of
The
SourceCharacter
immediately following a
NumericLiteral
must not be an
IdentifierStart
or
DecimalDigit
Note
For example:
3in
is an error and not the two input elements
and
in
A conforming implementation, when processing
strict mode code
, must not extend, as described in
B.1.1
, the syntax of
NumericLiteral
to include
LegacyOctalIntegerLiteral
, nor extend the syntax of
DecimalIntegerLiteral
to include
NonOctalDecimalIntegerLiteral
11.8.3.1
Static Semantics: MV
A numeric literal stands for a value of the Number type. This
value is determined in two steps: first, a mathematical value (MV) is
derived from the literal; second, this mathematical value is rounded as
described below.
The MV of
NumericLiteral
::
DecimalLiteral
is the MV of
DecimalLiteral
The MV of
NumericLiteral
::
BinaryIntegerLiteral
is the MV of
BinaryIntegerLiteral
The MV of
NumericLiteral
::
OctalIntegerLiteral
is the MV of
OctalIntegerLiteral
The MV of
NumericLiteral
::
HexIntegerLiteral
is the MV of
HexIntegerLiteral
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
is the MV of
DecimalIntegerLiteral
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
is the MV of
DecimalIntegerLiteral
plus (the MV of
DecimalDigits
× 10
), where
is the number of code points in
DecimalDigits
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
ExponentPart
is the MV of
DecimalIntegerLiteral
× 10
, where
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
ExponentPart
is (the MV of
DecimalIntegerLiteral
plus (the MV of
DecimalDigits
× 10
)) × 10
, where
is the number of code points in
DecimalDigits
and
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalDigits
is the MV of
DecimalDigits
× 10
, where
is the number of code points in
DecimalDigits
The MV of
DecimalLiteral
::
DecimalDigits
ExponentPart
is the MV of
DecimalDigits
× 10
, where
is the number of code points in
DecimalDigits
and
is the MV of
ExponentPart
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
is the MV of
DecimalIntegerLiteral
The MV of
DecimalLiteral
::
DecimalIntegerLiteral
ExponentPart
is the MV of
DecimalIntegerLiteral
× 10
, where
is the MV of
ExponentPart
The MV of
DecimalIntegerLiteral
::
is 0.
The MV of
DecimalIntegerLiteral
::
NonZeroDigit
is the MV of
NonZeroDigit
The MV of
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
is (the MV of
NonZeroDigit
× 10
) plus the MV of
DecimalDigits
, where
is the number of code points in
DecimalDigits
The MV of
DecimalDigits
::
DecimalDigit
is the MV of
DecimalDigit
The MV of
DecimalDigits
::
DecimalDigits
DecimalDigit
is (the MV of
DecimalDigits
× 10) plus the MV of
DecimalDigit
The MV of
ExponentPart
::
ExponentIndicator
SignedInteger
is the MV of
SignedInteger
The MV of
SignedInteger
::
DecimalDigits
is the MV of
DecimalDigits
The MV of
SignedInteger
::
DecimalDigits
is the MV of
DecimalDigits
The MV of
SignedInteger
::
DecimalDigits
is the negative of the MV of
DecimalDigits
The MV of
DecimalDigit
::
or of
HexDigit
::
or of
OctalDigit
::
or of
BinaryDigit
::
is 0.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
or of
BinaryDigit
::
is 1.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 2.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 3.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 4.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 5.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 6.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
or of
OctalDigit
::
is 7.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 8.
The MV of
DecimalDigit
::
or of
NonZeroDigit
::
or of
HexDigit
::
is 9.
The MV of
HexDigit
::
or of
HexDigit
::
is 10.
The MV of
HexDigit
::
or of
HexDigit
::
is 11.
The MV of
HexDigit
::
or of
HexDigit
::
is 12.
The MV of
HexDigit
::
or of
HexDigit
::
is 13.
The MV of
HexDigit
::
or of
HexDigit
::
is 14.
The MV of
HexDigit
::
or of
HexDigit
::
is 15.
The MV of
BinaryIntegerLiteral
::
0b
BinaryDigits
is the MV of
BinaryDigits
The MV of
BinaryIntegerLiteral
::
0B
BinaryDigits
is the MV of
BinaryDigits
The MV of
BinaryDigits
::
BinaryDigit
is the MV of
BinaryDigit
The MV of
BinaryDigits
::
BinaryDigits
BinaryDigit
is (the MV of
BinaryDigits
× 2) plus the MV of
BinaryDigit
The MV of
OctalIntegerLiteral
::
0o
OctalDigits
is the MV of
OctalDigits
The MV of
OctalIntegerLiteral
::
0O
OctalDigits
is the MV of
OctalDigits
The MV of
OctalDigits
::
OctalDigit
is the MV of
OctalDigit
The MV of
OctalDigits
::
OctalDigits
OctalDigit
is (the MV of
OctalDigits
× 8) plus the MV of
OctalDigit
The MV of
HexIntegerLiteral
::
0x
HexDigits
is the MV of
HexDigits
The MV of
HexIntegerLiteral
::
0X
HexDigits
is the MV of
HexDigits
The MV of
HexDigits
::
HexDigit
is the MV of
HexDigit
The MV of
HexDigits
::
HexDigits
HexDigit
is (the MV of
HexDigits
× 16) plus the MV of
HexDigit
Once the exact MV for a numeric literal has been determined,
it is then rounded to a value of the Number type. If the MV is 0, then
the rounded value is
+0
; otherwise, the rounded value must be the Number value for the MV (as specified in
6.1.6
), unless the literal is a
DecimalLiteral
and the literal has more than 20 significant digits, in which case the
Number value may be either the Number value for the MV of a literal
produced by replacing each significant digit after the 20th with a
digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a
digit and then incrementing the literal at the 20th significant digit position. A digit is
significant
if it is not part of an
ExponentPart
and
it is not
; or
there is a nonzero digit to its left and there is a nonzero digit, not in the
ExponentPart
, to its right.
11.8.4
String Literals
Note 1
A string literal is zero or more Unicode code points enclosed
in single or double quotes. Unicode code points may also be represented
by an escape sequence. All code points may appear literally in a string
literal except for the closing quote code points, U+005C (REVERSE
SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code
points may appear in the form of an escape sequence. String literals
evaluate to ECMAScript String values. When generating these String
values Unicode code points are UTF-16 encoded as defined in
10.1.1
Code points belonging to the Basic Multilingual Plane are encoded as a
single code unit element of the string. All other code points are
encoded as two code unit elements of the string.
Syntax
StringLiteral
::
DoubleStringCharacters
opt
SingleStringCharacters
opt
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
opt
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters
opt
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator


EscapeSequence
LineContinuation
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator


EscapeSequence
LineContinuation
LineContinuation
::
LineTerminatorSequence
EscapeSequence
::
CharacterEscapeSequence
[lookahead ∉
DecimalDigit
HexEscapeSequence
UnicodeEscapeSequence
A conforming implementation, when processing
strict mode code
, must not extend the syntax of
EscapeSequence
to include
LegacyOctalEscapeSequence
as described in
B.1.2
CharacterEscapeSequence
::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter
::
one of
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
EscapeCharacter
::
SingleEscapeCharacter
DecimalDigit
HexEscapeSequence
::
HexDigit
HexDigit
UnicodeEscapeSequence
::
Hex4Digits
u{
CodePoint
Hex4Digits
::
HexDigit
HexDigit
HexDigit
HexDigit
The definition of the nonterminal
HexDigit
is given in
11.8.3
SourceCharacter
is defined in
10.1
Note 2
and cannot appear in a string literal, except as part of a
LineContinuation
to produce the empty code points sequence. The proper way to include
either in the String value of a string literal is to use an escape
sequence such as
\n
or
\u000A
11.8.4.1
Static Semantics: StringValue
StringLiteral
::
DoubleStringCharacters
opt
SingleStringCharacters
opt
Return the String value whose code units are the SV of this
StringLiteral
11.8.4.2
Static Semantics: SV
A string literal stands for a value of the String type. The
String value (SV) of the literal is described in terms of code unit
values contributed by the various parts of the string literal. As part
of this process, some Unicode code points within the string literal are
interpreted as having a mathematical value (MV), as described below or
in
11.8.3
The SV of
StringLiteral
::
is the empty code unit sequence.
The SV of
StringLiteral
::
is the empty code unit sequence.
The SV of
StringLiteral
::
DoubleStringCharacters
is the SV of
DoubleStringCharacters
The SV of
StringLiteral
::
SingleStringCharacters
is the SV of
SingleStringCharacters
The SV of
DoubleStringCharacters
::
DoubleStringCharacter
is a sequence of up to two code units that is the SV of
DoubleStringCharacter
The SV of
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
is a sequence of up to two code units that is the SV of
DoubleStringCharacter
followed by the code units of the SV of
DoubleStringCharacters
in order.
The SV of
SingleStringCharacters
::
SingleStringCharacter
is a sequence of up to two code units that is the SV of
SingleStringCharacter
The SV of
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters
is a sequence of up to two code units that is the SV of
SingleStringCharacter
followed by the code units of the SV of
SingleStringCharacters
in order.
The SV of
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
is the
UTF16Encoding
of the code point value of
SourceCharacter
The SV of
DoubleStringCharacter
::

is the code unit 0x2028 (LINE SEPARATOR).
The SV of
DoubleStringCharacter
::

is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of
DoubleStringCharacter
::
EscapeSequence
is the SV of the
EscapeSequence
The SV of
DoubleStringCharacter
::
LineContinuation
is the empty code unit sequence.
The SV of
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator
is the
UTF16Encoding
of the code point value of
SourceCharacter
The SV of
SingleStringCharacter
::

is the code unit 0x2028 (LINE SEPARATOR).
The SV of
SingleStringCharacter
::

is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The SV of
SingleStringCharacter
::
EscapeSequence
is the SV of the
EscapeSequence
The SV of
SingleStringCharacter
::
LineContinuation
is the empty code unit sequence.
The SV of
EscapeSequence
::
CharacterEscapeSequence
is the SV of the
CharacterEscapeSequence
The SV of
EscapeSequence
::
is the code unit 0x0000 (NULL).
The SV of
EscapeSequence
::
HexEscapeSequence
is the SV of the
HexEscapeSequence
The SV of
EscapeSequence
::
UnicodeEscapeSequence
is the SV of the
UnicodeEscapeSequence
The SV of
CharacterEscapeSequence
::
SingleEscapeCharacter
is the code unit whose value is determined by the
SingleEscapeCharacter
according to
Table 34
Table 34: String Single Character Escape Sequences
Escape Sequence
Code Unit Value
Unicode Character Name
Symbol
\b
0x0008
BACKSPACE

\t
0x0009
CHARACTER TABULATION

\n
0x000A
LINE FEED (LF)

\v
0x000B
LINE TABULATION

\f
0x000C
FORM FEED (FF)

\r
0x000D
CARRIAGE RETURN (CR)

\"
0x0022
QUOTATION MARK
\'
0x0027
APOSTROPHE
\\
0x005C
REVERSE SOLIDUS
The SV of
CharacterEscapeSequence
::
NonEscapeCharacter
is the SV of the
NonEscapeCharacter
The SV of
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
is the
UTF16Encoding
of the code point value of
SourceCharacter
The SV of
HexEscapeSequence
::
HexDigit
HexDigit
is the code unit whose value is (16 times the MV of the first
HexDigit
) plus the MV of the second
HexDigit
The SV of
UnicodeEscapeSequence
::
Hex4Digits
is the SV of
Hex4Digits
The SV of
Hex4Digits
::
HexDigit
HexDigit
HexDigit
HexDigit
is the code unit whose value is (0x1000 times the MV of the first
HexDigit
) plus (0x100 times the MV of the second
HexDigit
) plus (0x10 times the MV of the third
HexDigit
) plus the MV of the fourth
HexDigit
The SV of
UnicodeEscapeSequence
::
u{
CodePoint
is the
UTF16Encoding
of the MV of
CodePoint
11.8.5
Regular Expression Literals
Note 1
A regular expression literal is an input element that is converted to a RegExp object (see
21.2
each time the literal is evaluated. Two regular expression literals in a
program evaluate to regular expression objects that never compare as
===
to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by
new RegExp
or calling the
RegExp
constructor
as a function (see
21.2.3
).
The productions below describe the syntax for a regular
expression literal and are used by the input element scanner to find the
end of the regular expression literal. The source text comprising the
RegularExpressionBody
and the
RegularExpressionFlags
are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (
21.2.1
).
An implementation may extend the ECMAScript Regular Expression grammar defined in
21.2.1
, but it must not extend the
RegularExpressionBody
and
RegularExpressionFlags
productions defined below or the productions used by these productions.
Syntax
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
RegularExpressionBody
::
RegularExpressionFirstChar
RegularExpressionChars
RegularExpressionChars
::
[empty]
RegularExpressionChars
RegularExpressionChar
RegularExpressionFirstChar
::
RegularExpressionNonTerminator
but not one of
or
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar
::
RegularExpressionNonTerminator
but not one of
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence
::
RegularExpressionNonTerminator
RegularExpressionNonTerminator
::
SourceCharacter
but not
LineTerminator
RegularExpressionClass
::
RegularExpressionClassChars
RegularExpressionClassChars
::
[empty]
RegularExpressionClassChars
RegularExpressionClassChar
RegularExpressionClassChar
::
RegularExpressionNonTerminator
but not one of
or
RegularExpressionBackslashSequence
RegularExpressionFlags
::
[empty]
RegularExpressionFlags
IdentifierPart
Note 2
Regular expression literals may not be empty; instead of
representing an empty regular expression literal, the code unit sequence
//
starts a single-line comment. To specify an empty regular expression, use:
/(?:)/
11.8.5.1
Static Semantics: Early Errors
RegularExpressionFlags
::
RegularExpressionFlags
IdentifierPart
It is a Syntax Error if
IdentifierPart
contains a Unicode escape sequence.
11.8.5.2
Static Semantics: BodyText
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
Return the source text that was recognized as
RegularExpressionBody
11.8.5.3
Static Semantics: FlagText
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
Return the source text that was recognized as
RegularExpressionFlags
11.8.6
Template Literal Lexical Components
Syntax
Template
::
NoSubstitutionTemplate
TemplateHead
NoSubstitutionTemplate
::
TemplateCharacters
opt
TemplateHead
::
TemplateCharacters
opt
${
TemplateSubstitutionTail
::
TemplateMiddle
TemplateTail
TemplateMiddle
::
TemplateCharacters
opt
${
TemplateTail
::
TemplateCharacters
opt
TemplateCharacters
::
TemplateCharacter
TemplateCharacters
opt
TemplateCharacter
::
[lookahead ≠
EscapeSequence
NotEscapeSequence
LineContinuation
LineTerminatorSequence
SourceCharacter
but not one of
or
or
or
LineTerminator
NotEscapeSequence
::
DecimalDigit
DecimalDigit
but not
[lookahead ∉
HexDigit
HexDigit
[lookahead ∉
HexDigit
[lookahead ∉
HexDigit
[lookahead ≠
HexDigit
[lookahead ∉
HexDigit
HexDigit
HexDigit
[lookahead ∉
HexDigit
HexDigit
HexDigit
HexDigit
[lookahead ∉
HexDigit
[lookahead ∉
HexDigit
NotCodePoint
[lookahead ∉
HexDigit
CodePoint
[lookahead ∉
HexDigit
[lookahead ≠
NotCodePoint
::
HexDigits
but only if MV of
HexDigits
> 0x10FFFF
CodePoint
::
HexDigits
but only if MV of
HexDigits
≤ 0x10FFFF
A conforming implementation must not use the extended definition of
EscapeSequence
described in
B.1.2
when parsing a
TemplateCharacter
Note
TemplateSubstitutionTail
is used by the
InputElementTemplateTail
alternative lexical goal.
11.8.6.1
Static Semantics: TV and TRV
A template literal component is interpreted as a sequence of
Unicode code points. The Template Value (TV) of a literal component is
described in terms of code unit values (SV,
11.8.4
contributed by the various parts of the template literal component. As
part of this process, some Unicode code points within the template
component are interpreted as having a mathematical value (MV,
11.8.3
).
In determining a TV, escape sequences are replaced by the UTF-16 code
unit(s) of the Unicode code point represented by the escape sequence.
The Template Raw Value (TRV) is similar to a Template Value with the
difference that in TRVs escape sequences are interpreted literally.
The TV and TRV of
NoSubstitutionTemplate
::
is the empty code unit sequence.
The TV and TRV of
TemplateHead
::
${
is the empty code unit sequence.
The TV and TRV of
TemplateMiddle
::
${
is the empty code unit sequence.
The TV and TRV of
TemplateTail
::
is the empty code unit sequence.
The TV of
NoSubstitutionTemplate
::
TemplateCharacters
is the TV of
TemplateCharacters
The TV of
TemplateHead
::
TemplateCharacters
${
is the TV of
TemplateCharacters
The TV of
TemplateMiddle
::
TemplateCharacters
${
is the TV of
TemplateCharacters
The TV of
TemplateTail
::
TemplateCharacters
is the TV of
TemplateCharacters
The TV of
TemplateCharacters
::
TemplateCharacter
is the TV of
TemplateCharacter
The TV of
TemplateCharacters
::
TemplateCharacter
TemplateCharacters
is
undefined
if either the TV of
TemplateCharacter
is
undefined
or the TV of
TemplateCharacters
is
undefined
. Otherwise, it is a sequence consisting of the code units of the TV of
TemplateCharacter
followed by the code units of the TV of
TemplateCharacters
The TV of
TemplateCharacter
::
SourceCharacter
but not one of
or
or
or
LineTerminator
is the
UTF16Encoding
of the code point value of
SourceCharacter
The TV of
TemplateCharacter
::
is the code unit 0x0024 (DOLLAR SIGN).
The TV of
TemplateCharacter
::
EscapeSequence
is the SV of
EscapeSequence
The TV of
TemplateCharacter
::
NotEscapeSequence
is
undefined
The TV of
TemplateCharacter
::
LineContinuation
is the TV of
LineContinuation
The TV of
TemplateCharacter
::
LineTerminatorSequence
is the TRV of
LineTerminatorSequence
The TV of
LineContinuation
::
LineTerminatorSequence
is the empty code unit sequence.
The TRV of
NoSubstitutionTemplate
::
TemplateCharacters
is the TRV of
TemplateCharacters
The TRV of
TemplateHead
::
TemplateCharacters
${
is the TRV of
TemplateCharacters
The TRV of
TemplateMiddle
::
TemplateCharacters
${
is the TRV of
TemplateCharacters
The TRV of
TemplateTail
::
TemplateCharacters
is the TRV of
TemplateCharacters
The TRV of
TemplateCharacters
::
TemplateCharacter
is the TRV of
TemplateCharacter
The TRV of
TemplateCharacters
::
TemplateCharacter
TemplateCharacters
is a sequence consisting of the code units of the TRV of
TemplateCharacter
followed by the code units of the TRV of
TemplateCharacters
The TRV of
TemplateCharacter
::
SourceCharacter
but not one of
or
or
or
LineTerminator
is the
UTF16Encoding
of the code point value of
SourceCharacter
The TRV of
TemplateCharacter
::
is the code unit 0x0024 (DOLLAR SIGN).
The TRV of
TemplateCharacter
::
EscapeSequence
is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of
EscapeSequence
The TRV of
TemplateCharacter
::
NotEscapeSequence
is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of
NotEscapeSequence
The TRV of
TemplateCharacter
::
LineContinuation
is the TRV of
LineContinuation
The TRV of
TemplateCharacter
::
LineTerminatorSequence
is the TRV of
LineTerminatorSequence
The TRV of
EscapeSequence
::
CharacterEscapeSequence
is the TRV of the
CharacterEscapeSequence
The TRV of
EscapeSequence
::
is the code unit 0x0030 (DIGIT ZERO).
The TRV of
EscapeSequence
::
HexEscapeSequence
is the TRV of the
HexEscapeSequence
The TRV of
EscapeSequence
::
UnicodeEscapeSequence
is the TRV of the
UnicodeEscapeSequence
The TRV of
NotEscapeSequence
::
DecimalDigit
is the sequence consisting of the code unit 0x0030 (DIGIT ZERO) followed by the code units of the TRV of
DecimalDigit
The TRV of
NotEscapeSequence
::
[lookahead ∉
HexDigit
is the code unit 0x0078 (LATIN SMALL LETTER X).
The TRV of
NotEscapeSequence
::
HexDigit
[lookahead ∉
HexDigit
is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by the code units of the TRV of
HexDigit
The TRV of
NotEscapeSequence
::
[lookahead ∉
HexDigit
[lookahead ≠
is the code unit 0x0075 (LATIN SMALL LETTER U).
The TRV of
NotEscapeSequence
::
HexDigit
[lookahead ∉
HexDigit
is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of
HexDigit
The TRV of
NotEscapeSequence
::
HexDigit
HexDigit
[lookahead ∉
HexDigit
is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first
HexDigit
followed by the code units of the TRV of the second
HexDigit
The TRV of
NotEscapeSequence
::
HexDigit
HexDigit
HexDigit
[lookahead ∉
HexDigit
is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by the code units of the TRV of the first
HexDigit
followed by the code units of the TRV of the second
HexDigit
followed by the code units of the TRV of the third
HexDigit
The TRV of
NotEscapeSequence
::
[lookahead ∉
HexDigit
is the sequence consisting of the code
unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B
(LEFT CURLY BRACKET).
The TRV of
NotEscapeSequence
::
NotCodePoint
[lookahead ∉
HexDigit
is the sequence consisting of the code
unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B
(LEFT CURLY BRACKET) followed by the code units of the TRV of
NotCodePoint
The TRV of
NotEscapeSequence
::
CodePoint
[lookahead ∉
HexDigit
[lookahead ≠
is the sequence consisting of the code
unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B
(LEFT CURLY BRACKET) followed by the code units of the TRV of
CodePoint
The TRV of
DecimalDigit
::
one of
is the SV of the
SourceCharacter
that is that single code point.
The TRV of
CharacterEscapeSequence
::
SingleEscapeCharacter
is the TRV of the
SingleEscapeCharacter
The TRV of
CharacterEscapeSequence
::
NonEscapeCharacter
is the SV of the
NonEscapeCharacter
The TRV of
SingleEscapeCharacter
::
one of
is the SV of the
SourceCharacter
that is that single code point.
The TRV of
HexEscapeSequence
::
HexDigit
HexDigit
is the sequence consisting of the code unit 0x0078 (LATIN SMALL LETTER X) followed by TRV of the first
HexDigit
followed by the TRV of the second
HexDigit
The TRV of
UnicodeEscapeSequence
::
Hex4Digits
is the sequence consisting of the code unit 0x0075 (LATIN SMALL LETTER U) followed by TRV of
Hex4Digits
The TRV of
UnicodeEscapeSequence
::
u{
CodePoint
is the sequence consisting of the code
unit 0x0075 (LATIN SMALL LETTER U) followed by the code unit 0x007B
(LEFT CURLY BRACKET) followed by TRV of
CodePoint
followed by the code unit 0x007D (RIGHT CURLY BRACKET).
The TRV of
Hex4Digits
::
HexDigit
HexDigit
HexDigit
HexDigit
is the sequence consisting of the TRV of the first
HexDigit
followed by the TRV of the second
HexDigit
followed by the TRV of the third
HexDigit
followed by the TRV of the fourth
HexDigit
The TRV of
HexDigits
::
HexDigit
is the TRV of
HexDigit
The TRV of
HexDigits
::
HexDigits
HexDigit
is the sequence consisting of TRV of
HexDigits
followed by TRV of
HexDigit
The TRV of a
HexDigit
is the SV of the
SourceCharacter
that is that
HexDigit
The TRV of
LineContinuation
::
LineTerminatorSequence
is the sequence consisting of the code unit 0x005C (REVERSE SOLIDUS) followed by the code units of TRV of
LineTerminatorSequence
The TRV of
LineTerminatorSequence
::

is the code unit 0x000A (LINE FEED).
The TRV of
LineTerminatorSequence
::

is the code unit 0x000A (LINE FEED).
The TRV of
LineTerminatorSequence
::

is the code unit 0x2028 (LINE SEPARATOR).
The TRV of
LineTerminatorSequence
::

is the code unit 0x2029 (PARAGRAPH SEPARATOR).
The TRV of
LineTerminatorSequence
::


is the sequence consisting of the code unit 0x000A (LINE FEED).
Note
TV excludes the code units of
LineContinuation
while TRV includes them. and
LineTerminatorSequence
s are normalized to for both TV and TRV. An explicit
EscapeSequence
is needed to include a or sequence.
11.9
Automatic Semicolon Insertion
Most ECMAScript statements and declarations must be terminated
with a semicolon. Such semicolons may always appear explicitly in the
source text. For convenience, however, such semicolons may be omitted
from the source text in certain situations. These situations are
described by saying that semicolons are automatically inserted into the
source code token stream in those situations.
11.9.1
Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current lexical
goal symbol
as described in clause
11
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the
offending token
is encountered that is not allowed by any production of the grammar,
then a semicolon is automatically inserted before the offending token if
one or more of the following conditions is true:
The offending token is separated from the previous token by at least one
LineTerminator
The offending token is
The previous token is
and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (
13.7.2
).
When, as the source text is parsed from left to right, the end
of the input stream of tokens is encountered and the parser is unable
to parse the input token stream as a single instance of the goal
nonterminal, then a semicolon is automatically inserted at the end of
the input stream.
When, as the source text is parsed from left to right, a token
is encountered that is allowed by some production of the grammar, but
the production is a
restricted production
and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no
LineTerminator
here]” within the restricted production (and therefore such a token is
called a restricted token), and the restricted token is separated from
the previous token by at least one
LineTerminator
, then a semicolon is automatically inserted before the restricted token.
However, there is an additional overriding condition on the
preceding rules: a semicolon is never inserted automatically if the
semicolon would then be parsed as an empty statement or if that
semicolon would become one of the two semicolons in the header of a
for
statement (see
13.7.4
).
Note
The following are the only restricted productions in the grammar:
UpdateExpression
[Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
++
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
--
ContinueStatement
[Yield, Await]
continue
continue
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
BreakStatement
[Yield, Await]
break
break
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
ReturnStatement
[Yield, Await]
return
return
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
ThrowStatement
[Yield, Await]
throw
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
ArrowFunction
[In, Yield, Await]
ArrowParameters
[?Yield, ?Await]
[no
LineTerminator
here]
=>
ConciseBody
[?In]
YieldExpression
[In, Await]
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
The practical effect of these restricted productions is as follows:
When a
++
or
--
token is encountered where the parser would treat it as a postfix operator, and at least one
LineTerminator
occurred between the preceding token and the
++
or
--
token, then a semicolon is automatically inserted before the
++
or
--
token.
When a
continue
break
return
throw
, or
yield
token is encountered and a
LineTerminator
is encountered before the next token, a semicolon is automatically inserted after the
continue
break
return
throw
, or
yield
token.
The resulting practical advice to ECMAScript programmers is:
A postfix
++
or
--
operator should appear on the same line as its operand.
An
Expression
in a
return
or
throw
statement or an
AssignmentExpression
in a
yield
expression should start on the same line as the
return
throw
, or
yield
token.
LabelIdentifier
in a
break
or
continue
statement should be on the same line as the
break
or
continue
token.
11.9.2
Examples of Automatic Semicolon Insertion
The source
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:
;}
which is a valid ECMAScript sentence.
The source
for
(a; b
is not a valid ECMAScript sentence and is not altered by
automatic semicolon insertion because the semicolon is needed for the
header of a
for
statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a
for
statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return
a + b;
Note 1
The expression
a + b
is not treated as a value to be returned by the
return
statement, because a
LineTerminator
separates it from the token
return
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token
++
is not treated as a postfix operator applying to the variable
, because a
LineTerminator
occurs between
and
++
The source
if
(a > b)
else
c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the
else
token, even though no production of the grammar applies at that point,
because an automatically inserted semicolon would then be parsed as an
empty statement.
The source
a = b + c
(d + e).print()
is
not
transformed by automatic semicolon insertion,
because the parenthesized expression that begins the second line can be
interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin
with a left parenthesis, it is a good idea for the programmer to provide
an explicit semicolon at the end of the preceding statement rather than
to rely on automatic semicolon insertion.
12
ECMAScript Language: Expressions
12.1
Identifiers
Syntax
IdentifierReference
[Yield, Await]
Identifier
[~Yield]
yield
[~Await]
await
BindingIdentifier
[Yield, Await]
Identifier
yield
await
LabelIdentifier
[Yield, Await]
Identifier
[~Yield]
yield
[~Await]
await
Identifier
IdentifierName
but not
ReservedWord
Note
yield
and
await
are permitted as
BindingIdentifier
in the grammar, and prohibited with
static semantics
below, to prohibit automatic semicolon insertion in cases such as
let
await
12.1.1
Static Semantics: Early Errors
BindingIdentifier
Identifier
It is a Syntax Error if the code matched by this production is contained in
strict mode code
and the StringValue of
Identifier
is
"arguments"
or
"eval"
IdentifierReference
yield
BindingIdentifier
yield
LabelIdentifier
yield
It is a Syntax Error if the code matched by this production is contained in
strict mode code
IdentifierReference
await
BindingIdentifier
await
LabelIdentifier
await
It is a Syntax Error if the
goal symbol
of the syntactic grammar is
Module
BindingIdentifier
yield
It is a Syntax Error if this production has a
[Yield]
parameter.
BindingIdentifier
await
It is a Syntax Error if this production has an
Await
parameter.
IdentifierReference
[Yield, Await]
Identifier
BindingIdentifier
[Yield, Await]
Identifier
LabelIdentifier
[Yield, Await]
Identifier
It is a Syntax Error if this production has a
[Yield]
parameter and StringValue of
Identifier
is
"yield"
It is a Syntax Error if this production has an
Await
parameter and StringValue of
Identifier
is
"await"
Identifier
IdentifierName
but not
ReservedWord
It is a Syntax Error if this phrase is contained in
strict mode code
and the StringValue of
IdentifierName
is:
"implements"
"interface"
"let"
"package"
"private"
"protected"
"public"
"static"
, or
"yield"
It is a Syntax Error if the
goal symbol
of the syntactic grammar is
Module
and the StringValue of
IdentifierName
is
"await"
It is a Syntax Error if StringValue of
IdentifierName
is the same String value as the StringValue of any
ReservedWord
except for
yield
or
await
Note
StringValue of
IdentifierName
normalizes any Unicode escape sequences in
IdentifierName
hence such escapes cannot be used to write an
Identifier
whose code point sequence is the same as a
ReservedWord
12.1.2
Static Semantics: BoundNames
BindingIdentifier
Identifier
Return a new
List
containing the StringValue of
Identifier
BindingIdentifier
yield
Return a new
List
containing
"yield"
BindingIdentifier
await
Return a new
List
containing
"await"
12.1.3
Static Semantics: AssignmentTargetType
IdentifierReference
Identifier
If this
IdentifierReference
is contained in
strict mode code
and StringValue of
Identifier
is
"eval"
or
"arguments"
, return
strict
Return
simple
IdentifierReference
yield
Return
simple
IdentifierReference
await
Return
simple
12.1.4
Static Semantics: StringValue
IdentifierReference
yield
BindingIdentifier
yield
LabelIdentifier
yield
Return
"yield"
IdentifierReference
await
BindingIdentifier
await
LabelIdentifier
await
Return
"await"
Identifier
IdentifierName
but not
ReservedWord
Return the StringValue of
IdentifierName
12.1.5
Runtime Semantics: BindingInitialization
With parameters
value
and
environment
Note
undefined
is passed for
environment
to indicate that a
PutValue
operation should be used to assign the initialization value. This is the case for
var
statements and formal parameter lists of some non-strict functions (See
9.2.15
). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
BindingIdentifier
Identifier
Let
name
be StringValue of
Identifier
Return ?
InitializeBoundName
name
value
environment
).
BindingIdentifier
yield
Return ?
InitializeBoundName
"yield"
value
environment
).
BindingIdentifier
await
Return ?
InitializeBoundName
"await"
value
environment
).
12.1.5.1
Runtime Semantics: InitializeBoundName (
name
value
environment
Assert
Type
name
) is String.
If
environment
is not
undefined
, then
Let
env
be the
EnvironmentRecord
component of
environment
Perform
env
.InitializeBinding(
name
value
).
Return
NormalCompletion
undefined
).
Else,
Let
lhs
be
ResolveBinding
name
).
Return ?
PutValue
lhs
value
).
12.1.6
Runtime Semantics: Evaluation
IdentifierReference
Identifier
Return ?
ResolveBinding
(StringValue of
Identifier
).
IdentifierReference
yield
Return ?
ResolveBinding
"yield"
).
IdentifierReference
await
Return ?
ResolveBinding
"await"
).
Note 1
The result of evaluating an
IdentifierReference
is always a value of type
Reference
Note 2
In
non-strict code
, the keyword
yield
may be used as an identifier. Evaluating the
IdentifierReference
resolves the binding of
yield
as if it was an
Identifier
. Early Error restriction ensures that such an evaluation only can occur for
non-strict code
12.2
Primary Expression
Syntax
PrimaryExpression
[Yield, Await]
this
IdentifierReference
[?Yield, ?Await]
Literal
ArrayLiteral
[?Yield, ?Await]
ObjectLiteral
[?Yield, ?Await]
FunctionExpression
ClassExpression
[?Yield, ?Await]
GeneratorExpression
AsyncFunctionExpression
AsyncGeneratorExpression
RegularExpressionLiteral
TemplateLiteral
[?Yield, ?Await, ~Tagged]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList
[Yield, Await]
Expression
[+In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingIdentifier
[?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingIdentifier
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
Supplemental Syntax
When processing an instance of the production
PrimaryExpression
[Yield, Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
the interpretation of
CoverParenthesizedExpressionAndArrowParameterList
is refined using the following grammar:
ParenthesizedExpression
[Yield, Await]
Expression
[+In, ?Yield, ?Await]
12.2.1
Semantics
12.2.1.1
Static Semantics: CoveredParenthesizedExpression
CoverParenthesizedExpressionAndArrowParameterList
Expression
Return the
ParenthesizedExpression
that is
covered
by
CoverParenthesizedExpressionAndArrowParameterList
12.2.1.2
Static Semantics: HasName
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
If IsFunctionDefinition of
expr
is
false
, return
false
Return HasName of
expr
12.2.1.3
Static Semantics: IsFunctionDefinition
PrimaryExpression
this
IdentifierReference
Literal
ArrayLiteral
ObjectLiteral
RegularExpressionLiteral
TemplateLiteral
Return
false
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
Return IsFunctionDefinition of
expr
12.2.1.4
Static Semantics: IsIdentifierRef
PrimaryExpression
IdentifierReference
Return
true
PrimaryExpression
this
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
AsyncGeneratorExpression
RegularExpressionLiteral
TemplateLiteral
CoverParenthesizedExpressionAndArrowParameterList
Return
false
12.2.1.5
Static Semantics: AssignmentTargetType
PrimaryExpression
this
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
AsyncGeneratorExpression
RegularExpressionLiteral
TemplateLiteral
Return
invalid
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
Return AssignmentTargetType of
expr
12.2.2
The
this
Keyword
12.2.2.1
Runtime Semantics: Evaluation
PrimaryExpression
this
Return ?
ResolveThisBinding
().
12.2.3
Identifier Reference
See
12.1
for
IdentifierReference
12.2.4
Literals
Syntax
Literal
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
12.2.4.1
Runtime Semantics: Evaluation
Literal
NullLiteral
Return
null
Literal
BooleanLiteral
If
BooleanLiteral
is the token
false
, return
false
If
BooleanLiteral
is the token
true
, return
true
Literal
NumericLiteral
Return the number whose value is MV of
NumericLiteral
as defined in
11.8.3
Literal
StringLiteral
Return the StringValue of
StringLiteral
as defined in
11.8.4.1
12.2.5
Array Initializer
Note
An
ArrayLiteral
is an expression describing the initialization of an Array object,
using a list, of zero or more expressions each of which represents an
array element, enclosed in square brackets. The elements need not be
literals; they are evaluated each time the array initializer is
evaluated.
Array elements may be elided at the beginning, middle or end of
the element list. Whenever a comma in the element list is not preceded
by an
AssignmentExpression
(i.e., a comma at the beginning or after another comma), the missing
array element contributes to the length of the Array and increases the
index of subsequent elements. Elided array elements are not defined. If
an element is elided at the end of an array, that element does not
contribute to the length of the Array.
Syntax
ArrayLiteral
[Yield, Await]
Elision
opt
ElementList
[?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
ElementList
[Yield, Await]
Elision
opt
AssignmentExpression
[+In, ?Yield, ?Await]
Elision
opt
SpreadElement
[?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
AssignmentExpression
[+In, ?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
SpreadElement
[?Yield, ?Await]
Elision
Elision
SpreadElement
[Yield, Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
12.2.5.1
Static Semantics: ElisionWidth
Elision
Return the numeric value 1.
Elision
Elision
Let
preceding
be the ElisionWidth of
Elision
Return
preceding
+ 1.
12.2.5.2
Runtime Semantics: ArrayAccumulation
With parameters
array
and
nextIndex
ElementList
Elision
opt
AssignmentExpression
Let
padding
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Let
initResult
be the result of evaluating
AssignmentExpression
Let
initValue
be ?
GetValue
initResult
).
Let
created
be
CreateDataProperty
array
ToString
ToUint32
nextIndex
padding
)),
initValue
).
Assert
created
is
true
Return
nextIndex
padding
+ 1.
ElementList
Elision
opt
SpreadElement
Let
padding
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Return the result of performing ArrayAccumulation for
SpreadElement
with arguments
array
and
nextIndex
padding
ElementList
ElementList
Elision
opt
AssignmentExpression
Let
postIndex
be the result of performing ArrayAccumulation for
ElementList
with arguments
array
and
nextIndex
ReturnIfAbrupt
postIndex
).
Let
padding
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Let
initResult
be the result of evaluating
AssignmentExpression
Let
initValue
be ?
GetValue
initResult
).
Let
created
be
CreateDataProperty
array
ToString
ToUint32
postIndex
padding
)),
initValue
).
Assert
created
is
true
Return
postIndex
padding
+ 1.
ElementList
ElementList
Elision
opt
SpreadElement
Let
postIndex
be the result of performing ArrayAccumulation for
ElementList
with arguments
array
and
nextIndex
ReturnIfAbrupt
postIndex
).
Let
padding
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Return the result of performing ArrayAccumulation for
SpreadElement
with arguments
array
and
postIndex
padding
SpreadElement
...
AssignmentExpression
Let
spreadRef
be the result of evaluating
AssignmentExpression
Let
spreadObj
be ?
GetValue
spreadRef
).
Let
iteratorRecord
be ?
GetIterator
spreadObj
).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
nextIndex
Let
nextValue
be ?
IteratorValue
next
).
Let
status
be
CreateDataProperty
array
ToString
ToUint32
nextIndex
)),
nextValue
).
Assert
status
is
true
Increase
nextIndex
by 1.
Note
CreateDataProperty
is used to ensure that own properties are defined for the array even if
the standard built-in Array prototype object has been modified in a
manner that would preclude the creation of new own properties using
[[Set]].
12.2.5.3
Runtime Semantics: Evaluation
ArrayLiteral
Elision
opt
Let
array
be !
ArrayCreate
(0).
Let
pad
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Perform
Set
array
"length"
ToUint32
pad
),
false
).
NOTE: The above Set cannot fail because of the nature of the object returned by
ArrayCreate
Return
array
ArrayLiteral
ElementList
Let
array
be !
ArrayCreate
(0).
Let
len
be the result of performing ArrayAccumulation for
ElementList
with arguments
array
and 0.
ReturnIfAbrupt
len
).
Perform
Set
array
"length"
ToUint32
len
),
false
).
NOTE: The above Set cannot fail because of the nature of the object returned by
ArrayCreate
Return
array
ArrayLiteral
ElementList
Elision
opt
Let
array
be !
ArrayCreate
(0).
Let
len
be the result of performing ArrayAccumulation for
ElementList
with arguments
array
and 0.
ReturnIfAbrupt
len
).
Let
padding
be the ElisionWidth of
Elision
; if
Elision
is not present, use the numeric value zero.
Perform
Set
array
"length"
ToUint32
padding
len
),
false
).
NOTE: The above Set cannot fail because of the nature of the object returned by
ArrayCreate
Return
array
12.2.6
Object Initializer
Note 1
An object initializer is an expression describing the
initialization of an Object, written in a form resembling a literal. It
is a list of zero or more pairs of property keys and associated values,
enclosed in curly brackets. The values need not be literals; they are
evaluated each time the object initializer is evaluated.
Syntax
ObjectLiteral
[Yield, Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinitionList
[Yield, Await]
PropertyDefinition
[?Yield, ?Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinition
[?Yield, ?Await]
PropertyDefinition
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
CoverInitializedName
[?Yield, ?Await]
PropertyName
[?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
MethodDefinition
[?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
PropertyName
[Yield, Await]
LiteralPropertyName
ComputedPropertyName
[?Yield, ?Await]
LiteralPropertyName
IdentifierName
StringLiteral
NumericLiteral
ComputedPropertyName
[Yield, Await]
AssignmentExpression
[+In, ?Yield, ?Await]
CoverInitializedName
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
Initializer
[In, Yield, Await]
AssignmentExpression
[?In, ?Yield, ?Await]
Note 2
MethodDefinition
is defined in
14.3
Note 3
In certain contexts,
ObjectLiteral
is used as a cover grammar for a more restricted secondary grammar. The
CoverInitializedName
production is necessary to fully cover these secondary grammars.
However, use of this production results in an early Syntax Error in
normal contexts where an actual
ObjectLiteral
is expected.
12.2.6.1
Static Semantics: Early Errors
PropertyDefinition
MethodDefinition
It is a Syntax Error if HasDirectSuper of
MethodDefinition
is
true
In addition to describing an actual object initializer the
ObjectLiteral
productions are also used as a cover grammar for
ObjectAssignmentPattern
and may be recognized as part of a
CoverParenthesizedExpressionAndArrowParameterList
. When
ObjectLiteral
appears in a context where
ObjectAssignmentPattern
is required the following Early Error rules are
not
applied. In addition, they are not applied when initially parsing a
CoverParenthesizedExpressionAndArrowParameterList
or
CoverCallExpressionAndAsyncArrowHead
PropertyDefinition
CoverInitializedName
Always throw a Syntax Error if code matches this production.
Note
This production exists so that
ObjectLiteral
can serve as a cover grammar for
ObjectAssignmentPattern
. It cannot occur in an actual object initializer.
12.2.6.2
Static Semantics: ComputedPropertyContains
With parameter
symbol
PropertyName
LiteralPropertyName
Return
false
PropertyName
ComputedPropertyName
Return the result of
ComputedPropertyName
Contains
symbol
12.2.6.3
Static Semantics: Contains
With parameter
symbol
PropertyDefinition
MethodDefinition
If
symbol
is
MethodDefinition
, return
true
Return the result of ComputedPropertyContains for
MethodDefinition
with argument
symbol
Note
Static semantic rules that depend upon substructure generally do not look into function definitions.
LiteralPropertyName
IdentifierName
If
symbol
is a
ReservedWord
, return
false
If
symbol
is an
Identifier
and StringValue of
symbol
is the same value as the StringValue of
IdentifierName
, return
true
Return
false
12.2.6.4
Static Semantics: IsComputedPropertyKey
PropertyName
LiteralPropertyName
Return
false
PropertyName
ComputedPropertyName
Return
true
12.2.6.5
Static Semantics: PropName
PropertyDefinition
IdentifierReference
Return StringValue of
IdentifierReference
PropertyDefinition
...
AssignmentExpression
Return
empty
PropertyDefinition
PropertyName
AssignmentExpression
Return PropName of
PropertyName
LiteralPropertyName
IdentifierName
Return StringValue of
IdentifierName
LiteralPropertyName
StringLiteral
Return the String value whose code units are the SV of the
StringLiteral
LiteralPropertyName
NumericLiteral
Let
nbr
be the result of forming the value of the
NumericLiteral
Return !
ToString
nbr
).
ComputedPropertyName
AssignmentExpression
Return
empty
12.2.6.6
Static Semantics: PropertyNameList
PropertyDefinitionList
PropertyDefinition
If PropName of
PropertyDefinition
is
empty
, return a new empty
List
Return a new
List
containing PropName of
PropertyDefinition
PropertyDefinitionList
PropertyDefinitionList
PropertyDefinition
Let
list
be PropertyNameList of
PropertyDefinitionList
If PropName of
PropertyDefinition
is
empty
, return
list
Append PropName of
PropertyDefinition
to the end of
list
Return
list
12.2.6.7
Runtime Semantics: Evaluation
ObjectLiteral
Return
ObjectCreate
%ObjectPrototype%
).
ObjectLiteral
PropertyDefinitionList
PropertyDefinitionList
Let
obj
be
ObjectCreate
%ObjectPrototype%
).
Perform ? PropertyDefinitionEvaluation of
PropertyDefinitionList
with arguments
obj
and
true
Return
obj
LiteralPropertyName
IdentifierName
Return StringValue of
IdentifierName
LiteralPropertyName
StringLiteral
Return the String value whose code units are the SV of the
StringLiteral
LiteralPropertyName
NumericLiteral
Let
nbr
be the result of forming the value of the
NumericLiteral
Return !
ToString
nbr
).
ComputedPropertyName
AssignmentExpression
Let
exprValue
be the result of evaluating
AssignmentExpression
Let
propName
be ?
GetValue
exprValue
).
Return ?
ToPropertyKey
propName
).
12.2.6.8
Runtime Semantics: PropertyDefinitionEvaluation
With parameters
object
and
enumerable
PropertyDefinitionList
PropertyDefinitionList
PropertyDefinition
Perform ? PropertyDefinitionEvaluation of
PropertyDefinitionList
with arguments
object
and
enumerable
Return the result of performing PropertyDefinitionEvaluation of
PropertyDefinition
with arguments
object
and
enumerable
PropertyDefinition
...
AssignmentExpression
Let
exprValue
be the result of evaluating
AssignmentExpression
Let
fromValue
be ?
GetValue
exprValue
).
Let
excludedNames
be a new empty
List
Return ?
CopyDataProperties
object
fromValue
excludedNames
).
PropertyDefinition
IdentifierReference
Let
propName
be StringValue of
IdentifierReference
Let
exprValue
be the result of evaluating
IdentifierReference
Let
propValue
be ?
GetValue
exprValue
).
Assert
enumerable
is
true
Assert
object
is an ordinary, extensible object with no non-configurable properties.
Return !
CreateDataPropertyOrThrow
object
propName
propValue
).
PropertyDefinition
PropertyName
AssignmentExpression
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If
IsAnonymousFunctionDefinition
AssignmentExpression
) is
true
, then
Let
propValue
be the result of performing NamedEvaluation for
AssignmentExpression
with argument
propKey
Else,
Let
exprValueRef
be the result of evaluating
AssignmentExpression
Let
propValue
be ?
GetValue
exprValueRef
).
Assert
enumerable
is
true
Assert
object
is an ordinary, extensible object with no non-configurable properties.
Return !
CreateDataPropertyOrThrow
object
propKey
propValue
).
Note
An alternative semantics for this production is given in
B.3.1
12.2.7
Function Defining Expressions
See
14.1
for
PrimaryExpression
FunctionExpression
See
14.4
for
PrimaryExpression
GeneratorExpression
See
14.6
for
PrimaryExpression
ClassExpression
See
14.7
for
PrimaryExpression
AsyncFunctionExpression
See
14.5
for
PrimaryExpression
AsyncGeneratorExpression
12.2.8
Regular Expression Literals
Syntax
See
11.8.5
12.2.8.1
Static Semantics: Early Errors
PrimaryExpression
RegularExpressionLiteral
It is a Syntax Error if BodyText of
RegularExpressionLiteral
cannot be recognized using the
goal symbol
Pattern
of the ECMAScript RegExp grammar specified in
21.2.1
It is a Syntax Error if FlagText of
RegularExpressionLiteral
contains any code points other than
"g"
"i"
"m"
"s"
"u"
, or
"y"
, or if it contains the same code point more than once.
12.2.8.2
Runtime Semantics: Evaluation
PrimaryExpression
RegularExpressionLiteral
Let
pattern
be the String value consisting of the
UTF16Encoding
of each code point of BodyText of
RegularExpressionLiteral
Let
flags
be the String value consisting of the
UTF16Encoding
of each code point of FlagText of
RegularExpressionLiteral
Return
RegExpCreate
pattern
flags
).
12.2.9
Template Literals
Syntax
TemplateLiteral
[Yield, Await, Tagged]
NoSubstitutionTemplate
SubstitutionTemplate
[?Yield, ?Await, ?Tagged]
SubstitutionTemplate
[Yield, Await, Tagged]
TemplateHead
Expression
[+In, ?Yield, ?Await]
TemplateSpans
[?Yield, ?Await, ?Tagged]
TemplateSpans
[Yield, Await, Tagged]
TemplateTail
TemplateMiddleList
[?Yield, ?Await, ?Tagged]
TemplateTail
TemplateMiddleList
[Yield, Await, Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
TemplateMiddleList
[?Yield, ?Await, ?Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
12.2.9.1
Static Semantics: Early Errors
TemplateLiteral
[Yield, Await, Tagged]
NoSubstitutionTemplate
It is a Syntax Error if the number of elements in the result of TemplateStrings of
TemplateLiteral
with argument
false
is greater than 2
32
- 1.
It is a Syntax Error if the
[Tagged]
parameter was not set and
NoSubstitutionTemplate
Contains
NotEscapeSequence
SubstitutionTemplate
[Yield, Await, Tagged]
TemplateHead
Expression
[+In, ?Yield, ?Await]
TemplateSpans
[?Yield, ?Await, ?Tagged]
It is a Syntax Error if the
[Tagged]
parameter was not set and
TemplateHead
Contains
NotEscapeSequence
TemplateSpans
[Yield, Await, Tagged]
TemplateTail
It is a Syntax Error if the
[Tagged]
parameter was not set and
TemplateTail
Contains
NotEscapeSequence
TemplateMiddleList
[Yield, Await, Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
TemplateMiddleList
[?Yield, ?Await, ?Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
It is a Syntax Error if the
[Tagged]
parameter was not set and
TemplateMiddle
Contains
NotEscapeSequence
12.2.9.2
Static Semantics: TemplateStrings
With parameter
raw
TemplateLiteral
NoSubstitutionTemplate
If
raw
is
false
, then
Let
string
be the TV of
NoSubstitutionTemplate
Else,
Let
string
be the TRV of
NoSubstitutionTemplate
Return a
List
containing the single element,
string
SubstitutionTemplate
TemplateHead
Expression
TemplateSpans
If
raw
is
false
, then
Let
head
be the TV of
TemplateHead
Else,
Let
head
be the TRV of
TemplateHead
Let
tail
be TemplateStrings of
TemplateSpans
with argument
raw
Return a
List
containing
head
followed by the elements, in order, of
tail
TemplateSpans
TemplateTail
If
raw
is
false
, then
Let
tail
be the TV of
TemplateTail
Else,
Let
tail
be the TRV of
TemplateTail
Return a
List
containing the single element,
tail
TemplateSpans
TemplateMiddleList
TemplateTail
Let
middle
be TemplateStrings of
TemplateMiddleList
with argument
raw
If
raw
is
false
, then
Let
tail
be the TV of
TemplateTail
Else,
Let
tail
be the TRV of
TemplateTail
Return a
List
containing the elements, in order, of
middle
followed by
tail
TemplateMiddleList
TemplateMiddle
Expression
If
raw
is
false
, then
Let
string
be the TV of
TemplateMiddle
Else,
Let
string
be the TRV of
TemplateMiddle
Return a
List
containing the single element,
string
TemplateMiddleList
TemplateMiddleList
TemplateMiddle
Expression
Let
front
be TemplateStrings of
TemplateMiddleList
with argument
raw
If
raw
is
false
, then
Let
last
be the TV of
TemplateMiddle
Else,
Let
last
be the TRV of
TemplateMiddle
Append
last
as the last element of the
List
front
Return
front
12.2.9.3
Runtime Semantics: ArgumentListEvaluation
TemplateLiteral
NoSubstitutionTemplate
Let
templateLiteral
be this
TemplateLiteral
Let
siteObj
be
GetTemplateObject
templateLiteral
).
Return a
List
containing the one element which is
siteObj
SubstitutionTemplate
TemplateHead
Expression
TemplateSpans
Let
templateLiteral
be this
TemplateLiteral
Let
siteObj
be
GetTemplateObject
templateLiteral
).
Let
firstSubRef
be the result of evaluating
Expression
Let
firstSub
be ?
GetValue
firstSubRef
).
Let
restSub
be SubstitutionEvaluation of
TemplateSpans
ReturnIfAbrupt
restSub
).
Assert
restSub
is a
List
Return a
List
whose first element is
siteObj
, whose second elements is
firstSub
, and whose subsequent elements are the elements of
restSub
, in order.
restSub
may contain no elements.
12.2.9.4
Runtime Semantics: GetTemplateObject (
templateLiteral
The abstract operation GetTemplateObject is called with a
Parse Node
templateLiteral
, as an argument. It performs the following steps:
Let
rawStrings
be TemplateStrings of
templateLiteral
with argument
true
Let
realm
be
the current Realm Record
Let
templateRegistry
be
realm
.[[TemplateMap]].
For each element
of
templateRegistry
, do
If
.[[Site]] is
the same Parse Node
as
templateLiteral
, then
Return
.[[Array]].
Let
cookedStrings
be TemplateStrings of
templateLiteral
with argument
false
Let
count
be the number of elements in the
List
cookedStrings
Assert
count
≤ 2
32
- 1.
Let
template
be !
ArrayCreate
count
).
Let
rawObj
be !
ArrayCreate
count
).
Let
index
be 0.
Repeat, while
index
count
Let
prop
be !
ToString
index
).
Let
cookedValue
be the String value
cookedStrings
index
].
Call
template
.[[DefineOwnProperty]](
prop
, PropertyDescriptor { [[Value]]:
cookedValue
, [[Writable]]:
false
, [[Enumerable]]:
true
, [[Configurable]]:
false
}).
Let
rawValue
be the String value
rawStrings
index
].
Call
rawObj
.[[DefineOwnProperty]](
prop
, PropertyDescriptor { [[Value]]:
rawValue
, [[Writable]]:
false
, [[Enumerable]]:
true
, [[Configurable]]:
false
}).
Increase
index
by 1.
Perform
SetIntegrityLevel
rawObj
"frozen"
).
Call
template
.[[DefineOwnProperty]](
"raw"
, PropertyDescriptor { [[Value]]:
rawObj
, [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetIntegrityLevel
template
"frozen"
).
Append the
Record
{ [[Site]]:
templateLiteral
, [[Array]]:
template
} to
templateRegistry
Return
template
Note 1
The creation of a template object cannot result in an
abrupt completion
Note 2
Each
TemplateLiteral
in the program code of a
realm
is associated with a unique template object that is used in the evaluation of tagged Templates (
12.2.9.6
).
The template objects are frozen and the same template object is used
each time a specific tagged Template is evaluated. Whether template
objects are created lazily upon first evaluation of the
TemplateLiteral
or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
12.2.9.5
Runtime Semantics: SubstitutionEvaluation
TemplateSpans
TemplateTail
Return a new empty
List
TemplateSpans
TemplateMiddleList
TemplateTail
Return the result of SubstitutionEvaluation of
TemplateMiddleList
TemplateMiddleList
TemplateMiddle
Expression
Let
subRef
be the result of evaluating
Expression
Let
sub
be ?
GetValue
subRef
).
Return a
List
containing only
sub
TemplateMiddleList
TemplateMiddleList
TemplateMiddle
Expression
Let
preceding
be the result of SubstitutionEvaluation of
TemplateMiddleList
ReturnIfAbrupt
preceding
).
Let
nextRef
be the result of evaluating
Expression
Let
next
be ?
GetValue
nextRef
).
Append
next
as the last element of the
List
preceding
Return
preceding
12.2.9.6
Runtime Semantics: Evaluation
TemplateLiteral
NoSubstitutionTemplate
Return the String value whose code units are the elements of the TV of
NoSubstitutionTemplate
as defined in
11.8.6
SubstitutionTemplate
TemplateHead
Expression
TemplateSpans
Let
head
be the TV of
TemplateHead
as defined in
11.8.6
Let
subRef
be the result of evaluating
Expression
Let
sub
be ?
GetValue
subRef
).
Let
middle
be ?
ToString
sub
).
Let
tail
be the result of evaluating
TemplateSpans
ReturnIfAbrupt
tail
).
Return the
string-concatenation
of
head
middle
, and
tail
Note 1
The string conversion semantics applied to the
Expression
value are like
String.prototype.concat
rather than the
operator.
TemplateSpans
TemplateTail
Let
tail
be the TV of
TemplateTail
as defined in
11.8.6
Return the String value consisting of the code units of
tail
TemplateSpans
TemplateMiddleList
TemplateTail
Let
head
be the result of evaluating
TemplateMiddleList
ReturnIfAbrupt
head
).
Let
tail
be the TV of
TemplateTail
as defined in
11.8.6
Return the
string-concatenation
of
head
and
tail
TemplateMiddleList
TemplateMiddle
Expression
Let
head
be the TV of
TemplateMiddle
as defined in
11.8.6
Let
subRef
be the result of evaluating
Expression
Let
sub
be ?
GetValue
subRef
).
Let
middle
be ?
ToString
sub
).
Return the sequence of code units consisting of the code units of
head
followed by the elements of
middle
Note 2
The string conversion semantics applied to the
Expression
value are like
String.prototype.concat
rather than the
operator.
TemplateMiddleList
TemplateMiddleList
TemplateMiddle
Expression
Let
rest
be the result of evaluating
TemplateMiddleList
ReturnIfAbrupt
rest
).
Let
middle
be the TV of
TemplateMiddle
as defined in
11.8.6
Let
subRef
be the result of evaluating
Expression
Let
sub
be ?
GetValue
subRef
).
Let
last
be ?
ToString
sub
).
Return the sequence of code units consisting of the elements of
rest
followed by the code units of
middle
followed by the elements of
last
Note 3
The string conversion semantics applied to the
Expression
value are like
String.prototype.concat
rather than the
operator.
12.2.10
The Grouping Operator
12.2.10.1
Static Semantics: Early Errors
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
It is a Syntax Error if
CoverParenthesizedExpressionAndArrowParameterList
is not
covering
ParenthesizedExpression
All Early Error rules for
ParenthesizedExpression
and its derived productions also apply to CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
12.2.10.2
Static Semantics: IsFunctionDefinition
ParenthesizedExpression
Expression
Return IsFunctionDefinition of
Expression
12.2.10.3
Static Semantics: AssignmentTargetType
ParenthesizedExpression
Expression
Return AssignmentTargetType of
Expression
12.2.10.4
Runtime Semantics: NamedEvaluation
With parameter
name
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
Return the result of performing NamedEvaluation for
expr
with argument
name
ParenthesizedExpression
Expression
Assert
IsAnonymousFunctionDefinition
Expression
) is
true
Return the result of performing NamedEvaluation for
Expression
with argument
name
12.2.10.5
Runtime Semantics: Evaluation
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
Return the result of evaluating
expr
ParenthesizedExpression
Expression
Return the result of evaluating
Expression
. This may be of type
Reference
Note
This algorithm does not apply
GetValue
to the result of evaluating
Expression
. The principal motivation for this is so that operators such as
delete
and
typeof
may be applied to parenthesized expressions.
12.3
Left-Hand-Side Expressions
Syntax
MemberExpression
[Yield, Await]
PrimaryExpression
[?Yield, ?Await]
MemberExpression
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
MemberExpression
[?Yield, ?Await]
IdentifierName
MemberExpression
[?Yield, ?Await]
TemplateLiteral
[?Yield, ?Await, +Tagged]
SuperProperty
[?Yield, ?Await]
MetaProperty
new
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
SuperProperty
[Yield, Await]
super
Expression
[+In, ?Yield, ?Await]
super
IdentifierName
MetaProperty
NewTarget
NewTarget
new
target
NewExpression
[Yield, Await]
MemberExpression
[?Yield, ?Await]
new
NewExpression
[?Yield, ?Await]
CallExpression
[Yield, Await]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
SuperCall
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
CallExpression
[?Yield, ?Await]
IdentifierName
CallExpression
[?Yield, ?Await]
TemplateLiteral
[?Yield, ?Await, +Tagged]
SuperCall
[Yield, Await]
super
Arguments
[?Yield, ?Await]
Arguments
[Yield, Await]
ArgumentList
[?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
ArgumentList
[Yield, Await]
AssignmentExpression
[+In, ?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
LeftHandSideExpression
[Yield, Await]
NewExpression
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
Supplemental Syntax
When processing an instance of the production
CallExpression
CoverCallExpressionAndAsyncArrowHead
the interpretation of
CoverCallExpressionAndAsyncArrowHead
is refined using the following grammar:
CallMemberExpression
[Yield, Await]
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
12.3.1
Static Semantics
12.3.1.1
Static Semantics: CoveredCallExpression
CallExpression
CoverCallExpressionAndAsyncArrowHead
Return the
CallMemberExpression
that is
covered
by
CoverCallExpressionAndAsyncArrowHead
12.3.1.2
Static Semantics: Contains
With parameter
symbol
MemberExpression
MemberExpression
IdentifierName
If
MemberExpression
Contains
symbol
is
true
, return
true
If
symbol
is a
ReservedWord
, return
false
If
symbol
is an
Identifier
and StringValue of
symbol
is the same value as the StringValue of
IdentifierName
, return
true
Return
false
SuperProperty
super
IdentifierName
If
symbol
is the
ReservedWord
super
, return
true
If
symbol
is a
ReservedWord
, return
false
If
symbol
is an
Identifier
and StringValue of
symbol
is the same value as the StringValue of
IdentifierName
, return
true
Return
false
CallExpression
CallExpression
IdentifierName
If
CallExpression
Contains
symbol
is
true
, return
true
If
symbol
is a
ReservedWord
, return
false
If
symbol
is an
Identifier
and StringValue of
symbol
is the same value as the StringValue of
IdentifierName
, return
true
Return
false
12.3.1.3
Static Semantics: IsFunctionDefinition
MemberExpression
MemberExpression
Expression
MemberExpression
IdentifierName
MemberExpression
TemplateLiteral
SuperProperty
MetaProperty
new
MemberExpression
Arguments
NewExpression
new
NewExpression
LeftHandSideExpression
CallExpression
Return
false
12.3.1.4
Static Semantics: IsDestructuring
MemberExpression
PrimaryExpression
If
PrimaryExpression
is either an
ObjectLiteral
or an
ArrayLiteral
, return
true
Return
false
MemberExpression
MemberExpression
Expression
MemberExpression
IdentifierName
MemberExpression
TemplateLiteral
SuperProperty
MetaProperty
new
MemberExpression
Arguments
NewExpression
new
NewExpression
LeftHandSideExpression
CallExpression
Return
false
12.3.1.5
Static Semantics: IsIdentifierRef
MemberExpression
MemberExpression
Expression
MemberExpression
IdentifierName
MemberExpression
TemplateLiteral
SuperProperty
MetaProperty
new
MemberExpression
Arguments
NewExpression
new
NewExpression
LeftHandSideExpression
CallExpression
Return
false
12.3.1.6
Static Semantics: AssignmentTargetType
CallExpression
CallExpression
Expression
CallExpression
IdentifierName
MemberExpression
MemberExpression
Expression
MemberExpression
IdentifierName
SuperProperty
Return
simple
CallExpression
CoverCallExpressionAndAsyncArrowHead
SuperCall
CallExpression
Arguments
CallExpression
TemplateLiteral
NewExpression
new
NewExpression
MemberExpression
MemberExpression
TemplateLiteral
new
MemberExpression
Arguments
NewTarget
new
target
Return
invalid
12.3.2
Property Accessors
Note
Properties are accessed by name, using either the dot notation:
MemberExpression
IdentifierName
CallExpression
IdentifierName
or the bracket notation:
MemberExpression
Expression
CallExpression
Expression
The dot notation is explained by the following syntactic conversion:
MemberExpression
IdentifierName
is identical in its behaviour to
MemberExpression
identifier-name-string
and similarly
CallExpression
IdentifierName
is identical in its behaviour to
CallExpression
identifier-name-string
where <
identifier-name-string
> is the result of evaluating StringValue of
IdentifierName
12.3.2.1
Runtime Semantics: Evaluation
MemberExpression
MemberExpression
Expression
Let
baseReference
be the result of evaluating
MemberExpression
Let
baseValue
be ?
GetValue
baseReference
).
Let
propertyNameReference
be the result of evaluating
Expression
Let
propertyNameValue
be ?
GetValue
propertyNameReference
).
Let
bv
be ?
RequireObjectCoercible
baseValue
).
Let
propertyKey
be ?
ToPropertyKey
propertyNameValue
).
If the code matched by this
MemberExpression
is
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return a value of type
Reference
whose base value component is
bv
, whose referenced name component is
propertyKey
, and whose strict reference flag is
strict
MemberExpression
MemberExpression
IdentifierName
Let
baseReference
be the result of evaluating
MemberExpression
Let
baseValue
be ?
GetValue
baseReference
).
Let
bv
be ?
RequireObjectCoercible
baseValue
).
Let
propertyNameString
be StringValue of
IdentifierName
If the code matched by this
MemberExpression
is
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return a value of type
Reference
whose base value component is
bv
, whose referenced name component is
propertyNameString
, and whose strict reference flag is
strict
CallExpression
CallExpression
Expression
Is evaluated in exactly the same manner as
MemberExpression
MemberExpression
Expression
except that the contained
CallExpression
is evaluated in step 1.
CallExpression
CallExpression
IdentifierName
Is evaluated in exactly the same manner as
MemberExpression
MemberExpression
IdentifierName
except that the contained
CallExpression
is evaluated in step 1.
12.3.3
The
new
Operator
12.3.3.1
Runtime Semantics: Evaluation
NewExpression
new
NewExpression
Return ?
EvaluateNew
NewExpression
empty
).
MemberExpression
new
MemberExpression
Arguments
Return ?
EvaluateNew
MemberExpression
Arguments
).
12.3.3.1.1
Runtime Semantics: EvaluateNew (
constructExpr
arguments
The abstract operation EvaluateNew with arguments
constructExpr
, and
arguments
performs the following steps:
Assert
constructExpr
is either a
NewExpression
or a
MemberExpression
Assert
arguments
is either
empty
or an
Arguments
Let
ref
be the result of evaluating
constructExpr
Let
constructor
be ?
GetValue
ref
).
If
arguments
is
empty
, let
argList
be a new empty
List
Else,
Let
argList
be ArgumentListEvaluation of
arguments
ReturnIfAbrupt
argList
).
If
IsConstructor
constructor
) is
false
, throw a
TypeError
exception.
Return ?
Construct
constructor
argList
).
12.3.4
Function Calls
12.3.4.1
Runtime Semantics: Evaluation
CallExpression
CoverCallExpressionAndAsyncArrowHead
Let
expr
be CoveredCallExpression of
CoverCallExpressionAndAsyncArrowHead
Let
memberExpr
be the
MemberExpression
of
expr
Let
arguments
be the
Arguments
of
expr
Let
ref
be the result of evaluating
memberExpr
Let
func
be ?
GetValue
ref
).
If
Type
ref
) is
Reference
and
IsPropertyReference
ref
) is
false
and
GetReferencedName
ref
) is
"eval"
, then
If
SameValue
func
%eval%
) is
true
, then
Let
argList
be ? ArgumentListEvaluation of
arguments
If
argList
has no elements, return
undefined
Let
evalText
be the first element of
argList
If the source code matching this
CallExpression
is
strict mode code
, let
strictCaller
be
true
. Otherwise let
strictCaller
be
false
Let
evalRealm
be
the current Realm Record
Perform ?
HostEnsureCanCompileStrings
evalRealm
evalRealm
).
Return ?
PerformEval
evalText
evalRealm
strictCaller
true
).
Let
thisCall
be this
CallExpression
Let
tailCall
be
IsInTailPosition
thisCall
).
Return ?
EvaluateCall
func
ref
arguments
tailCall
).
CallExpression
evaluation that executes step 6.a.vii is a
direct eval
CallExpression
CallExpression
Arguments
Let
ref
be the result of evaluating
CallExpression
Let
func
be ?
GetValue
ref
).
Let
thisCall
be this
CallExpression
Let
tailCall
be
IsInTailPosition
thisCall
).
Return ?
EvaluateCall
func
ref
Arguments
tailCall
).
12.3.4.2
Runtime Semantics: EvaluateCall (
func
ref
arguments
tailPosition
The abstract operation EvaluateCall takes as arguments a value
func
, a value
ref
, a
Parse Node
arguments
, and a Boolean argument
tailPosition
. It performs the following steps:
If
Type
ref
) is
Reference
, then
If
IsPropertyReference
ref
) is
true
, then
Let
thisValue
be
GetThisValue
ref
).
Else the base of
ref
is an
Environment Record
Let
refEnv
be
GetBase
ref
).
Let
thisValue
be
refEnv
.WithBaseObject().
Else
Type
ref
) is not
Reference
Let
thisValue
be
undefined
Let
argList
be ArgumentListEvaluation of
arguments
ReturnIfAbrupt
argList
).
If
Type
func
) is not Object, throw a
TypeError
exception.
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
If
tailPosition
is
true
, perform
PrepareForTailCall
().
Let
result
be
Call
func
thisValue
argList
).
Assert
: If
tailPosition
is
true
, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
Assert
: If
result
is not an
abrupt completion
, then
Type
result
) is an
ECMAScript language type
Return
result
12.3.5
The
super
Keyword
12.3.5.1
Runtime Semantics: Evaluation
SuperProperty
super
Expression
Let
env
be
GetThisEnvironment
().
Let
actualThis
be ?
env
.GetThisBinding().
Let
propertyNameReference
be the result of evaluating
Expression
Let
propertyNameValue
be ?
GetValue
propertyNameReference
).
Let
propertyKey
be ?
ToPropertyKey
propertyNameValue
).
If the code matched by this
SuperProperty
is
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return ?
MakeSuperPropertyReference
actualThis
propertyKey
strict
).
SuperProperty
super
IdentifierName
Let
env
be
GetThisEnvironment
().
Let
actualThis
be ?
env
.GetThisBinding().
Let
propertyKey
be StringValue of
IdentifierName
If the code matched by this
SuperProperty
is
strict mode code
, let
strict
be
true
, else let
strict
be
false
Return ?
MakeSuperPropertyReference
actualThis
propertyKey
strict
).
SuperCall
super
Arguments
Let
newTarget
be
GetNewTarget
().
Assert
Type
newTarget
) is Object.
Let
func
be ?
GetSuperConstructor
().
Let
argList
be ArgumentListEvaluation of
Arguments
ReturnIfAbrupt
argList
).
Let
result
be ?
Construct
func
argList
newTarget
).
Let
thisER
be
GetThisEnvironment
().
Return ?
thisER
.BindThisValue(
result
).
12.3.5.2
Runtime Semantics: GetSuperConstructor ( )
The abstract operation GetSuperConstructor performs the following steps:
Let
envRec
be
GetThisEnvironment
().
Assert
envRec
is a
function Environment Record
Let
activeFunction
be
envRec
.[[FunctionObject]].
Assert
activeFunction
is an ECMAScript
function object
Let
superConstructor
be !
activeFunction
.[[GetPrototypeOf]]().
If
IsConstructor
superConstructor
) is
false
, throw a
TypeError
exception.
Return
superConstructor
12.3.5.3
Runtime Semantics: MakeSuperPropertyReference (
actualThis
propertyKey
strict
The abstract operation MakeSuperPropertyReference with arguments
actualThis
propertyKey
, and
strict
performs the following steps:
Let
env
be
GetThisEnvironment
().
Assert
env
.HasSuperBinding() is
true
Let
baseValue
be ?
env
.GetSuperBase().
Let
bv
be ?
RequireObjectCoercible
baseValue
).
Return a value of type
Reference
that is a
Super Reference
whose base value component is
bv
, whose referenced name component is
propertyKey
, whose thisValue component is
actualThis
, and whose strict reference flag is
strict
12.3.6
Argument Lists
Note
The evaluation of an argument list produces a
List
of values.
12.3.6.1
Runtime Semantics: ArgumentListEvaluation
Arguments
Return a new empty
List
ArgumentList
AssignmentExpression
Let
ref
be the result of evaluating
AssignmentExpression
Let
arg
be ?
GetValue
ref
).
Return a
List
whose sole item is
arg
ArgumentList
...
AssignmentExpression
Let
list
be a new empty
List
Let
spreadRef
be the result of evaluating
AssignmentExpression
Let
spreadObj
be ?
GetValue
spreadRef
).
Let
iteratorRecord
be ?
GetIterator
spreadObj
).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
list
Let
nextArg
be ?
IteratorValue
next
).
Append
nextArg
as the last element of
list
ArgumentList
ArgumentList
AssignmentExpression
Let
precedingArgs
be ArgumentListEvaluation of
ArgumentList
ReturnIfAbrupt
precedingArgs
).
Let
ref
be the result of evaluating
AssignmentExpression
Let
arg
be ?
GetValue
ref
).
Append
arg
to the end of
precedingArgs
Return
precedingArgs
ArgumentList
ArgumentList
...
AssignmentExpression
Let
precedingArgs
be ArgumentListEvaluation of
ArgumentList
ReturnIfAbrupt
precedingArgs
).
Let
spreadRef
be the result of evaluating
AssignmentExpression
Let
iteratorRecord
be ?
GetIterator
(?
GetValue
spreadRef
)).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
precedingArgs
Let
nextArg
be ?
IteratorValue
next
).
Append
nextArg
as the last element of
precedingArgs
12.3.7
Tagged Templates
Note
A tagged template is a function call where the arguments of the call are derived from a
TemplateLiteral
12.2.9
). The actual arguments include a template object (
12.2.9.4
) and the values produced by evaluating the expressions embedded within the
TemplateLiteral
12.3.7.1
Runtime Semantics: Evaluation
MemberExpression
MemberExpression
TemplateLiteral
Let
tagRef
be the result of evaluating
MemberExpression
Let
tagFunc
be ?
GetValue
tagRef
).
Let
thisCall
be this
MemberExpression
Let
tailCall
be
IsInTailPosition
thisCall
).
Return ?
EvaluateCall
tagFunc
tagRef
TemplateLiteral
tailCall
).
CallExpression
CallExpression
TemplateLiteral
Let
tagRef
be the result of evaluating
CallExpression
Let
tagFunc
be ?
GetValue
tagRef
).
Let
thisCall
be this
CallExpression
Let
tailCall
be
IsInTailPosition
thisCall
).
Return ?
EvaluateCall
tagFunc
tagRef
TemplateLiteral
tailCall
).
12.3.8
Meta Properties
12.3.8.1
Runtime Semantics: Evaluation
NewTarget
new
target
Return
GetNewTarget
().
12.4
Update Expressions
Syntax
UpdateExpression
[Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
++
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
--
++
UnaryExpression
[?Yield, ?Await]
--
UnaryExpression
[?Yield, ?Await]
12.4.1
Static Semantics: Early Errors
UpdateExpression
LeftHandSideExpression
++
LeftHandSideExpression
--
It is an early
Reference
Error if AssignmentTargetType of
LeftHandSideExpression
is
invalid
It is an early Syntax Error if AssignmentTargetType of
LeftHandSideExpression
is
strict
UpdateExpression
++
UnaryExpression
--
UnaryExpression
It is an early
Reference
Error if AssignmentTargetType of
UnaryExpression
is
invalid
It is an early Syntax Error if AssignmentTargetType of
UnaryExpression
is
strict
12.4.2
Static Semantics: IsFunctionDefinition
UpdateExpression
LeftHandSideExpression
++
LeftHandSideExpression
--
++
UnaryExpression
--
UnaryExpression
Return
false
12.4.3
Static Semantics: AssignmentTargetType
UpdateExpression
LeftHandSideExpression
++
LeftHandSideExpression
--
++
UnaryExpression
--
UnaryExpression
Return
invalid
12.4.4
Postfix Increment Operator
12.4.4.1
Runtime Semantics: Evaluation
UpdateExpression
LeftHandSideExpression
++
Let
lhs
be the result of evaluating
LeftHandSideExpression
Let
oldValue
be ?
ToNumber
(?
GetValue
lhs
)).
Let
newValue
be the result of adding the value 1 to
oldValue
, using the same rules as for the
operator (see
12.8.5
).
Perform ?
PutValue
lhs
newValue
).
Return
oldValue
12.4.5
Postfix Decrement Operator
12.4.5.1
Runtime Semantics: Evaluation
UpdateExpression
LeftHandSideExpression
--
Let
lhs
be the result of evaluating
LeftHandSideExpression
Let
oldValue
be ?
ToNumber
(?
GetValue
lhs
)).
Let
newValue
be the result of subtracting the value 1 from
oldValue
, using the same rules as for the
operator (see
12.8.5
).
Perform ?
PutValue
lhs
newValue
).
Return
oldValue
12.4.6
Prefix Increment Operator
12.4.6.1
Runtime Semantics: Evaluation
UpdateExpression
++
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be ?
ToNumber
(?
GetValue
expr
)).
Let
newValue
be the result of adding the value 1 to
oldValue
, using the same rules as for the
operator (see
12.8.5
).
Perform ?
PutValue
expr
newValue
).
Return
newValue
12.4.7
Prefix Decrement Operator
12.4.7.1
Runtime Semantics: Evaluation
UpdateExpression
--
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be ?
ToNumber
(?
GetValue
expr
)).
Let
newValue
be the result of subtracting the value 1 from
oldValue
, using the same rules as for the
operator (see
12.8.5
).
Perform ?
PutValue
expr
newValue
).
Return
newValue
12.5
Unary Operators
Syntax
UnaryExpression
[Yield, Await]
UpdateExpression
[?Yield, ?Await]
delete
UnaryExpression
[?Yield, ?Await]
void
UnaryExpression
[?Yield, ?Await]
typeof
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
[+Await]
AwaitExpression
[?Yield]
12.5.1
Static Semantics: IsFunctionDefinition
UnaryExpression
delete
UnaryExpression
void
UnaryExpression
typeof
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
AwaitExpression
Return
false
12.5.2
Static Semantics: AssignmentTargetType
UnaryExpression
delete
UnaryExpression
void
UnaryExpression
typeof
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
AwaitExpression
Return
invalid
12.5.3
The
delete
Operator
12.5.3.1
Static Semantics: Early Errors
UnaryExpression
delete
UnaryExpression
It is a Syntax Error if the
UnaryExpression
is contained in
strict mode code
and the derived
UnaryExpression
is
PrimaryExpression
IdentifierReference
It is a Syntax Error if the derived
UnaryExpression
is
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
and
CoverParenthesizedExpressionAndArrowParameterList
ultimately derives a phrase that, if used in place of
UnaryExpression
, would produce a Syntax Error according to these rules. This rule is recursively applied.
Note
The last rule means that expressions such as
delete (((foo)))
produce early errors because of recursive application of the first rule.
12.5.3.2
Runtime Semantics: Evaluation
UnaryExpression
delete
UnaryExpression
Let
ref
be the result of evaluating
UnaryExpression
ReturnIfAbrupt
ref
).
If
Type
ref
) is not
Reference
, return
true
If
IsUnresolvableReference
ref
) is
true
, then
Assert
IsStrictReference
ref
) is
false
Return
true
If
IsPropertyReference
ref
) is
true
, then
If
IsSuperReference
ref
) is
true
, throw a
ReferenceError
exception.
Let
baseObj
be !
ToObject
GetBase
ref
)).
Let
deleteStatus
be ?
baseObj
.[[Delete]](
GetReferencedName
ref
)).
If
deleteStatus
is
false
and
IsStrictReference
ref
) is
true
, throw a
TypeError
exception.
Return
deleteStatus
Else
ref
is a
Reference
to an
Environment Record
binding,
Let
bindings
be
GetBase
ref
).
Return ?
bindings
.DeleteBinding(
GetReferencedName
ref
)).
Note
When a
delete
operator occurs within
strict mode code
, a
SyntaxError
exception is thrown if its
UnaryExpression
is a direct reference to a variable, function argument, or function name. In addition, if a
delete
operator occurs within
strict mode code
and the property to be deleted has the attribute { [[Configurable]]:
false
}, a
TypeError
exception is thrown.
12.5.4
The
void
Operator
12.5.4.1
Runtime Semantics: Evaluation
UnaryExpression
void
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Perform ?
GetValue
expr
).
Return
undefined
Note
GetValue
must be called even though its value is not used because it may have observable side-effects.
12.5.5
The
typeof
Operator
12.5.5.1
Runtime Semantics: Evaluation
UnaryExpression
typeof
UnaryExpression
Let
val
be the result of evaluating
UnaryExpression
If
Type
val
) is
Reference
, then
If
IsUnresolvableReference
val
) is
true
, return
"undefined"
Set
val
to ?
GetValue
val
).
Return a String according to
Table 35
Table 35: typeof Operator Results
Type of
val
Result
Undefined
"undefined"
Null
"object"
Boolean
"boolean"
Number
"number"
String
"string"
Symbol
"symbol"
Object (ordinary and does not implement [[Call]])
"object"
Object (standard exotic and does not implement [[Call]])
"object"
Object (implements [[Call]])
"function"
Object (non-standard exotic and does not implement [[Call]])
Implementation-defined. Must not be
"undefined"
"boolean"
"function"
"number"
"symbol"
, or
"string"
Note
Implementations are discouraged from defining new
typeof
result values for non-standard exotic objects. If possible
"object"
should be used for such objects.
12.5.6
Unary
Operator
Note
The unary + operator converts its operand to Number type.
12.5.6.1
Runtime Semantics: Evaluation
UnaryExpression
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Return ?
ToNumber
(?
GetValue
expr
)).
12.5.7
Unary
Operator
Note
The unary
operator converts its operand to Number type and then negates it. Negating
+0
produces
-0
, and negating
-0
produces
+0
12.5.7.1
Runtime Semantics: Evaluation
UnaryExpression
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be ?
ToNumber
(?
GetValue
expr
)).
If
oldValue
is
NaN
, return
NaN
Return the result of negating
oldValue
; that is, compute a Number with the same magnitude but opposite sign.
12.5.8
Bitwise NOT Operator (
12.5.8.1
Runtime Semantics: Evaluation
UnaryExpression
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be ?
ToInt32
(?
GetValue
expr
)).
Return the result of applying bitwise complement to
oldValue
. The result is a signed 32-bit integer.
12.5.9
Logical NOT Operator (
12.5.9.1
Runtime Semantics: Evaluation
UnaryExpression
UnaryExpression
Let
expr
be the result of evaluating
UnaryExpression
Let
oldValue
be
ToBoolean
(?
GetValue
expr
)).
If
oldValue
is
true
, return
false
Return
true
12.6
Exponentiation Operator
Syntax
ExponentiationExpression
[Yield, Await]
UnaryExpression
[?Yield, ?Await]
UpdateExpression
[?Yield, ?Await]
**
ExponentiationExpression
[?Yield, ?Await]
12.6.1
Static Semantics: IsFunctionDefinition
ExponentiationExpression
UpdateExpression
**
ExponentiationExpression
Return
false
12.6.2
Static Semantics: AssignmentTargetType
ExponentiationExpression
UpdateExpression
**
ExponentiationExpression
Return
invalid
12.6.3
Runtime Semantics: Evaluation
ExponentiationExpression
UpdateExpression
**
ExponentiationExpression
Let
left
be the result of evaluating
UpdateExpression
Let
leftValue
be ?
GetValue
left
).
Let
right
be the result of evaluating
ExponentiationExpression
Let
rightValue
be ?
GetValue
right
).
Let
base
be ?
ToNumber
leftValue
).
Let
exponent
be ?
ToNumber
rightValue
).
Return the result of
Applying the ** operator
with
base
and
exponent
as specified in
12.6.4
12.6.4
Applying the
**
Operator
Returns an implementation-dependent approximation of the result of raising
base
to the power
exponent
If
exponent
is
NaN
, the result is
NaN
If
exponent
is
+0
, the result is 1, even if
base
is
NaN
If
exponent
is
-0
, the result is 1, even if
base
is
NaN
If
base
is
NaN
and
exponent
is nonzero, the result is
NaN
If
abs
base
) > 1 and
exponent
is
+∞
, the result is
+∞
If
abs
base
) > 1 and
exponent
is
-∞
, the result is
+0
If
abs
base
) is 1 and
exponent
is
+∞
, the result is
NaN
If
abs
base
) is 1 and
exponent
is
-∞
, the result is
NaN
If
abs
base
) < 1 and
exponent
is
+∞
, the result is
+0
If
abs
base
) < 1 and
exponent
is
-∞
, the result is
+∞
If
base
is
+∞
and
exponent
> 0, the result is
+∞
If
base
is
+∞
and
exponent
< 0, the result is
+0
If
base
is
-∞
and
exponent
> 0 and
exponent
is an odd integer, the result is
-∞
If
base
is
-∞
and
exponent
> 0 and
exponent
is not an odd integer, the result is
+∞
If
base
is
-∞
and
exponent
< 0 and
exponent
is an odd integer, the result is
-0
If
base
is
-∞
and
exponent
< 0 and
exponent
is not an odd integer, the result is
+0
If
base
is
+0
and
exponent
> 0, the result is
+0
If
base
is
+0
and
exponent
< 0, the result is
+∞
If
base
is
-0
and
exponent
> 0 and
exponent
is an odd integer, the result is
-0
If
base
is
-0
and
exponent
> 0 and
exponent
is not an odd integer, the result is
+0
If
base
is
-0
and
exponent
< 0 and
exponent
is an odd integer, the result is
-∞
If
base
is
-0
and
exponent
< 0 and
exponent
is not an odd integer, the result is
+∞
If
base
< 0 and
base
is finite and
exponent
is finite and
exponent
is not an integer, the result is
NaN
Note
The result of
base
**
exponent
when
base
is
or
-1
and
exponent
is
+Infinity
or
-Infinity
differs from IEEE 754-2008. The first edition of ECMAScript specified a result of
NaN
for this operation, whereas later versions of IEEE 754-2008 specified
. The historical ECMAScript behaviour is preserved for compatibility reasons.
12.7
Multiplicative Operators
Syntax
MultiplicativeExpression
[Yield, Await]
ExponentiationExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
MultiplicativeOperator
ExponentiationExpression
[?Yield, ?Await]
MultiplicativeOperator
one of
12.7.1
Static Semantics: IsFunctionDefinition
MultiplicativeExpression
MultiplicativeExpression
MultiplicativeOperator
ExponentiationExpression
Return
false
12.7.2
Static Semantics: AssignmentTargetType
MultiplicativeExpression
MultiplicativeExpression
MultiplicativeOperator
ExponentiationExpression
Return
invalid
12.7.3
Runtime Semantics: Evaluation
MultiplicativeExpression
MultiplicativeExpression
MultiplicativeOperator
ExponentiationExpression
Let
left
be the result of evaluating
MultiplicativeExpression
Let
leftValue
be ?
GetValue
left
).
Let
right
be the result of evaluating
ExponentiationExpression
Let
rightValue
be ?
GetValue
right
).
Let
lnum
be ?
ToNumber
leftValue
).
Let
rnum
be ?
ToNumber
rightValue
).
Return the result of applying the
MultiplicativeOperator
, or
) to
lnum
and
rnum
as specified in
12.7.3.1
12.7.3.2
, or
12.7.3.3
12.7.3.1
Applying the
Operator
The
MultiplicativeOperator
performs multiplication, producing the product of its operands.
Multiplication is commutative. Multiplication is not always associative
in ECMAScript, because of finite precision.
The result of a floating-point multiplication is governed by the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is
NaN
, the result is
NaN
The sign of the result is positive if both operands have the
same sign, negative if the operands have different signs.
Multiplication of an infinity by a zero results in
NaN
Multiplication of an infinity by an infinity results in an
infinity. The sign is determined by the rule already stated above.
Multiplication of an infinity by a finite nonzero value
results in a signed infinity. The sign is determined by the rule already
stated above.
In the remaining cases, where neither an infinity nor
NaN
is involved, the product is computed and rounded to the nearest
representable value using IEEE 754-2008 round to nearest, ties to even
mode. If the magnitude is too large to represent, the result is then an
infinity of appropriate sign. If the magnitude is too small to
represent, the result is then a zero of appropriate sign. The ECMAScript
language requires support of gradual underflow as defined by IEEE
754-2008.
12.7.3.2
Applying the
Operator
The
MultiplicativeOperator
performs division, producing the quotient of its operands. The left
operand is the dividend and the right operand is the divisor. ECMAScript
does not perform integer division. The operands and result of all
division operations are double-precision floating-point numbers. The
result of division is determined by the specification of IEEE 754-2008
arithmetic:
If either operand is
NaN
, the result is
NaN
The sign of the result is positive if both operands have the
same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in
NaN
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in
a signed infinity. The sign is determined by the rule already stated
above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in
NaN
; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.
Division of a nonzero finite value by a zero results in a
signed infinity. The sign is determined by the rule already stated
above.
In the remaining cases, where neither an infinity, nor a zero, nor
NaN
is involved, the quotient is computed and rounded to the nearest
representable value using IEEE 754-2008 round to nearest, ties to even
mode. If the magnitude is too large to represent, the operation
overflows; the result is then an infinity of appropriate sign. If the
magnitude is too small to represent, the operation underflows and the
result is a zero of the appropriate sign. The ECMAScript language
requires support of gradual underflow as defined by IEEE 754-2008.
12.7.3.3
Applying the
Operator
The
MultiplicativeOperator
yields the remainder of its operands from an implied division; the left
operand is the dividend and the right operand is the divisor.
Note
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
The result of a floating-point remainder operation as computed by the
operator is not the same as the “remainder” operation defined by IEEE
754-2008. The IEEE 754-2008 “remainder” operation computes the remainder
from a rounding division, not a truncating division, and so its
behaviour is not analogous to that of the usual integer remainder
operator. Instead the ECMAScript language defines
on
floating-point operations to behave in a manner analogous to that of the
Java integer remainder operator; this may be compared with the C
library function fmod.
The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:
If either operand is
NaN
, the result is
NaN
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is
NaN
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor
NaN
is involved, the floating-point remainder r from a dividend n and a
divisor d is defined by the mathematical relation r = n - (d × q) where q
is an integer that is negative only if n/d is negative and positive
only if n/d is positive, and whose magnitude is as large as possible
without exceeding the magnitude of the true mathematical quotient of n
and d. r is computed and rounded to the nearest representable value
using IEEE 754-2008 round to nearest, ties to even mode.
12.8
Additive Operators
Syntax
AdditiveExpression
[Yield, Await]
MultiplicativeExpression
[?Yield, ?Await]
AdditiveExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
AdditiveExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
12.8.1
Static Semantics: IsFunctionDefinition
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
Return
false
12.8.2
Static Semantics: AssignmentTargetType
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
Return
invalid
12.8.3
The Addition Operator (
Note
The addition operator either performs string concatenation or numeric addition.
12.8.3.1
Runtime Semantics: Evaluation
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
Let
lref
be the result of evaluating
AdditiveExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
MultiplicativeExpression
Let
rval
be ?
GetValue
rref
).
Let
lprim
be ?
ToPrimitive
lval
).
Let
rprim
be ?
ToPrimitive
rval
).
If
Type
lprim
) is String or
Type
rprim
) is String, then
Let
lstr
be ?
ToString
lprim
).
Let
rstr
be ?
ToString
rprim
).
Return the
string-concatenation
of
lstr
and
rstr
Let
lnum
be ?
ToNumber
lprim
).
Let
rnum
be ?
ToNumber
rprim
).
Return the result of applying the addition operation to
lnum
and
rnum
. See the Note below
12.8.5
Note 1
No hint is provided in the calls to
ToPrimitive
in steps 5 and 6. All standard objects except Date objects handle the
absence of a hint as if the hint Number were given; Date objects handle
the absence of a hint as if the hint String were given. Exotic objects
may handle the absence of a hint in some other manner.
Note 2
Step 7 differs from step 3 of the
Abstract Relational Comparison
algorithm, by using the logical-or operation instead of the logical-and operation.
12.8.4
The Subtraction Operator (
12.8.4.1
Runtime Semantics: Evaluation
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
Let
lref
be the result of evaluating
AdditiveExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
MultiplicativeExpression
Let
rval
be ?
GetValue
rref
).
Let
lnum
be ?
ToNumber
lval
).
Let
rnum
be ?
ToNumber
rval
).
Return the result of applying the subtraction operation to
lnum
and
rnum
. See the note below
12.8.5
12.8.5
Applying the Additive Operators to Numbers
The
operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The
operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is
NaN
, the result is
NaN
The sum of two infinities of opposite sign is
NaN
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is
-0
. The sum of two positive zeroes, or of two zeroes of opposite sign, is
+0
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is
+0
In the remaining cases, where neither an infinity, nor a zero, nor
NaN
is involved, and the operands have the same sign or have different
magnitudes, the sum is computed and rounded to the nearest representable
value using IEEE 754-2008 round to nearest, ties to even mode. If the
magnitude is too large to represent, the operation overflows and the
result is then an infinity of appropriate sign. The ECMAScript language
requires support of gradual underflow as defined by IEEE 754-2008.
Note
The
operator performs subtraction when applied
to two operands of numeric type, producing the difference of its
operands; the left operand is the minuend and the right operand is the
subtrahend. Given numeric operands
and
, it is always the case that
a - b
produces the same result as
a + (-b)
12.9
Bitwise Shift Operators
Syntax
ShiftExpression
[Yield, Await]
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
<<
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
>>
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
>>>
AdditiveExpression
[?Yield, ?Await]
12.9.1
Static Semantics: IsFunctionDefinition
ShiftExpression
ShiftExpression
<<
AdditiveExpression
ShiftExpression
>>
AdditiveExpression
ShiftExpression
>>>
AdditiveExpression
Return
false
12.9.2
Static Semantics: AssignmentTargetType
ShiftExpression
ShiftExpression
<<
AdditiveExpression
ShiftExpression
>>
AdditiveExpression
ShiftExpression
>>>
AdditiveExpression
Return
invalid
12.9.3
The Left Shift Operator (
<<
Note
Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.
12.9.3.1
Runtime Semantics: Evaluation
ShiftExpression
ShiftExpression
<<
AdditiveExpression
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be ?
GetValue
rref
).
Let
lnum
be ?
ToInt32
lval
).
Let
rnum
be ?
ToUint32
rval
).
Let
shiftCount
be the result of masking out all but the least significant 5 bits of
rnum
, that is, compute
rnum
& 0x1F.
Return the result of left shifting
lnum
by
shiftCount
bits. The result is a signed 32-bit integer.
12.9.4
The Signed Right Shift Operator (
>>
Note
Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
12.9.4.1
Runtime Semantics: Evaluation
ShiftExpression
ShiftExpression
>>
AdditiveExpression
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be ?
GetValue
rref
).
Let
lnum
be ?
ToInt32
lval
).
Let
rnum
be ?
ToUint32
rval
).
Let
shiftCount
be the result of masking out all but the least significant 5 bits of
rnum
, that is, compute
rnum
& 0x1F.
Return the result of performing a sign-extending right shift of
lnum
by
shiftCount
bits. The most significant bit is propagated. The result is a signed 32-bit integer.
12.9.5
The Unsigned Right Shift Operator (
>>>
Note
Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
12.9.5.1
Runtime Semantics: Evaluation
ShiftExpression
ShiftExpression
>>>
AdditiveExpression
Let
lref
be the result of evaluating
ShiftExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
AdditiveExpression
Let
rval
be ?
GetValue
rref
).
Let
lnum
be ?
ToUint32
lval
).
Let
rnum
be ?
ToUint32
rval
).
Let
shiftCount
be the result of masking out all but the least significant 5 bits of
rnum
, that is, compute
rnum
& 0x1F.
Return the result of performing a zero-filling right shift of
lnum
by
shiftCount
bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.
12.10
Relational Operators
Note 1
The result of evaluating a relational operator is always of
type Boolean, reflecting whether the relationship named by the operator
holds between its two operands.
Syntax
RelationalExpression
[In, Yield, Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
<=
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
>=
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
instanceof
ShiftExpression
[?Yield, ?Await]
[+In]
RelationalExpression
[+In, ?Yield, ?Await]
in
ShiftExpression
[?Yield, ?Await]
Note 2
The
[In]
grammar parameter is needed to avoid confusing the
in
operator in a relational expression with the
in
operator in a
for
statement.
12.10.1
Static Semantics: IsFunctionDefinition
RelationalExpression
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
<=
ShiftExpression
RelationalExpression
>=
ShiftExpression
RelationalExpression
instanceof
ShiftExpression
RelationalExpression
in
ShiftExpression
Return
false
12.10.2
Static Semantics: AssignmentTargetType
RelationalExpression
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
<=
ShiftExpression
RelationalExpression
>=
ShiftExpression
RelationalExpression
instanceof
ShiftExpression
RelationalExpression
in
ShiftExpression
Return
invalid
12.10.3
Runtime Semantics: Evaluation
RelationalExpression
RelationalExpression
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Abstract Relational Comparison
lval
rval
ReturnIfAbrupt
).
If
is
undefined
, return
false
. Otherwise, return
RelationalExpression
RelationalExpression
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Abstract Relational Comparison
rval
lval
with
LeftFirst
equal to
false
ReturnIfAbrupt
).
If
is
undefined
, return
false
. Otherwise, return
RelationalExpression
RelationalExpression
<=
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Abstract Relational Comparison
rval
lval
with
LeftFirst
equal to
false
ReturnIfAbrupt
).
If
is
true
or
undefined
, return
false
. Otherwise, return
true
RelationalExpression
RelationalExpression
>=
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Abstract Relational Comparison
lval
rval
ReturnIfAbrupt
).
If
is
true
or
undefined
, return
false
. Otherwise, return
true
RelationalExpression
RelationalExpression
instanceof
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
Return ?
InstanceofOperator
lval
rval
).
RelationalExpression
RelationalExpression
in
ShiftExpression
Let
lref
be the result of evaluating
RelationalExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
ShiftExpression
Let
rval
be ?
GetValue
rref
).
If
Type
rval
) is not Object, throw a
TypeError
exception.
Return ?
HasProperty
rval
ToPropertyKey
lval
)).
12.10.4
Runtime Semantics: InstanceofOperator (
target
The abstract operation InstanceofOperator(
target
) implements the generic algorithm for determining if ECMAScript value
is an instance of object
target
either by consulting
target
's @@hasinstance method or, if absent, determining whether the value of
target
's
prototype
property is present in
's prototype chain. This abstract operation performs the following steps:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
instOfHandler
be ?
GetMethod
target
, @@hasInstance).
If
instOfHandler
is not
undefined
, then
Return
ToBoolean
(?
Call
instOfHandler
target
, «
»)).
If
IsCallable
target
) is
false
, throw a
TypeError
exception.
Return ?
OrdinaryHasInstance
target
).
Note
Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the
instanceof
operator semantics. If an object does not define or inherit @@hasInstance it uses the default
instanceof
semantics.
12.11
Equality Operators
Note
The result of evaluating an equality operator is always of type
Boolean, reflecting whether the relationship named by the operator
holds between its two operands.
Syntax
EqualityExpression
[In, Yield, Await]
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
==
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
!=
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
===
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
!==
RelationalExpression
[?In, ?Yield, ?Await]
12.11.1
Static Semantics: IsFunctionDefinition
EqualityExpression
EqualityExpression
==
RelationalExpression
EqualityExpression
!=
RelationalExpression
EqualityExpression
===
RelationalExpression
EqualityExpression
!==
RelationalExpression
Return
false
12.11.2
Static Semantics: AssignmentTargetType
EqualityExpression
EqualityExpression
==
RelationalExpression
EqualityExpression
!=
RelationalExpression
EqualityExpression
===
RelationalExpression
EqualityExpression
!==
RelationalExpression
Return
invalid
12.11.3
Runtime Semantics: Evaluation
EqualityExpression
EqualityExpression
==
RelationalExpression
Let
lref
be the result of evaluating
EqualityExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
RelationalExpression
Let
rval
be ?
GetValue
rref
).
Return the result of performing
Abstract Equality Comparison
rval
==
lval
EqualityExpression
EqualityExpression
!=
RelationalExpression
Let
lref
be the result of evaluating
EqualityExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
RelationalExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Abstract Equality Comparison
rval
==
lval
If
is
true
, return
false
. Otherwise, return
true
EqualityExpression
EqualityExpression
===
RelationalExpression
Let
lref
be the result of evaluating
EqualityExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
RelationalExpression
Let
rval
be ?
GetValue
rref
).
Return the result of performing
Strict Equality Comparison
rval
===
lval
EqualityExpression
EqualityExpression
!==
RelationalExpression
Let
lref
be the result of evaluating
EqualityExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
RelationalExpression
Let
rval
be ?
GetValue
rref
).
Let
be the result of performing
Strict Equality Comparison
rval
===
lval
If
is
true
, return
false
. Otherwise, return
true
Note 1
Given the above definition of equality:
String comparison can be forced by:
"" + a == "" + b
Numeric comparison can be forced by:
+a == +b
Boolean comparison can be forced by:
!a == !b
Note 2
The equality operators maintain the following invariants:
A != B
is equivalent to
!(A == B)
A == B
is equivalent to
B == A
, except in the order of evaluation of
and
Note 3
The equality operator is not always transitive. For example,
there might be two distinct String objects, each representing the same
String value; each String object would be considered equal to the String
value by the
==
operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a"
and
"a" == new String("a")
are both
true
new String("a") == new String("a")
is
false
Note 4
Comparison of Strings uses a simple equality test on
sequences of code unit values. There is no attempt to use the more
complex, semantically oriented definitions of character or string
equality and collating order defined in the Unicode specification.
Therefore Strings values that are canonically equal according to the
Unicode standard could test as unequal. In effect this algorithm assumes
that both Strings are already in normalized form.
12.12
Binary Bitwise Operators
Syntax
BitwiseANDExpression
[In, Yield, Await]
EqualityExpression
[?In, ?Yield, ?Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[In, Yield, Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
BitwiseORExpression
[In, Yield, Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
BitwiseORExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
12.12.1
Static Semantics: IsFunctionDefinition
BitwiseANDExpression
BitwiseANDExpression
EqualityExpression
BitwiseXORExpression
BitwiseXORExpression
BitwiseANDExpression
BitwiseORExpression
BitwiseORExpression
BitwiseXORExpression
Return
false
12.12.2
Static Semantics: AssignmentTargetType
BitwiseANDExpression
BitwiseANDExpression
EqualityExpression
BitwiseXORExpression
BitwiseXORExpression
BitwiseANDExpression
BitwiseORExpression
BitwiseORExpression
BitwiseXORExpression
Return
invalid
12.12.3
Runtime Semantics: Evaluation
The production
, where @ is one of the bitwise operators in the productions above, is evaluated as follows:
Let
lref
be the result of evaluating
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
Let
rval
be ?
GetValue
rref
).
Let
lnum
be ?
ToInt32
lval
).
Let
rnum
be ?
ToInt32
rval
).
Return the result of applying the bitwise operator @ to
lnum
and
rnum
. The result is a signed 32-bit integer.
12.13
Binary Logical Operators
Syntax
LogicalANDExpression
[In, Yield, Await]
BitwiseORExpression
[?In, ?Yield, ?Await]
LogicalANDExpression
[?In, ?Yield, ?Await]
&&
BitwiseORExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[In, Yield, Await]
LogicalANDExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[?In, ?Yield, ?Await]
||
LogicalANDExpression
[?In, ?Yield, ?Await]
Note
The value produced by a
&&
or
||
operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
12.13.1
Static Semantics: IsFunctionDefinition
LogicalANDExpression
LogicalANDExpression
&&
BitwiseORExpression
LogicalORExpression
LogicalORExpression
||
LogicalANDExpression
Return
false
12.13.2
Static Semantics: AssignmentTargetType
LogicalANDExpression
LogicalANDExpression
&&
BitwiseORExpression
LogicalORExpression
LogicalORExpression
||
LogicalANDExpression
Return
invalid
12.13.3
Runtime Semantics: Evaluation
LogicalANDExpression
LogicalANDExpression
&&
BitwiseORExpression
Let
lref
be the result of evaluating
LogicalANDExpression
Let
lval
be ?
GetValue
lref
).
Let
lbool
be
ToBoolean
lval
).
If
lbool
is
false
, return
lval
Let
rref
be the result of evaluating
BitwiseORExpression
Return ?
GetValue
rref
).
LogicalORExpression
LogicalORExpression
||
LogicalANDExpression
Let
lref
be the result of evaluating
LogicalORExpression
Let
lval
be ?
GetValue
lref
).
Let
lbool
be
ToBoolean
lval
).
If
lbool
is
true
, return
lval
Let
rref
be the result of evaluating
LogicalANDExpression
Return ?
GetValue
rref
).
12.14
Conditional Operator (
? :
Syntax
ConditionalExpression
[In, Yield, Await]
LogicalORExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[?In, ?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
Note
The grammar for a
ConditionalExpression
in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an
Expression
but restrict the third expression to be a
ConditionalExpression
The motivation for this difference in ECMAScript is to allow an
assignment expression to be governed by either arm of a conditional and
to eliminate the confusing and fairly useless case of a comma expression
as the centre expression.
12.14.1
Static Semantics: IsFunctionDefinition
ConditionalExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
Return
false
12.14.2
Static Semantics: AssignmentTargetType
ConditionalExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
Return
invalid
12.14.3
Runtime Semantics: Evaluation
ConditionalExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
Let
lref
be the result of evaluating
LogicalORExpression
Let
lval
be
ToBoolean
(?
GetValue
lref
)).
If
lval
is
true
, then
Let
trueRef
be the result of evaluating the first
AssignmentExpression
Return ?
GetValue
trueRef
).
Else,
Let
falseRef
be the result of evaluating the second
AssignmentExpression
Return ?
GetValue
falseRef
).
12.15
Assignment Operators
Syntax
AssignmentExpression
[In, Yield, Await]
ConditionalExpression
[?In, ?Yield, ?Await]
[+Yield]
YieldExpression
[?In, ?Await]
ArrowFunction
[?In, ?Yield, ?Await]
AsyncArrowFunction
[?In, ?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentOperator
AssignmentExpression
[?In, ?Yield, ?Await]
AssignmentOperator
one of
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
**=
12.15.1
Static Semantics: Early Errors
AssignmentExpression
LeftHandSideExpression
AssignmentExpression
It is a Syntax Error if
LeftHandSideExpression
is either an
ObjectLiteral
or an
ArrayLiteral
and
LeftHandSideExpression
is not
covering
an
AssignmentPattern
It is an early
Reference
Error if
LeftHandSideExpression
is neither an
ObjectLiteral
nor an
ArrayLiteral
and AssignmentTargetType of
LeftHandSideExpression
is
invalid
It is an early Syntax Error if
LeftHandSideExpression
is neither an
ObjectLiteral
nor an
ArrayLiteral
and AssignmentTargetType of
LeftHandSideExpression
is
strict
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
It is an early
Reference
Error if AssignmentTargetType of
LeftHandSideExpression
is
invalid
It is an early Syntax Error if AssignmentTargetType of
LeftHandSideExpression
is
strict
12.15.2
Static Semantics: IsFunctionDefinition
AssignmentExpression
ArrowFunction
AsyncArrowFunction
Return
true
AssignmentExpression
YieldExpression
LeftHandSideExpression
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
Return
false
12.15.3
Static Semantics: AssignmentTargetType
AssignmentExpression
YieldExpression
ArrowFunction
AsyncArrowFunction
LeftHandSideExpression
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
Return
invalid
12.15.4
Runtime Semantics: Evaluation
AssignmentExpression
LeftHandSideExpression
AssignmentExpression
If
LeftHandSideExpression
is neither an
ObjectLiteral
nor an
ArrayLiteral
, then
Let
lref
be the result of evaluating
LeftHandSideExpression
ReturnIfAbrupt
lref
).
If
IsAnonymousFunctionDefinition
AssignmentExpression
) and IsIdentifierRef of
LeftHandSideExpression
are both
true
, then
Let
rval
be the result of performing NamedEvaluation for
AssignmentExpression
with argument
GetReferencedName
lref
).
Else,
Let
rref
be the result of evaluating
AssignmentExpression
Let
rval
be ?
GetValue
rref
).
Perform ?
PutValue
lref
rval
).
Return
rval
Let
assignmentPattern
be the
AssignmentPattern
that is
covered
by
LeftHandSideExpression
Let
rref
be the result of evaluating
AssignmentExpression
Let
rval
be ?
GetValue
rref
).
Perform ? DestructuringAssignmentEvaluation of
assignmentPattern
using
rval
as the argument.
Return
rval
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
Let
lref
be the result of evaluating
LeftHandSideExpression
Let
lval
be ?
GetValue
lref
).
Let
rref
be the result of evaluating
AssignmentExpression
Let
rval
be ?
GetValue
rref
).
Let
op
be the
where
AssignmentOperator
is
@=
Let
be the result of applying
op
to
lval
and
rval
as if evaluating the expression
lval
op
rval
Perform ?
PutValue
lref
).
Return
Note
When an assignment occurs within
strict mode code
, it is a runtime error if
lref
in step 1.f of the first algorithm or step 7 of the second algorithm it is an unresolvable reference. If it is, a
ReferenceError
exception is thrown. The
LeftHandSideExpression
also may not be a reference to a
data property
with the attribute value { [[Writable]]:
false
}, to an
accessor property
with the attribute value { [[Set]]:
undefined
}, nor to a non-existent property of an object for which the
IsExtensible
predicate returns the value
false
. In these cases a
TypeError
exception is thrown.
12.15.5
Destructuring Assignment
Supplemental Syntax
In certain circumstances when processing an instance of the production
AssignmentExpression
LeftHandSideExpression
AssignmentExpression
the following grammar is used to refine the interpretation of
LeftHandSideExpression
AssignmentPattern
[Yield, Await]
ObjectAssignmentPattern
[?Yield, ?Await]
ArrayAssignmentPattern
[?Yield, ?Await]
ObjectAssignmentPattern
[Yield, Await]
AssignmentRestProperty
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentRestProperty
[?Yield, ?Await]
opt
ArrayAssignmentPattern
[Yield, Await]
Elision
opt
AssignmentRestElement
[?Yield, ?Await]
opt
AssignmentElementList
[?Yield, ?Await]
AssignmentElementList
[?Yield, ?Await]
Elision
opt
AssignmentRestElement
[?Yield, ?Await]
opt
AssignmentRestProperty
[Yield, Await]
...
DestructuringAssignmentTarget
[?Yield, ?Await]
AssignmentPropertyList
[Yield, Await]
AssignmentProperty
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentProperty
[?Yield, ?Await]
AssignmentElementList
[Yield, Await]
AssignmentElisionElement
[?Yield, ?Await]
AssignmentElementList
[?Yield, ?Await]
AssignmentElisionElement
[?Yield, ?Await]
AssignmentElisionElement
[Yield, Await]
Elision
opt
AssignmentElement
[?Yield, ?Await]
AssignmentProperty
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
PropertyName
[?Yield, ?Await]
AssignmentElement
[?Yield, ?Await]
AssignmentElement
[Yield, Await]
DestructuringAssignmentTarget
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
AssignmentRestElement
[Yield, Await]
...
DestructuringAssignmentTarget
[?Yield, ?Await]
DestructuringAssignmentTarget
[Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
12.15.5.1
Static Semantics: Early Errors
AssignmentProperty
IdentifierReference
Initializer
opt
It is a Syntax Error if AssignmentTargetType of
IdentifierReference
is not
simple
AssignmentRestProperty
...
DestructuringAssignmentTarget
It is a Syntax Error if
DestructuringAssignmentTarget
is an
ArrayLiteral
or an
ObjectLiteral
DestructuringAssignmentTarget
LeftHandSideExpression
It is a Syntax Error if
LeftHandSideExpression
is either an
ObjectLiteral
or an
ArrayLiteral
and if
LeftHandSideExpression
is not
covering
an
AssignmentPattern
It is a Syntax Error if
LeftHandSideExpression
is neither an
ObjectLiteral
nor an
ArrayLiteral
and AssignmentTargetType(
LeftHandSideExpression
) is not
simple
12.15.5.2
Runtime Semantics: DestructuringAssignmentEvaluation
With parameter
value
ObjectAssignmentPattern
Perform ?
RequireObjectCoercible
value
).
Return
NormalCompletion
empty
).
ObjectAssignmentPattern
AssignmentPropertyList
AssignmentPropertyList
Perform ?
RequireObjectCoercible
value
).
Perform ? PropertyDestructuringAssignmentEvaluation for
AssignmentPropertyList
using
value
as the argument.
Return
NormalCompletion
empty
).
ArrayAssignmentPattern
Let
iteratorRecord
be ?
GetIterator
value
).
Return ?
IteratorClose
iteratorRecord
NormalCompletion
empty
)).
ArrayAssignmentPattern
Elision
Let
iteratorRecord
be ?
GetIterator
value
).
Let
result
be the result of performing IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
result
).
Return
result
ArrayAssignmentPattern
Elision
opt
AssignmentRestElement
Let
iteratorRecord
be ?
GetIterator
value
).
If
Elision
is present, then
Let
status
be the result of performing IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
If
status
is an
abrupt completion
, then
Assert
iteratorRecord
.[[Done]] is
true
Return
Completion
status
).
Let
result
be the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentRestElement
with
iteratorRecord
as the argument.
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
result
).
Return
result
ArrayAssignmentPattern
AssignmentElementList
Let
iteratorRecord
be ?
GetIterator
value
).
Let
result
be the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElementList
using
iteratorRecord
as the argument.
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
result
).
Return
result
ArrayAssignmentPattern
AssignmentElementList
Elision
opt
AssignmentRestElement
opt
Let
iteratorRecord
be ?
GetIterator
value
).
Let
status
be the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElementList
using
iteratorRecord
as the argument.
If
status
is an
abrupt completion
, then
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
status
).
Return
Completion
status
).
If
Elision
is present, then
Set
status
to the result of performing IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
If
status
is an
abrupt completion
, then
Assert
iteratorRecord
.[[Done]] is
true
Return
Completion
status
).
If
AssignmentRestElement
is present, then
Set
status
to the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentRestElement
with
iteratorRecord
as the argument.
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
status
).
Return
Completion
status
).
ObjectAssignmentPattern
AssignmentRestProperty
Perform ?
RequireObjectCoercible
value
).
Let
excludedNames
be a new empty
List
Return the result of performing RestDestructuringAssignmentEvaluation of
AssignmentRestProperty
with
value
and
excludedNames
as the arguments.
ObjectAssignmentPattern
AssignmentPropertyList
AssignmentRestProperty
Perform ?
RequireObjectCoercible
value
).
Let
excludedNames
be the result of performing ? PropertyDestructuringAssignmentEvaluation for
AssignmentPropertyList
using
value
as the argument.
Return the result of performing RestDestructuringAssignmentEvaluation of
AssignmentRestProperty
with
value
and
excludedNames
as the arguments.
12.15.5.3
Runtime Semantics: PropertyDestructuringAssignmentEvaluation
With parameter
value
Note
The following operations collect a list of all destructured property names.
AssignmentPropertyList
AssignmentPropertyList
AssignmentProperty
Let
propertyNames
be the result of performing ? PropertyDestructuringAssignmentEvaluation for
AssignmentPropertyList
using
value
as the argument.
Let
nextNames
be the result of performing ? PropertyDestructuringAssignmentEvaluation for
AssignmentProperty
using
value
as the argument.
Append each item in
nextNames
to the end of
propertyNames
Return
propertyNames
AssignmentProperty
IdentifierReference
Initializer
opt
Let
be StringValue of
IdentifierReference
Let
lref
be ?
ResolveBinding
).
Let
be ?
GetV
value
).
If
Initializer
opt
is present and
is
undefined
, then
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Set
to the result of performing NamedEvaluation for
Initializer
with argument
Else,
Let
defaultValue
be the result of evaluating
Initializer
Set
to ?
GetValue
defaultValue
).
Perform ?
PutValue
lref
).
Return a new
List
containing
AssignmentProperty
PropertyName
AssignmentElement
Let
name
be the result of evaluating
PropertyName
ReturnIfAbrupt
name
).
Perform ? KeyedDestructuringAssignmentEvaluation of
AssignmentElement
with
value
and
name
as the arguments.
Return a new
List
containing
name
12.15.5.4
Runtime Semantics: RestDestructuringAssignmentEvaluation
With parameters
value
and
excludedNames
AssignmentRestProperty
...
DestructuringAssignmentTarget
Let
lref
be the result of evaluating
DestructuringAssignmentTarget
ReturnIfAbrupt
lref
).
Let
restObj
be
ObjectCreate
%ObjectPrototype%
).
Perform ?
CopyDataProperties
restObj
value
excludedNames
).
Return
PutValue
lref
restObj
).
12.15.5.5
Runtime Semantics: IteratorDestructuringAssignmentEvaluation
With parameter
iteratorRecord
AssignmentElementList
AssignmentElisionElement
Return the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElisionElement
using
iteratorRecord
as the argument.
AssignmentElementList
AssignmentElementList
AssignmentElisionElement
Perform ? IteratorDestructuringAssignmentEvaluation of
AssignmentElementList
using
iteratorRecord
as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElisionElement
using
iteratorRecord
as the argument.
AssignmentElisionElement
AssignmentElement
Return the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElement
with
iteratorRecord
as the argument.
AssignmentElisionElement
Elision
AssignmentElement
Perform ? IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
Return the result of performing IteratorDestructuringAssignmentEvaluation of
AssignmentElement
with
iteratorRecord
as the argument.
Elision
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Return
NormalCompletion
empty
).
Elision
Elision
Perform ? IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Return
NormalCompletion
empty
).
AssignmentElement
DestructuringAssignmentTarget
Initializer
opt
If
DestructuringAssignmentTarget
is neither an
ObjectLiteral
nor an
ArrayLiteral
, then
Let
lref
be the result of evaluating
DestructuringAssignmentTarget
ReturnIfAbrupt
lref
).
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
value
be
IteratorValue
next
).
If
value
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
value
).
If
iteratorRecord
.[[Done]] is
true
, let
value
be
undefined
If
Initializer
is present and
value
is
undefined
, then
If
IsAnonymousFunctionDefinition
Initializer
) and IsIdentifierRef of
DestructuringAssignmentTarget
are both
true
, then
Let
be the result of performing NamedEvaluation for
Initializer
with argument
GetReferencedName
lref
).
Else,
Let
defaultValue
be the result of evaluating
Initializer
Let
be ?
GetValue
defaultValue
).
Else, let
be
value
If
DestructuringAssignmentTarget
is an
ObjectLiteral
or an
ArrayLiteral
, then
Let
nestedAssignmentPattern
be the
AssignmentPattern
that is
covered
by
DestructuringAssignmentTarget
Return the result of performing DestructuringAssignmentEvaluation of
nestedAssignmentPattern
with
as the argument.
Return ?
PutValue
lref
).
Note
Left to right evaluation order is maintained by evaluating a
DestructuringAssignmentTarget
that is not a destructuring pattern prior to accessing the iterator or evaluating the
Initializer
AssignmentRestElement
...
DestructuringAssignmentTarget
If
DestructuringAssignmentTarget
is neither an
ObjectLiteral
nor an
ArrayLiteral
, then
Let
lref
be the result of evaluating
DestructuringAssignmentTarget
ReturnIfAbrupt
lref
).
Let
be !
ArrayCreate
(0).
Let
be 0.
Repeat, while
iteratorRecord
.[[Done]] is
false
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
nextValue
be
IteratorValue
next
).
If
nextValue
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
nextValue
).
Let
status
be
CreateDataProperty
, !
ToString
),
nextValue
).
Assert
status
is
true
Increment
by 1.
If
DestructuringAssignmentTarget
is neither an
ObjectLiteral
nor an
ArrayLiteral
, then
Return ?
PutValue
lref
).
Let
nestedAssignmentPattern
be the
AssignmentPattern
that is
covered
by
DestructuringAssignmentTarget
Return the result of performing DestructuringAssignmentEvaluation of
nestedAssignmentPattern
with
as the argument.
12.15.5.6
Runtime Semantics: KeyedDestructuringAssignmentEvaluation
With parameters
value
and
propertyName
AssignmentElement
DestructuringAssignmentTarget
Initializer
opt
If
DestructuringAssignmentTarget
is neither an
ObjectLiteral
nor an
ArrayLiteral
, then
Let
lref
be the result of evaluating
DestructuringAssignmentTarget
ReturnIfAbrupt
lref
).
Let
be ?
GetV
value
propertyName
).
If
Initializer
is present and
is
undefined
, then
If
IsAnonymousFunctionDefinition
Initializer
) and IsIdentifierRef of
DestructuringAssignmentTarget
are both
true
, then
Let
rhsValue
be the result of performing NamedEvaluation for
Initializer
with argument
GetReferencedName
lref
).
Else,
Let
defaultValue
be the result of evaluating
Initializer
Let
rhsValue
be ?
GetValue
defaultValue
).
Else, let
rhsValue
be
If
DestructuringAssignmentTarget
is an
ObjectLiteral
or an
ArrayLiteral
, then
Let
assignmentPattern
be the
AssignmentPattern
that is
covered
by
DestructuringAssignmentTarget
Return the result of performing DestructuringAssignmentEvaluation of
assignmentPattern
with
rhsValue
as the argument.
Return ?
PutValue
lref
rhsValue
).
12.16
Comma Operator (
Syntax
Expression
[In, Yield, Await]
AssignmentExpression
[?In, ?Yield, ?Await]
Expression
[?In, ?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
12.16.1
Static Semantics: IsFunctionDefinition
Expression
Expression
AssignmentExpression
Return
false
12.16.2
Static Semantics: AssignmentTargetType
Expression
Expression
AssignmentExpression
Return
invalid
12.16.3
Runtime Semantics: Evaluation
Expression
Expression
AssignmentExpression
Let
lref
be the result of evaluating
Expression
Perform ?
GetValue
lref
).
Let
rref
be the result of evaluating
AssignmentExpression
Return ?
GetValue
rref
).
Note
GetValue
must be called even though its value is not used because it may have observable side-effects.
13
ECMAScript Language: Statements and Declarations
Syntax
Statement
[Yield, Await, Return]
BlockStatement
[?Yield, ?Await, ?Return]
VariableStatement
[?Yield, ?Await]
EmptyStatement
ExpressionStatement
[?Yield, ?Await]
IfStatement
[?Yield, ?Await, ?Return]
BreakableStatement
[?Yield, ?Await, ?Return]
ContinueStatement
[?Yield, ?Await]
BreakStatement
[?Yield, ?Await]
[+Return]
ReturnStatement
[?Yield, ?Await]
WithStatement
[?Yield, ?Await, ?Return]
LabelledStatement
[?Yield, ?Await, ?Return]
ThrowStatement
[?Yield, ?Await]
TryStatement
[?Yield, ?Await, ?Return]
DebuggerStatement
Declaration
[Yield, Await]
HoistableDeclaration
[?Yield, ?Await, ~Default]
ClassDeclaration
[?Yield, ?Await, ~Default]
LexicalDeclaration
[+In, ?Yield, ?Await]
HoistableDeclaration
[Yield, Await, Default]
FunctionDeclaration
[?Yield, ?Await, ?Default]
GeneratorDeclaration
[?Yield, ?Await, ?Default]
AsyncFunctionDeclaration
[?Yield, ?Await, ?Default]
AsyncGeneratorDeclaration
[?Yield, ?Await, ?Default]
BreakableStatement
[Yield, Await, Return]
IterationStatement
[?Yield, ?Await, ?Return]
SwitchStatement
[?Yield, ?Await, ?Return]
13.1
Statement Semantics
13.1.1
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
Statement
VariableStatement
EmptyStatement
ExpressionStatement
ContinueStatement
BreakStatement
ReturnStatement
ThrowStatement
DebuggerStatement
Return
false
13.1.2
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
Statement
VariableStatement
EmptyStatement
ExpressionStatement
ContinueStatement
ReturnStatement
ThrowStatement
DebuggerStatement
Return
false
13.1.3
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
Statement
VariableStatement
EmptyStatement
ExpressionStatement
BreakStatement
ReturnStatement
ThrowStatement
DebuggerStatement
Return
false
BreakableStatement
IterationStatement
Let
newIterationSet
be a copy of
iterationSet
with all the elements of
labelSet
appended.
Return ContainsUndefinedContinueTarget of
IterationStatement
with arguments
newIterationSet
and « ».
13.1.4
Static Semantics: DeclarationPart
HoistableDeclaration
FunctionDeclaration
Return
FunctionDeclaration
HoistableDeclaration
GeneratorDeclaration
Return
GeneratorDeclaration
HoistableDeclaration
AsyncFunctionDeclaration
Return
AsyncFunctionDeclaration
HoistableDeclaration
AsyncGeneratorDeclaration
Return
AsyncGeneratorDeclaration
Declaration
ClassDeclaration
Return
ClassDeclaration
Declaration
LexicalDeclaration
Return
LexicalDeclaration
13.1.5
Static Semantics: VarDeclaredNames
Statement
EmptyStatement
ExpressionStatement
ContinueStatement
BreakStatement
ReturnStatement
ThrowStatement
DebuggerStatement
Return a new empty
List
13.1.6
Static Semantics: VarScopedDeclarations
Statement
EmptyStatement
ExpressionStatement
ContinueStatement
BreakStatement
ReturnStatement
ThrowStatement
DebuggerStatement
Return a new empty
List
13.1.7
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
BreakableStatement
IterationStatement
Let
stmtResult
be the result of performing LabelledEvaluation of
IterationStatement
with argument
labelSet
If
stmtResult
.[[Type]] is
break
, then
If
stmtResult
.[[Target]] is
empty
, then
If
stmtResult
.[[Value]] is
empty
, set
stmtResult
to
NormalCompletion
undefined
).
Else, set
stmtResult
to
NormalCompletion
stmtResult
.[[Value]]).
Return
Completion
stmtResult
).
BreakableStatement
SwitchStatement
Let
stmtResult
be the result of evaluating
SwitchStatement
If
stmtResult
.[[Type]] is
break
, then
If
stmtResult
.[[Target]] is
empty
, then
If
stmtResult
.[[Value]] is
empty
, set
stmtResult
to
NormalCompletion
undefined
).
Else, set
stmtResult
to
NormalCompletion
stmtResult
.[[Value]]).
Return
Completion
stmtResult
).
Note
BreakableStatement
is one that can be exited via an unlabelled
BreakStatement
13.1.8
Runtime Semantics: Evaluation
HoistableDeclaration
GeneratorDeclaration
AsyncFunctionDeclaration
AsyncGeneratorDeclaration
Return
NormalCompletion
empty
).
HoistableDeclaration
FunctionDeclaration
Return the result of evaluating
FunctionDeclaration
BreakableStatement
IterationStatement
SwitchStatement
Let
newLabelSet
be a new empty
List
Return the result of performing LabelledEvaluation of this
BreakableStatement
with argument
newLabelSet
13.2
Block
Syntax
BlockStatement
[Yield, Await, Return]
Block
[?Yield, ?Await, ?Return]
Block
[Yield, Await, Return]
StatementList
[?Yield, ?Await, ?Return]
opt
StatementList
[Yield, Await, Return]
StatementListItem
[?Yield, ?Await, ?Return]
StatementList
[?Yield, ?Await, ?Return]
StatementListItem
[?Yield, ?Await, ?Return]
StatementListItem
[Yield, Await, Return]
Statement
[?Yield, ?Await, ?Return]
Declaration
[?Yield, ?Await]
13.2.1
Static Semantics: Early Errors
Block
StatementList
It is a Syntax Error if the LexicallyDeclaredNames of
StatementList
contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
StatementList
also occurs in the VarDeclaredNames of
StatementList
13.2.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
Block
Return
false
StatementList
StatementList
StatementListItem
Let
hasDuplicates
be ContainsDuplicateLabels of
StatementList
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
StatementListItem
with argument
labelSet
StatementListItem
Declaration
Return
false
13.2.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
Block
Return
false
StatementList
StatementList
StatementListItem
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
StatementList
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
StatementListItem
with argument
labelSet
StatementListItem
Declaration
Return
false
13.2.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
Block
Return
false
StatementList
StatementList
StatementListItem
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
StatementList
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
StatementListItem
with arguments
iterationSet
and « ».
StatementListItem
Declaration
Return
false
13.2.5
Static Semantics: LexicallyDeclaredNames
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
names
be LexicallyDeclaredNames of
StatementList
Append to
names
the elements of the LexicallyDeclaredNames of
StatementListItem
Return
names
StatementListItem
Statement
If
Statement
is
Statement
LabelledStatement
, return LexicallyDeclaredNames of
LabelledStatement
Return a new empty
List
StatementListItem
Declaration
Return the BoundNames of
Declaration
13.2.6
Static Semantics: LexicallyScopedDeclarations
StatementList
StatementList
StatementListItem
Let
declarations
be LexicallyScopedDeclarations of
StatementList
Append to
declarations
the elements of the LexicallyScopedDeclarations of
StatementListItem
Return
declarations
StatementListItem
Statement
If
Statement
is
Statement
LabelledStatement
, return LexicallyScopedDeclarations of
LabelledStatement
Return a new empty
List
StatementListItem
Declaration
Return a new
List
containing DeclarationPart of
Declaration
13.2.7
Static Semantics: TopLevelLexicallyDeclaredNames
StatementList
StatementList
StatementListItem
Let
names
be TopLevelLexicallyDeclaredNames of
StatementList
Append to
names
the elements of the TopLevelLexicallyDeclaredNames of
StatementListItem
Return
names
StatementListItem
Statement
Return a new empty
List
StatementListItem
Declaration
If
Declaration
is
Declaration
HoistableDeclaration
, then
Return « ».
Return the BoundNames of
Declaration
Note
At the top level of a function, or script, function
declarations are treated like var declarations rather than like lexical
declarations.
13.2.8
Static Semantics: TopLevelLexicallyScopedDeclarations
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
declarations
be TopLevelLexicallyScopedDeclarations of
StatementList
Append to
declarations
the elements of the TopLevelLexicallyScopedDeclarations of
StatementListItem
Return
declarations
StatementListItem
Statement
Return a new empty
List
StatementListItem
Declaration
If
Declaration
is
Declaration
HoistableDeclaration
, then
Return « ».
Return a new
List
containing
Declaration
13.2.9
Static Semantics: TopLevelVarDeclaredNames
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
names
be TopLevelVarDeclaredNames of
StatementList
Append to
names
the elements of the TopLevelVarDeclaredNames of
StatementListItem
Return
names
StatementListItem
Declaration
If
Declaration
is
Declaration
HoistableDeclaration
, then
Return the BoundNames of
HoistableDeclaration
Return a new empty
List
StatementListItem
Statement
If
Statement
is
Statement
LabelledStatement
, return TopLevelVarDeclaredNames of
Statement
Return VarDeclaredNames of
Statement
Note
At the top level of a function or script, inner function declarations are treated like var declarations.
13.2.10
Static Semantics: TopLevelVarScopedDeclarations
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
declarations
be TopLevelVarScopedDeclarations of
StatementList
Append to
declarations
the elements of the TopLevelVarScopedDeclarations of
StatementListItem
Return
declarations
StatementListItem
Statement
If
Statement
is
Statement
LabelledStatement
, return TopLevelVarScopedDeclarations of
Statement
Return VarScopedDeclarations of
Statement
StatementListItem
Declaration
If
Declaration
is
Declaration
HoistableDeclaration
, then
Let
declaration
be DeclarationPart of
HoistableDeclaration
Return «
declaration
».
Return a new empty
List
13.2.11
Static Semantics: VarDeclaredNames
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
names
be VarDeclaredNames of
StatementList
Append to
names
the elements of the VarDeclaredNames of
StatementListItem
Return
names
StatementListItem
Declaration
Return a new empty
List
13.2.12
Static Semantics: VarScopedDeclarations
Block
Return a new empty
List
StatementList
StatementList
StatementListItem
Let
declarations
be VarScopedDeclarations of
StatementList
Append to
declarations
the elements of the VarScopedDeclarations of
StatementListItem
Return
declarations
StatementListItem
Declaration
Return a new empty
List
13.2.13
Runtime Semantics: Evaluation
Block
Return
NormalCompletion
empty
).
Block
StatementList
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
blockEnv
be
NewDeclarativeEnvironment
oldEnv
).
Perform
BlockDeclarationInstantiation
StatementList
blockEnv
).
Set the
running execution context
's LexicalEnvironment to
blockEnv
Let
blockValue
be the result of evaluating
StatementList
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
blockValue
Note 1
No matter how control leaves the
Block
the LexicalEnvironment is always restored to its former state.
StatementList
StatementList
StatementListItem
Let
sl
be the result of evaluating
StatementList
ReturnIfAbrupt
sl
).
Let
be the result of evaluating
StatementListItem
Return
Completion
UpdateEmpty
sl
)).
Note 2
The value of a
StatementList
is the value of the last value-producing item in the
StatementList
. For example, the following calls to the
eval
function all return the value 1:
eval
"1;;;;;"
eval
"1;{}"
eval
"1;var a;"
13.2.14
Runtime Semantics: BlockDeclarationInstantiation (
code
env
Note
When a
Block
or
CaseBlock
is evaluated a new declarative
Environment Record
is created and bindings for each block scoped variable, constant,
function, or class declared in the block are instantiated in the
Environment Record
BlockDeclarationInstantiation is performed as follows using arguments
code
and
env
code
is the
Parse Node
corresponding to the body of the block.
env
is the
Lexical Environment
in which bindings are to be created.
Let
envRec
be
env
's
EnvironmentRecord
Assert
envRec
is a declarative
Environment Record
Let
declarations
be the LexicallyScopedDeclarations of
code
For each element
in
declarations
, do
For each element
dn
of the BoundNames of
, do
If IsConstantDeclaration of
is
true
, then
Perform !
envRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform !
envRec
.CreateMutableBinding(
dn
false
).
If
is a
FunctionDeclaration
, a
GeneratorDeclaration
, an
AsyncFunctionDeclaration
, or an
AsyncGeneratorDeclaration
, then
Let
fn
be the sole element of the BoundNames of
Let
fo
be the result of performing InstantiateFunctionObject for
with argument
env
Perform
envRec
.InitializeBinding(
fn
fo
).
13.3
Declarations and the Variable Statement
13.3.1
Let and Const Declarations
Note
let
and
const
declarations define variables that are scoped to the
running execution context
's LexicalEnvironment. The variables are created when their containing
Lexical Environment
is instantiated but may not be accessed in any way until the variable's
LexicalBinding
is evaluated. A variable defined by a
LexicalBinding
with an
Initializer
is assigned the value of its
Initializer
's
AssignmentExpression
when the
LexicalBinding
is evaluated, not when the variable is created. If a
LexicalBinding
in a
let
declaration does not have an
Initializer
the variable is assigned the value
undefined
when the
LexicalBinding
is evaluated.
Syntax
LexicalDeclaration
[In, Yield, Await]
LetOrConst
BindingList
[?In, ?Yield, ?Await]
LetOrConst
let
const
BindingList
[In, Yield, Await]
LexicalBinding
[?In, ?Yield, ?Await]
BindingList
[?In, ?Yield, ?Await]
LexicalBinding
[?In, ?Yield, ?Await]
LexicalBinding
[In, Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
opt
BindingPattern
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
13.3.1.1
Static Semantics: Early Errors
LexicalDeclaration
LetOrConst
BindingList
It is a Syntax Error if the BoundNames of
BindingList
contains
"let"
It is a Syntax Error if the BoundNames of
BindingList
contains any duplicate entries.
LexicalBinding
BindingIdentifier
Initializer
opt
It is a Syntax Error if
Initializer
is not present and IsConstantDeclaration of the
LexicalDeclaration
containing this
LexicalBinding
is
true
13.3.1.2
Static Semantics: BoundNames
LexicalDeclaration
LetOrConst
BindingList
Return the BoundNames of
BindingList
BindingList
BindingList
LexicalBinding
Let
names
be the BoundNames of
BindingList
Append to
names
the elements of the BoundNames of
LexicalBinding
Return
names
LexicalBinding
BindingIdentifier
Initializer
opt
Return the BoundNames of
BindingIdentifier
LexicalBinding
BindingPattern
Initializer
Return the BoundNames of
BindingPattern
13.3.1.3
Static Semantics: IsConstantDeclaration
LexicalDeclaration
LetOrConst
BindingList
Return IsConstantDeclaration of
LetOrConst
LetOrConst
let
Return
false
LetOrConst
const
Return
true
13.3.1.4
Runtime Semantics: Evaluation
LexicalDeclaration
LetOrConst
BindingList
Let
next
be the result of evaluating
BindingList
ReturnIfAbrupt
next
).
Return
NormalCompletion
empty
).
BindingList
BindingList
LexicalBinding
Let
next
be the result of evaluating
BindingList
ReturnIfAbrupt
next
).
Return the result of evaluating
LexicalBinding
LexicalBinding
BindingIdentifier
Let
lhs
be
ResolveBinding
(StringValue of
BindingIdentifier
).
Return
InitializeReferencedBinding
lhs
undefined
).
Note
static semantics
rule ensures that this form of
LexicalBinding
never occurs in a
const
declaration.
LexicalBinding
BindingIdentifier
Initializer
Let
bindingId
be StringValue of
BindingIdentifier
Let
lhs
be
ResolveBinding
bindingId
).
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Let
value
be the result of performing NamedEvaluation for
Initializer
with argument
bindingId
Else,
Let
rhs
be the result of evaluating
Initializer
Let
value
be ?
GetValue
rhs
).
Return
InitializeReferencedBinding
lhs
value
).
LexicalBinding
BindingPattern
Initializer
Let
rhs
be the result of evaluating
Initializer
Let
value
be ?
GetValue
rhs
).
Let
env
be the
running execution context
's LexicalEnvironment.
Return the result of performing BindingInitialization for
BindingPattern
using
value
and
env
as the arguments.
13.3.2
Variable Statement
Note
var
statement declares variables that are scoped to the
running execution context
's VariableEnvironment. Var variables are created when their containing
Lexical Environment
is instantiated and are initialized to
undefined
when created. Within the scope of any VariableEnvironment a common
BindingIdentifier
may appear in more than one
VariableDeclaration
but those declarations collectively define only one variable. A variable defined by a
VariableDeclaration
with an
Initializer
is assigned the value of its
Initializer
's
AssignmentExpression
when the
VariableDeclaration
is executed, not when the variable is created.
Syntax
VariableStatement
[Yield, Await]
var
VariableDeclarationList
[+In, ?Yield, ?Await]
VariableDeclarationList
[In, Yield, Await]
VariableDeclaration
[?In, ?Yield, ?Await]
VariableDeclarationList
[?In, ?Yield, ?Await]
VariableDeclaration
[?In, ?Yield, ?Await]
VariableDeclaration
[In, Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
opt
BindingPattern
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
13.3.2.1
Static Semantics: BoundNames
VariableDeclarationList
VariableDeclarationList
VariableDeclaration
Let
names
be BoundNames of
VariableDeclarationList
Append to
names
the elements of BoundNames of
VariableDeclaration
Return
names
VariableDeclaration
BindingIdentifier
Initializer
opt
Return the BoundNames of
BindingIdentifier
VariableDeclaration
BindingPattern
Initializer
Return the BoundNames of
BindingPattern
13.3.2.2
Static Semantics: VarDeclaredNames
VariableStatement
var
VariableDeclarationList
Return BoundNames of
VariableDeclarationList
13.3.2.3
Static Semantics: VarScopedDeclarations
VariableDeclarationList
VariableDeclaration
Return a new
List
containing
VariableDeclaration
VariableDeclarationList
VariableDeclarationList
VariableDeclaration
Let
declarations
be VarScopedDeclarations of
VariableDeclarationList
Append
VariableDeclaration
to
declarations
Return
declarations
13.3.2.4
Runtime Semantics: Evaluation
VariableStatement
var
VariableDeclarationList
Let
next
be the result of evaluating
VariableDeclarationList
ReturnIfAbrupt
next
).
Return
NormalCompletion
empty
).
VariableDeclarationList
VariableDeclarationList
VariableDeclaration
Let
next
be the result of evaluating
VariableDeclarationList
ReturnIfAbrupt
next
).
Return the result of evaluating
VariableDeclaration
VariableDeclaration
BindingIdentifier
Return
NormalCompletion
empty
).
VariableDeclaration
BindingIdentifier
Initializer
Let
bindingId
be StringValue of
BindingIdentifier
Let
lhs
be ?
ResolveBinding
bindingId
).
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Let
value
be the result of performing NamedEvaluation for
Initializer
with argument
bindingId
Else,
Let
rhs
be the result of evaluating
Initializer
Let
value
be ?
GetValue
rhs
).
Return ?
PutValue
lhs
value
).
Note
If a
VariableDeclaration
is nested within a with statement and the
BindingIdentifier
in the
VariableDeclaration
is the same as a
property name
of the binding object of the with statement's object
Environment Record
, then step 6 will assign
value
to the property instead of assigning to the VariableEnvironment binding of the
Identifier
VariableDeclaration
BindingPattern
Initializer
Let
rhs
be the result of evaluating
Initializer
Let
rval
be ?
GetValue
rhs
).
Return the result of performing BindingInitialization for
BindingPattern
passing
rval
and
undefined
as arguments.
13.3.3
Destructuring Binding Patterns
Syntax
BindingPattern
[Yield, Await]
ObjectBindingPattern
[?Yield, ?Await]
ArrayBindingPattern
[?Yield, ?Await]
ObjectBindingPattern
[Yield, Await]
BindingRestProperty
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingRestProperty
[?Yield, ?Await]
opt
ArrayBindingPattern
[Yield, Await]
Elision
opt
BindingRestElement
[?Yield, ?Await]
opt
BindingElementList
[?Yield, ?Await]
BindingElementList
[?Yield, ?Await]
Elision
opt
BindingRestElement
[?Yield, ?Await]
opt
BindingRestProperty
[Yield, Await]
...
BindingIdentifier
[?Yield, ?Await]
BindingPropertyList
[Yield, Await]
BindingProperty
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingProperty
[?Yield, ?Await]
BindingElementList
[Yield, Await]
BindingElisionElement
[?Yield, ?Await]
BindingElementList
[?Yield, ?Await]
BindingElisionElement
[?Yield, ?Await]
BindingElisionElement
[Yield, Await]
Elision
opt
BindingElement
[?Yield, ?Await]
BindingProperty
[Yield, Await]
SingleNameBinding
[?Yield, ?Await]
PropertyName
[?Yield, ?Await]
BindingElement
[?Yield, ?Await]
BindingElement
[Yield, Await]
SingleNameBinding
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
SingleNameBinding
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
BindingRestElement
[Yield, Await]
...
BindingIdentifier
[?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
13.3.3.1
Static Semantics: BoundNames
ObjectBindingPattern
Return a new empty
List
ArrayBindingPattern
Elision
opt
Return a new empty
List
ArrayBindingPattern
Elision
opt
BindingRestElement
Return the BoundNames of
BindingRestElement
ArrayBindingPattern
BindingElementList
Elision
opt
Return the BoundNames of
BindingElementList
ArrayBindingPattern
BindingElementList
Elision
opt
BindingRestElement
Let
names
be BoundNames of
BindingElementList
Append to
names
the elements of BoundNames of
BindingRestElement
Return
names
BindingPropertyList
BindingPropertyList
BindingProperty
Let
names
be BoundNames of
BindingPropertyList
Append to
names
the elements of BoundNames of
BindingProperty
Return
names
BindingElementList
BindingElementList
BindingElisionElement
Let
names
be BoundNames of
BindingElementList
Append to
names
the elements of BoundNames of
BindingElisionElement
Return
names
BindingElisionElement
Elision
opt
BindingElement
Return BoundNames of
BindingElement
BindingProperty
PropertyName
BindingElement
Return the BoundNames of
BindingElement
SingleNameBinding
BindingIdentifier
Initializer
opt
Return the BoundNames of
BindingIdentifier
BindingElement
BindingPattern
Initializer
opt
Return the BoundNames of
BindingPattern
13.3.3.2
Static Semantics: ContainsExpression
ObjectBindingPattern
Return
false
ArrayBindingPattern
Elision
opt
Return
false
ArrayBindingPattern
Elision
opt
BindingRestElement
Return ContainsExpression of
BindingRestElement
ArrayBindingPattern
BindingElementList
Elision
opt
Return ContainsExpression of
BindingElementList
ArrayBindingPattern
BindingElementList
Elision
opt
BindingRestElement
Let
has
be ContainsExpression of
BindingElementList
If
has
is
true
, return
true
Return ContainsExpression of
BindingRestElement
BindingPropertyList
BindingPropertyList
BindingProperty
Let
has
be ContainsExpression of
BindingPropertyList
If
has
is
true
, return
true
Return ContainsExpression of
BindingProperty
BindingElementList
BindingElementList
BindingElisionElement
Let
has
be ContainsExpression of
BindingElementList
If
has
is
true
, return
true
Return ContainsExpression of
BindingElisionElement
BindingElisionElement
Elision
opt
BindingElement
Return ContainsExpression of
BindingElement
BindingProperty
PropertyName
BindingElement
Let
has
be IsComputedPropertyKey of
PropertyName
If
has
is
true
, return
true
Return ContainsExpression of
BindingElement
BindingElement
BindingPattern
Initializer
Return
true
SingleNameBinding
BindingIdentifier
Return
false
SingleNameBinding
BindingIdentifier
Initializer
Return
true
BindingRestElement
...
BindingIdentifier
Return
false
BindingRestElement
...
BindingPattern
Return ContainsExpression of
BindingPattern
13.3.3.3
Static Semantics: HasInitializer
BindingElement
BindingPattern
Return
false
BindingElement
BindingPattern
Initializer
Return
true
SingleNameBinding
BindingIdentifier
Return
false
SingleNameBinding
BindingIdentifier
Initializer
Return
true
13.3.3.4
Static Semantics: IsSimpleParameterList
BindingElement
BindingPattern
Return
false
BindingElement
BindingPattern
Initializer
Return
false
SingleNameBinding
BindingIdentifier
Return
true
SingleNameBinding
BindingIdentifier
Initializer
Return
false
13.3.3.5
Runtime Semantics: BindingInitialization
With parameters
value
and
environment
Note
When
undefined
is passed for
environment
it indicates that a
PutValue
operation should be used to assign the initialization value. This is
the case for formal parameter lists of non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal
with the possibility of multiple parameters with the same name.
BindingPattern
ObjectBindingPattern
Perform ?
RequireObjectCoercible
value
).
Return the result of performing BindingInitialization for
ObjectBindingPattern
using
value
and
environment
as arguments.
BindingPattern
ArrayBindingPattern
Let
iteratorRecord
be ?
GetIterator
value
).
Let
result
be IteratorBindingInitialization for
ArrayBindingPattern
using
iteratorRecord
and
environment
as arguments.
If
iteratorRecord
.[[Done]] is
false
, return ?
IteratorClose
iteratorRecord
result
).
Return
result
ObjectBindingPattern
Return
NormalCompletion
empty
).
ObjectBindingPattern
BindingPropertyList
BindingPropertyList
Perform ? PropertyBindingInitialization for
BindingPropertyList
using
value
and
environment
as the arguments.
Return
NormalCompletion
empty
).
ObjectBindingPattern
BindingRestProperty
Let
excludedNames
be a new empty
List
Return the result of performing RestBindingInitialization of
BindingRestProperty
with
value
environment
, and
excludedNames
as the arguments.
ObjectBindingPattern
BindingPropertyList
BindingRestProperty
Let
excludedNames
be the result of performing ? PropertyBindingInitialization of
BindingPropertyList
using
value
and
environment
as arguments.
Return the result of performing RestBindingInitialization of
BindingRestProperty
with
value
environment
, and
excludedNames
as the arguments.
13.3.3.6
Runtime Semantics: PropertyBindingInitialization
With parameters
value
and
environment
Note
These collect a list of all bound property names rather than just empty completion.
BindingPropertyList
BindingPropertyList
BindingProperty
Let
boundNames
be the result of performing ? PropertyBindingInitialization for
BindingPropertyList
using
value
and
environment
as arguments.
Let
nextNames
be the result of performing ? PropertyBindingInitialization for
BindingProperty
using
value
and
environment
as arguments.
Append each item in
nextNames
to the end of
boundNames
Return
boundNames
BindingProperty
SingleNameBinding
Let
name
be the string that is the only element of BoundNames of
SingleNameBinding
Perform ? KeyedBindingInitialization for
SingleNameBinding
using
value
environment
, and
name
as the arguments.
Return a new
List
containing
name
BindingProperty
PropertyName
BindingElement
Let
be the result of evaluating
PropertyName
ReturnIfAbrupt
).
Perform ? KeyedBindingInitialization of
BindingElement
with
value
environment
, and
as the arguments.
Return a new
List
containing
13.3.3.7
Runtime Semantics: RestBindingInitialization
With parameters
value
environment
, and
excludedNames
BindingRestProperty
...
BindingIdentifier
Let
lhs
be ?
ResolveBinding
(StringValue of
BindingIdentifier
environment
).
Let
restObj
be
ObjectCreate
%ObjectPrototype%
).
Perform ?
CopyDataProperties
restObj
value
excludedNames
).
If
environment
is
undefined
, return
PutValue
lhs
restObj
).
Return
InitializeReferencedBinding
lhs
restObj
).
13.3.3.8
Runtime Semantics: IteratorBindingInitialization
With parameters
iteratorRecord
and
environment
Note
When
undefined
is passed for
environment
it indicates that a
PutValue
operation should be used to assign the initialization value. This is
the case for formal parameter lists of non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal
with the possibility of multiple parameters with the same name.
ArrayBindingPattern
Return
NormalCompletion
empty
).
ArrayBindingPattern
Elision
Return the result of performing IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
ArrayBindingPattern
Elision
opt
BindingRestElement
If
Elision
is present, then
Perform ? IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
Return the result of performing IteratorBindingInitialization for
BindingRestElement
with
iteratorRecord
and
environment
as arguments.
ArrayBindingPattern
BindingElementList
Return the result of performing IteratorBindingInitialization for
BindingElementList
with
iteratorRecord
and
environment
as arguments.
ArrayBindingPattern
BindingElementList
Return the result of performing IteratorBindingInitialization for
BindingElementList
with
iteratorRecord
and
environment
as arguments.
ArrayBindingPattern
BindingElementList
Elision
Perform ? IteratorBindingInitialization for
BindingElementList
with
iteratorRecord
and
environment
as arguments.
Return the result of performing IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
ArrayBindingPattern
BindingElementList
Elision
opt
BindingRestElement
Perform ? IteratorBindingInitialization for
BindingElementList
with
iteratorRecord
and
environment
as arguments.
If
Elision
is present, then
Perform ? IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
Return the result of performing IteratorBindingInitialization for
BindingRestElement
with
iteratorRecord
and
environment
as arguments.
BindingElementList
BindingElisionElement
Return the result of performing IteratorBindingInitialization for
BindingElisionElement
with
iteratorRecord
and
environment
as arguments.
BindingElementList
BindingElementList
BindingElisionElement
Perform ? IteratorBindingInitialization for
BindingElementList
with
iteratorRecord
and
environment
as arguments.
Return the result of performing IteratorBindingInitialization for
BindingElisionElement
using
iteratorRecord
and
environment
as arguments.
BindingElisionElement
BindingElement
Return the result of performing IteratorBindingInitialization of
BindingElement
with
iteratorRecord
and
environment
as the arguments.
BindingElisionElement
Elision
BindingElement
Perform ? IteratorDestructuringAssignmentEvaluation of
Elision
with
iteratorRecord
as the argument.
Return the result of performing IteratorBindingInitialization of
BindingElement
with
iteratorRecord
and
environment
as the arguments.
BindingElement
SingleNameBinding
Return the result of performing IteratorBindingInitialization for
SingleNameBinding
with
iteratorRecord
and
environment
as the arguments.
SingleNameBinding
BindingIdentifier
Initializer
opt
Let
bindingId
be StringValue of
BindingIdentifier
Let
lhs
be ?
ResolveBinding
bindingId
environment
).
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
be
IteratorValue
next
).
If
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
).
If
iteratorRecord
.[[Done]] is
true
, let
be
undefined
If
Initializer
is present and
is
undefined
, then
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Set
to the result of performing NamedEvaluation for
Initializer
with argument
bindingId
Else,
Let
defaultValue
be the result of evaluating
Initializer
Set
to ?
GetValue
defaultValue
).
If
environment
is
undefined
, return ?
PutValue
lhs
).
Return
InitializeReferencedBinding
lhs
).
BindingElement
BindingPattern
Initializer
opt
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
be
IteratorValue
next
).
If
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
).
If
iteratorRecord
.[[Done]] is
true
, let
be
undefined
If
Initializer
is present and
is
undefined
, then
Let
defaultValue
be the result of evaluating
Initializer
Set
to ?
GetValue
defaultValue
).
Return the result of performing BindingInitialization of
BindingPattern
with
and
environment
as the arguments.
BindingRestElement
...
BindingIdentifier
Let
lhs
be ?
ResolveBinding
(StringValue of
BindingIdentifier
environment
).
Let
be !
ArrayCreate
(0).
Let
be 0.
Repeat,
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
If
iteratorRecord
.[[Done]] is
true
, then
If
environment
is
undefined
, return ?
PutValue
lhs
).
Return
InitializeReferencedBinding
lhs
).
Let
nextValue
be
IteratorValue
next
).
If
nextValue
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
nextValue
).
Let
status
be
CreateDataProperty
, !
ToString
),
nextValue
).
Assert
status
is
true
Increment
by 1.
BindingRestElement
...
BindingPattern
Let
be !
ArrayCreate
(0).
Let
be 0.
Repeat,
If
iteratorRecord
.[[Done]] is
false
, then
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
If
iteratorRecord
.[[Done]] is
true
, then
Return the result of performing BindingInitialization of
BindingPattern
with
and
environment
as the arguments.
Let
nextValue
be
IteratorValue
next
).
If
nextValue
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
nextValue
).
Let
status
be
CreateDataProperty
, !
ToString
),
nextValue
).
Assert
status
is
true
Increment
by 1.
13.3.3.9
Runtime Semantics: KeyedBindingInitialization
With parameters
value
environment
, and
propertyName
Note
When
undefined
is passed for
environment
it indicates that a
PutValue
operation should be used to assign the initialization value. This is
the case for formal parameter lists of non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal
with the possibility of multiple parameters with the same name.
BindingElement
BindingPattern
Initializer
opt
Let
be ?
GetV
value
propertyName
).
If
Initializer
is present and
is
undefined
, then
Let
defaultValue
be the result of evaluating
Initializer
Set
to ?
GetValue
defaultValue
).
Return the result of performing BindingInitialization for
BindingPattern
passing
and
environment
as arguments.
SingleNameBinding
BindingIdentifier
Initializer
opt
Let
bindingId
be StringValue of
BindingIdentifier
Let
lhs
be ?
ResolveBinding
bindingId
environment
).
Let
be ?
GetV
value
propertyName
).
If
Initializer
is present and
is
undefined
, then
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Set
to the result of performing NamedEvaluation for
Initializer
with argument
bindingId
Else,
Let
defaultValue
be the result of evaluating
Initializer
Set
to ?
GetValue
defaultValue
).
If
environment
is
undefined
, return ?
PutValue
lhs
).
Return
InitializeReferencedBinding
lhs
).
13.4
Empty Statement
Syntax
EmptyStatement
13.4.1
Runtime Semantics: Evaluation
EmptyStatement
Return
NormalCompletion
empty
).
13.5
Expression Statement
Syntax
ExpressionStatement
[Yield, Await]
[lookahead ∉ {
function
async
[no
LineTerminator
here]
function
class
let
}]
Expression
[+In, ?Yield, ?Await]
Note
An
ExpressionStatement
cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a
Block
. An
ExpressionStatement
cannot start with the
function
or
class
keywords because that would make it ambiguous with a
FunctionDeclaration
, a
GeneratorDeclaration
, or a
ClassDeclaration
. An
ExpressionStatement
cannot start with
async function
because that would make it ambiguous with an
AsyncFunctionDeclaration
or a
AsyncGeneratorDeclaration
. An
ExpressionStatement
cannot start with the two token sequence
let [
because that would make it ambiguous with a
let
LexicalDeclaration
whose first
LexicalBinding
was an
ArrayBindingPattern
13.5.1
Runtime Semantics: Evaluation
ExpressionStatement
Expression
Let
exprRef
be the result of evaluating
Expression
Return ?
GetValue
exprRef
).
13.6
The
if
Statement
Syntax
IfStatement
[Yield, Await, Return]
if
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
else
Statement
[?Yield, ?Await, ?Return]
if
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
Each
else
for which the choice of associated
if
is ambiguous shall be associated with the nearest possible
if
that would otherwise have no corresponding
else
13.6.1
Static Semantics: Early Errors
IfStatement
if
Expression
Statement
else
Statement
if
Expression
Statement
It is a Syntax Error if
IsLabelledFunction
Statement
) is
true
Note
It is only necessary to apply this rule if the extension specified in
B.3.2
is implemented.
13.6.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
IfStatement
if
Expression
Statement
else
Statement
Let
hasDuplicate
be ContainsDuplicateLabels of the first
Statement
with argument
labelSet
If
hasDuplicate
is
true
, return
true
Return ContainsDuplicateLabels of the second
Statement
with argument
labelSet
IfStatement
if
Expression
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
13.6.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
IfStatement
if
Expression
Statement
else
Statement
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of the first
Statement
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of the second
Statement
with argument
labelSet
IfStatement
if
Expression
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
13.6.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
IfStatement
if
Expression
Statement
else
Statement
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of the first
Statement
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of the second
Statement
with arguments
iterationSet
and « ».
IfStatement
if
Expression
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
13.6.5
Static Semantics: VarDeclaredNames
IfStatement
if
Expression
Statement
else
Statement
Let
names
be VarDeclaredNames of the first
Statement
Append to
names
the elements of the VarDeclaredNames of the second
Statement
Return
names
IfStatement
if
Expression
Statement
Return the VarDeclaredNames of
Statement
13.6.6
Static Semantics: VarScopedDeclarations
IfStatement
if
Expression
Statement
else
Statement
Let
declarations
be VarScopedDeclarations of the first
Statement
Append to
declarations
the elements of the VarScopedDeclarations of the second
Statement
Return
declarations
IfStatement
if
Expression
Statement
Return the VarScopedDeclarations of
Statement
13.6.7
Runtime Semantics: Evaluation
IfStatement
if
Expression
Statement
else
Statement
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be
ToBoolean
(?
GetValue
exprRef
)).
If
exprValue
is
true
, then
Let
stmtCompletion
be the result of evaluating the first
Statement
Else,
Let
stmtCompletion
be the result of evaluating the second
Statement
Return
Completion
UpdateEmpty
stmtCompletion
undefined
)).
IfStatement
if
Expression
Statement
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be
ToBoolean
(?
GetValue
exprRef
)).
If
exprValue
is
false
, then
Return
NormalCompletion
undefined
).
Else,
Let
stmtCompletion
be the result of evaluating
Statement
Return
Completion
UpdateEmpty
stmtCompletion
undefined
)).
13.7
Iteration Statements
Syntax
IterationStatement
[Yield, Await, Return]
do
Statement
[?Yield, ?Await, ?Return]
while
Expression
[+In, ?Yield, ?Await]
while
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ∉ {
let
}]
Expression
[~In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
var
VariableDeclarationList
[~In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
LexicalDeclaration
[~In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ∉ {
let
}]
LeftHandSideExpression
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
var
ForBinding
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
ForDeclaration
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ≠
let
LeftHandSideExpression
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
var
ForBinding
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
ForDeclaration
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
[lookahead ≠
let
LeftHandSideExpression
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
var
ForBinding
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
ForDeclaration
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
ForDeclaration
[Yield, Await]
LetOrConst
ForBinding
[?Yield, ?Await]
ForBinding
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
Note
This section is extended by Annex
B.3.6
13.7.1
Semantics
13.7.1.1
Static Semantics: Early Errors
IterationStatement
do
Statement
while
Expression
while
Expression
Statement
for
Expression
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
for
LeftHandSideExpression
in
Expression
Statement
for
var
ForBinding
in
Expression
Statement
for
ForDeclaration
in
Expression
Statement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
var
ForBinding
of
AssignmentExpression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
It is a Syntax Error if
IsLabelledFunction
Statement
) is
true
Note
It is only necessary to apply this rule if the extension specified in
B.3.2
is implemented.
13.7.1.2
Runtime Semantics: LoopContinues (
completion
labelSet
The abstract operation LoopContinues with arguments
completion
and
labelSet
is defined by the following steps:
If
completion
.[[Type]] is
normal
, return
true
If
completion
.[[Type]] is not
continue
, return
false
If
completion
.[[Target]] is
empty
, return
true
If
completion
.[[Target]] is an element of
labelSet
, return
true
Return
false
Note
Within the
Statement
part of an
IterationStatement
ContinueStatement
may be used to begin a new iteration.
13.7.2
The
do
while
Statement
13.7.2.1
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
IterationStatement
do
Statement
while
Expression
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
13.7.2.2
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
IterationStatement
do
Statement
while
Expression
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
13.7.2.3
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
IterationStatement
do
Statement
while
Expression
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
13.7.2.4
Static Semantics: VarDeclaredNames
IterationStatement
do
Statement
while
Expression
Return the VarDeclaredNames of
Statement
13.7.2.5
Static Semantics: VarScopedDeclarations
IterationStatement
do
Statement
while
Expression
Return the VarScopedDeclarations of
Statement
13.7.2.6
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
IterationStatement
do
Statement
while
Expression
Let
be
undefined
Repeat,
Let
stmtResult
be the result of evaluating
Statement
If
LoopContinues
stmtResult
labelSet
) is
false
, return
Completion
UpdateEmpty
stmtResult
)).
If
stmtResult
.[[Value]] is not
empty
, set
to
stmtResult
.[[Value]].
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be ?
GetValue
exprRef
).
If
ToBoolean
exprValue
) is
false
, return
NormalCompletion
).
13.7.3
The
while
Statement
13.7.3.1
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
IterationStatement
while
Expression
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
13.7.3.2
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
IterationStatement
while
Expression
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
13.7.3.3
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
IterationStatement
while
Expression
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
13.7.3.4
Static Semantics: VarDeclaredNames
IterationStatement
while
Expression
Statement
Return the VarDeclaredNames of
Statement
13.7.3.5
Static Semantics: VarScopedDeclarations
IterationStatement
while
Expression
Statement
Return the VarScopedDeclarations of
Statement
13.7.3.6
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
IterationStatement
while
Expression
Statement
Let
be
undefined
Repeat,
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be ?
GetValue
exprRef
).
If
ToBoolean
exprValue
) is
false
, return
NormalCompletion
).
Let
stmtResult
be the result of evaluating
Statement
If
LoopContinues
stmtResult
labelSet
) is
false
, return
Completion
UpdateEmpty
stmtResult
)).
If
stmtResult
.[[Value]] is not
empty
, set
to
stmtResult
.[[Value]].
13.7.4
The
for
Statement
13.7.4.1
Static Semantics: Early Errors
IterationStatement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
It is a Syntax Error if any element of the BoundNames of
LexicalDeclaration
also occurs in the VarDeclaredNames of
Statement
13.7.4.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
13.7.4.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
13.7.4.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
13.7.4.5
Static Semantics: VarDeclaredNames
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
Return the VarDeclaredNames of
Statement
IterationStatement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
Let
names
be BoundNames of
VariableDeclarationList
Append to
names
the elements of the VarDeclaredNames of
Statement
Return
names
IterationStatement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Return the VarDeclaredNames of
Statement
13.7.4.6
Static Semantics: VarScopedDeclarations
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
Return the VarScopedDeclarations of
Statement
IterationStatement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
Let
declarations
be VarScopedDeclarations of
VariableDeclarationList
Append to
declarations
the elements of the VarScopedDeclarations of
Statement
Return
declarations
IterationStatement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Return the VarScopedDeclarations of
Statement
13.7.4.7
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
IterationStatement
for
Expression
opt
Expression
opt
Expression
opt
Statement
If the first
Expression
is present, then
Let
exprRef
be the result of evaluating the first
Expression
Perform ?
GetValue
exprRef
).
Return ?
ForBodyEvaluation
(the second
Expression
, the third
Expression
Statement
, « »,
labelSet
).
IterationStatement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
Let
varDcl
be the result of evaluating
VariableDeclarationList
ReturnIfAbrupt
varDcl
).
Return ?
ForBodyEvaluation
(the first
Expression
, the second
Expression
Statement
, « »,
labelSet
).
IterationStatement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
loopEnv
be
NewDeclarativeEnvironment
oldEnv
).
Let
loopEnvRec
be
loopEnv
's
EnvironmentRecord
Let
isConst
be the result of performing IsConstantDeclaration of
LexicalDeclaration
Let
boundNames
be the BoundNames of
LexicalDeclaration
For each element
dn
of
boundNames
, do
If
isConst
is
true
, then
Perform !
loopEnvRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform !
loopEnvRec
.CreateMutableBinding(
dn
false
).
Set the
running execution context
's LexicalEnvironment to
loopEnv
Let
forDcl
be the result of evaluating
LexicalDeclaration
If
forDcl
is an
abrupt completion
, then
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Completion
forDcl
).
If
isConst
is
false
, let
perIterationLets
be
boundNames
; otherwise let
perIterationLets
be « ».
Let
bodyResult
be
ForBodyEvaluation
(the first
Expression
, the second
Expression
Statement
perIterationLets
labelSet
).
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Completion
bodyResult
).
13.7.4.8
Runtime Semantics: ForBodyEvaluation (
test
increment
stmt
perIterationBindings
labelSet
The abstract operation ForBodyEvaluation with arguments
test
increment
stmt
perIterationBindings
, and
labelSet
is performed as follows:
Let
be
undefined
Perform ?
CreatePerIterationEnvironment
perIterationBindings
).
Repeat,
If
test
is not
[empty]
, then
Let
testRef
be the result of evaluating
test
Let
testValue
be ?
GetValue
testRef
).
If
ToBoolean
testValue
) is
false
, return
NormalCompletion
).
Let
result
be the result of evaluating
stmt
If
LoopContinues
result
labelSet
) is
false
, return
Completion
UpdateEmpty
result
)).
If
result
.[[Value]] is not
empty
, set
to
result
.[[Value]].
Perform ?
CreatePerIterationEnvironment
perIterationBindings
).
If
increment
is not
[empty]
, then
Let
incRef
be the result of evaluating
increment
Perform ?
GetValue
incRef
).
13.7.4.9
Runtime Semantics: CreatePerIterationEnvironment (
perIterationBindings
The abstract operation CreatePerIterationEnvironment with argument
perIterationBindings
is performed as follows:
If
perIterationBindings
has any elements, then
Let
lastIterationEnv
be the
running execution context
's LexicalEnvironment.
Let
lastIterationEnvRec
be
lastIterationEnv
's
EnvironmentRecord
Let
outer
be
lastIterationEnv
's outer environment reference.
Assert
outer
is not
null
Let
thisIterationEnv
be
NewDeclarativeEnvironment
outer
).
Let
thisIterationEnvRec
be
thisIterationEnv
's
EnvironmentRecord
For each element
bn
of
perIterationBindings
, do
Perform !
thisIterationEnvRec
.CreateMutableBinding(
bn
false
).
Let
lastValue
be ?
lastIterationEnvRec
.GetBindingValue(
bn
true
).
Perform
thisIterationEnvRec
.InitializeBinding(
bn
lastValue
).
Set the
running execution context
's LexicalEnvironment to
thisIterationEnv
Return
undefined
13.7.5
The
for
in
for
of
, and
for
await
of
Statements
13.7.5.1
Static Semantics: Early Errors
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
It is a Syntax Error if
LeftHandSideExpression
is either an
ObjectLiteral
or an
ArrayLiteral
and if
LeftHandSideExpression
is not
covering
an
AssignmentPattern
If
LeftHandSideExpression
is either an
ObjectLiteral
or an
ArrayLiteral
and if
LeftHandSideExpression
is
covering
an
AssignmentPattern
then the following rules are not applied. Instead, the Early Error rules for
AssignmentPattern
are used.
It is a Syntax Error if AssignmentTargetType of
LeftHandSideExpression
is not
simple
It is a Syntax Error if the
LeftHandSideExpression
is
CoverParenthesizedExpressionAndArrowParameterList
Expression
and
Expression
derives a phrase that would produce a Syntax Error according to these rules if that phrase were substituted for
LeftHandSideExpression
. This rule is recursively applied.
Note
The last rule means that the other rules are applied even if parentheses surround
Expression
IterationStatement
for
ForDeclaration
in
Expression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
It is a Syntax Error if the BoundNames of
ForDeclaration
contains
"let"
It is a Syntax Error if any element of the BoundNames of
ForDeclaration
also occurs in the VarDeclaredNames of
Statement
It is a Syntax Error if the BoundNames of
ForDeclaration
contains any duplicate entries.
13.7.5.2
Static Semantics: BoundNames
ForDeclaration
LetOrConst
ForBinding
Return the BoundNames of
ForBinding
13.7.5.3
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
for
var
ForBinding
in
Expression
Statement
for
ForDeclaration
in
Expression
Statement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
var
ForBinding
of
AssignmentExpression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
Note
This section is extended by Annex
B.3.6
13.7.5.4
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
for
var
ForBinding
in
Expression
Statement
for
ForDeclaration
in
Expression
Statement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
var
ForBinding
of
AssignmentExpression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
Note
This section is extended by Annex
B.3.6
13.7.5.5
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
for
var
ForBinding
in
Expression
Statement
for
ForDeclaration
in
Expression
Statement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
var
ForBinding
of
AssignmentExpression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
Note
This section is extended by Annex
B.3.6
13.7.5.6
Static Semantics: IsDestructuring
ForDeclaration
LetOrConst
ForBinding
Return IsDestructuring of
ForBinding
ForBinding
BindingIdentifier
Return
false
ForBinding
BindingPattern
Return
true
Note
This section is extended by Annex
B.3.6
13.7.5.7
Static Semantics: VarDeclaredNames
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
Return the VarDeclaredNames of
Statement
IterationStatement
for
var
ForBinding
in
Expression
Statement
Let
names
be the BoundNames of
ForBinding
Append to
names
the elements of the VarDeclaredNames of
Statement
Return
names
IterationStatement
for
ForDeclaration
in
Expression
Statement
Return the VarDeclaredNames of
Statement
IterationStatement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
Return the VarDeclaredNames of
Statement
IterationStatement
for
var
ForBinding
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
Let
names
be the BoundNames of
ForBinding
Append to
names
the elements of the VarDeclaredNames of
Statement
Return
names
IterationStatement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Return the VarDeclaredNames of
Statement
Note
This section is extended by Annex
B.3.6
13.7.5.8
Static Semantics: VarScopedDeclarations
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
Return the VarScopedDeclarations of
Statement
IterationStatement
for
var
ForBinding
in
Expression
Statement
Let
declarations
be a
List
containing
ForBinding
Append to
declarations
the elements of the VarScopedDeclarations of
Statement
Return
declarations
IterationStatement
for
ForDeclaration
in
Expression
Statement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
Return the VarScopedDeclarations of
Statement
IterationStatement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
Return the VarScopedDeclarations of
Statement
IterationStatement
for
var
ForBinding
of
AssignmentExpression
Statement
for
await
var
ForBinding
of
AssignmentExpression
Statement
Let
declarations
be a
List
containing
ForBinding
Append to
declarations
the elements of the VarScopedDeclarations of
Statement
Return
declarations
IterationStatement
for
ForDeclaration
of
AssignmentExpression
Statement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Return the VarScopedDeclarations of
Statement
Note
This section is extended by Annex
B.3.6
13.7.5.9
Runtime Semantics: BindingInitialization
With parameters
value
and
environment
Note
undefined
is passed for
environment
to indicate that a
PutValue
operation should be used to assign the initialization value. This is the case for
var
statements and the formal parameter lists of some non-strict functions (see
9.2.15
). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
ForDeclaration
LetOrConst
ForBinding
Return the result of performing BindingInitialization for
ForBinding
passing
value
and
environment
as the arguments.
13.7.5.10
Runtime Semantics: BindingInstantiation
With parameter
environment
ForDeclaration
LetOrConst
ForBinding
Let
envRec
be
environment
's
EnvironmentRecord
Assert
envRec
is a declarative
Environment Record
For each element
name
of the BoundNames of
ForBinding
, do
If IsConstantDeclaration of
LetOrConst
is
true
, then
Perform !
envRec
.CreateImmutableBinding(
name
true
).
Else,
Perform !
envRec
.CreateMutableBinding(
name
false
).
13.7.5.11
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
IterationStatement
for
LeftHandSideExpression
in
Expression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
Expression
enumerate
).
Return ?
ForIn/OfBodyEvaluation
LeftHandSideExpression
Statement
keyResult
enumerate
assignment
labelSet
).
IterationStatement
for
var
ForBinding
in
Expression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
Expression
enumerate
).
Return ?
ForIn/OfBodyEvaluation
ForBinding
Statement
keyResult
enumerate
varBinding
labelSet
).
IterationStatement
for
ForDeclaration
in
Expression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(BoundNames of
ForDeclaration
Expression
enumerate
).
Return ?
ForIn/OfBodyEvaluation
ForDeclaration
Statement
keyResult
enumerate
lexicalBinding
labelSet
).
IterationStatement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
AssignmentExpression
iterate
).
Return ?
ForIn/OfBodyEvaluation
LeftHandSideExpression
Statement
keyResult
iterate
assignment
labelSet
).
IterationStatement
for
var
ForBinding
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
AssignmentExpression
iterate
).
Return ?
ForIn/OfBodyEvaluation
ForBinding
Statement
keyResult
iterate
varBinding
labelSet
).
IterationStatement
for
ForDeclaration
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(BoundNames of
ForDeclaration
AssignmentExpression
iterate
).
Return ?
ForIn/OfBodyEvaluation
ForDeclaration
Statement
keyResult
iterate
lexicalBinding
labelSet
).
IterationStatement
for
await
LeftHandSideExpression
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
AssignmentExpression
async-iterate
).
Return ?
ForIn/OfBodyEvaluation
LeftHandSideExpression
Statement
keyResult
iterate
assignment
labelSet
async
).
IterationStatement
for
await
var
ForBinding
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
AssignmentExpression
async-iterate
).
Return ?
ForIn/OfBodyEvaluation
ForBinding
Statement
keyResult
iterate
varBinding
labelSet
async
).
IterationStatement
for
await
ForDeclaration
of
AssignmentExpression
Statement
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(BoundNames of
ForDeclaration
AssignmentExpression
async-iterate
).
Return ?
ForIn/OfBodyEvaluation
ForDeclaration
Statement
keyResult
iterate
lexicalBinding
labelSet
async
).
Note
This section is extended by Annex
B.3.6
13.7.5.12
Runtime Semantics: ForIn/OfHeadEvaluation (
TDZnames
expr
iterationKind
The abstract operation ForIn/OfHeadEvaluation is called with arguments
TDZnames
expr
, and
iterationKind
. The value of
iterationKind
is either
enumerate
iterate
, or
async-iterate
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
If
TDZnames
is not an empty
List
, then
Assert
TDZnames
has no duplicate entries.
Let
TDZ
be
NewDeclarativeEnvironment
oldEnv
).
Let
TDZEnvRec
be
TDZ
's
EnvironmentRecord
For each string
name
in
TDZnames
, do
Perform !
TDZEnvRec
.CreateMutableBinding(
name
false
).
Set the
running execution context
's LexicalEnvironment to
TDZ
Let
exprRef
be the result of evaluating
expr
Set the
running execution context
's LexicalEnvironment to
oldEnv
Let
exprValue
be ?
GetValue
exprRef
).
If
iterationKind
is
enumerate
, then
If
exprValue
is
undefined
or
null
, then
Return
Completion
{ [[Type]]:
break
, [[Value]]:
empty
, [[Target]]:
empty
}.
Let
obj
be !
ToObject
exprValue
).
Return ?
EnumerateObjectProperties
obj
).
Else,
Assert
iterationKind
is
iterate
If
iterationKind
is
async-iterate
, let
iteratorHint
be
async
Else, let
iteratorHint
be
sync
Return ?
GetIterator
exprValue
iteratorHint
).
13.7.5.13
Runtime Semantics: ForIn/OfBodyEvaluation (
lhs
stmt
iteratorRecord
iterationKind
lhsKind
labelSet
[ ,
iteratorKind
] )
The abstract operation ForIn/OfBodyEvaluation is called with arguments
lhs
stmt
iteratorRecord
iterationKind
lhsKind
labelSet
, and optional argument
iteratorKind
. The value of
lhsKind
is either
assignment
varBinding
or
lexicalBinding
. The value of
iteratorKind
is either
sync
or
async
If
iteratorKind
is not present, set
iteratorKind
to
sync
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
be
undefined
Let
destructuring
be IsDestructuring of
lhs
If
destructuring
is
true
and if
lhsKind
is
assignment
, then
Assert
lhs
is a
LeftHandSideExpression
Let
assignmentPattern
be the
AssignmentPattern
that is
covered
by
lhs
Repeat,
Let
nextResult
be ?
Call
iteratorRecord
.[[NextMethod]],
iteratorRecord
.[[Iterator]], « »).
If
iteratorKind
is
async
, then set
nextResult
to ?
Await
nextResult
).
If
Type
nextResult
) is not Object, throw a
TypeError
exception.
Let
done
be ?
IteratorComplete
nextResult
).
If
done
is
true
, return
NormalCompletion
).
Let
nextValue
be ?
IteratorValue
nextResult
).
If
lhsKind
is either
assignment
or
varBinding
, then
If
destructuring
is
false
, then
Let
lhsRef
be the result of evaluating
lhs
. (It may be evaluated repeatedly.)
Else,
Assert
lhsKind
is
lexicalBinding
Assert
lhs
is a
ForDeclaration
Let
iterationEnv
be
NewDeclarativeEnvironment
oldEnv
).
Perform BindingInstantiation for
lhs
passing
iterationEnv
as the argument.
Set the
running execution context
's LexicalEnvironment to
iterationEnv
If
destructuring
is
false
, then
Assert
lhs
binds a single name.
Let
lhsName
be the sole element of BoundNames of
lhs
Let
lhsRef
be !
ResolveBinding
lhsName
).
If
destructuring
is
false
, then
If
lhsRef
is an
abrupt completion
, then
Let
status
be
lhsRef
Else if
lhsKind
is
lexicalBinding
, then
Let
status
be
InitializeReferencedBinding
lhsRef
nextValue
).
Else,
Let
status
be
PutValue
lhsRef
nextValue
).
Else,
If
lhsKind
is
assignment
, then
Let
status
be the result of performing DestructuringAssignmentEvaluation of
assignmentPattern
using
nextValue
as the argument.
Else if
lhsKind
is
varBinding
, then
Assert
lhs
is a
ForBinding
Let
status
be the result of performing BindingInitialization for
lhs
passing
nextValue
and
undefined
as the arguments.
Else,
Assert
lhsKind
is
lexicalBinding
Assert
lhs
is a
ForDeclaration
Let
status
be the result of performing BindingInitialization for
lhs
passing
nextValue
and
iterationEnv
as arguments.
If
status
is an
abrupt completion
, then
Set the
running execution context
's LexicalEnvironment to
oldEnv
If
iteratorKind
is
async
, return ?
AsyncIteratorClose
iteratorRecord
status
).
If
iterationKind
is
enumerate
, then
Return
status
Else,
Assert
iterationKind
is
iterate
Return ?
IteratorClose
iteratorRecord
status
).
Let
result
be the result of evaluating
stmt
Set the
running execution context
's LexicalEnvironment to
oldEnv
If
LoopContinues
result
labelSet
) is
false
, then
If
iterationKind
is
enumerate
, then
Return
Completion
UpdateEmpty
result
)).
Else,
Assert
iterationKind
is
iterate
Set
status
to
UpdateEmpty
result
).
If
iteratorKind
is
async
, return ?
AsyncIteratorClose
iteratorRecord
status
).
Return ?
IteratorClose
iteratorRecord
status
).
If
result
.[[Value]] is not
empty
, set
to
result
.[[Value]].
13.7.5.14
Runtime Semantics: Evaluation
ForBinding
BindingIdentifier
Let
bindingId
be StringValue of
BindingIdentifier
Return ?
ResolveBinding
bindingId
).
13.7.5.15
EnumerateObjectProperties (
When the abstract operation EnumerateObjectProperties is called with argument
, the following steps are taken:
Assert
Type
) is Object.
Return an Iterator object (
25.1.1.2
) whose
next
method iterates over all the String-valued keys of enumerable properties of
The iterator object is never directly accessible to ECMAScript code.
The mechanics and order of enumerating the properties is not specified
but must conform to the rules specified below.
The iterator's
throw
and
return
methods are
null
and are never invoked. The iterator's
next
method processes object properties to determine whether the property
key should be returned as an iterator value. Returned property keys do
not include keys that are Symbols. Properties of the target object may
be deleted during enumeration. A property that is deleted before it is
processed by the iterator's
next
method is ignored. If new
properties are added to the target object during enumeration, the newly
added properties are not guaranteed to be processed in the active
enumeration. A
property name
will be returned by the iterator's
next
method at most once in any enumeration.
Enumerating the properties of the target object includes
enumerating properties of its prototype, and the prototype of the
prototype, and so on, recursively; but a property of a prototype is not
processed if it has the same name as a property that has already been
processed by the iterator's
next
method. The values of
[[Enumerable]] attributes are not considered when determining if a
property of a prototype object has already been processed. The
enumerable property names of prototype objects must be obtained by
invoking EnumerateObjectProperties passing the prototype object as the
argument. EnumerateObjectProperties must obtain the own property keys of
the target object by calling its [[OwnPropertyKeys]] internal method.
Property attributes of the target object must be obtained by calling its
[[GetOwnProperty]] internal method.
Note
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function
EnumerateObjectProperties
obj
const
visited =
new
Set
();
for
const
key
of
Reflect
.ownKeys(obj)) {
if
typeof
key ===
"symbol"
continue
const
desc =
Reflect
.getOwnPropertyDescriptor(obj, key);
if
(desc) {
visited.add(key);
if
(desc.enumerable)
yield
key;
const
proto =
Reflect
.getPrototypeOf(obj);
if
(proto ===
null
return
for
const
protoKey
of
EnumerateObjectProperties(proto)) {
if
(!visited.has(protoKey))
yield
protoKey;
13.8
The
continue
Statement
Syntax
ContinueStatement
[Yield, Await]
continue
continue
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
13.8.1
Static Semantics: Early Errors
ContinueStatement
continue
ContinueStatement
continue
LabelIdentifier
It is a Syntax Error if this
ContinueStatement
is not nested, directly or indirectly (but not crossing function boundaries), within an
IterationStatement
13.8.2
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
ContinueStatement
continue
Return
false
ContinueStatement
continue
LabelIdentifier
If the StringValue of
LabelIdentifier
is not an element of
iterationSet
, return
true
Return
false
13.8.3
Runtime Semantics: Evaluation
ContinueStatement
continue
Return
Completion
{ [[Type]]:
continue
, [[Value]]:
empty
, [[Target]]:
empty
}.
ContinueStatement
continue
LabelIdentifier
Let
label
be the StringValue of
LabelIdentifier
Return
Completion
{ [[Type]]:
continue
, [[Value]]:
empty
, [[Target]]:
label
}.
13.9
The
break
Statement
Syntax
BreakStatement
[Yield, Await]
break
break
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
13.9.1
Static Semantics: Early Errors
BreakStatement
break
It is a Syntax Error if this
BreakStatement
is not nested, directly or indirectly (but not crossing function boundaries), within an
IterationStatement
or a
SwitchStatement
13.9.2
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
BreakStatement
break
Return
false
BreakStatement
break
LabelIdentifier
If the StringValue of
LabelIdentifier
is not an element of
labelSet
, return
true
Return
false
13.9.3
Runtime Semantics: Evaluation
BreakStatement
break
Return
Completion
{ [[Type]]:
break
, [[Value]]:
empty
, [[Target]]:
empty
}.
BreakStatement
break
LabelIdentifier
Let
label
be the StringValue of
LabelIdentifier
Return
Completion
{ [[Type]]:
break
, [[Value]]:
empty
, [[Target]]:
label
}.
13.10
The
return
Statement
Syntax
ReturnStatement
[Yield, Await]
return
return
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
Note
return
statement causes a function to cease execution and, in most cases, returns a value to the caller. If
Expression
is omitted, the return value is
undefined
. Otherwise, the return value is the value of
Expression
. A
return
statement may not actually return a value to the caller depending on surrounding context. For example, in a
try
block, a
return
statement's completion record may be replaced with another completion record during evaluation of the
finally
block.
13.10.1
Runtime Semantics: Evaluation
ReturnStatement
return
Return
Completion
{ [[Type]]:
return
, [[Value]]:
undefined
, [[Target]]:
empty
}.
ReturnStatement
return
Expression
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be ?
GetValue
exprRef
).
If !
GetGeneratorKind
() is
async
, set
exprValue
to ?
Await
exprValue
).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
exprValue
, [[Target]]:
empty
}.
13.11
The
with
Statement
Syntax
WithStatement
[Yield, Await, Return]
with
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
Note
The
with
statement adds an object
Environment Record
for a computed object to the lexical environment of the
running execution context
. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
13.11.1
Static Semantics: Early Errors
WithStatement
with
Expression
Statement
It is a Syntax Error if the code that matches this production is contained in
strict mode code
It is a Syntax Error if
IsLabelledFunction
Statement
) is
true
Note
It is only necessary to apply the second rule if the extension specified in
B.3.2
is implemented.
13.11.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
WithStatement
with
Expression
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
13.11.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
WithStatement
with
Expression
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
13.11.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
WithStatement
with
Expression
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
13.11.5
Static Semantics: VarDeclaredNames
WithStatement
with
Expression
Statement
Return the VarDeclaredNames of
Statement
13.11.6
Static Semantics: VarScopedDeclarations
WithStatement
with
Expression
Statement
Return the VarScopedDeclarations of
Statement
13.11.7
Runtime Semantics: Evaluation
WithStatement
with
Expression
Statement
Let
val
be the result of evaluating
Expression
Let
obj
be ?
ToObject
(?
GetValue
val
)).
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
newEnv
be
NewObjectEnvironment
obj
oldEnv
).
Set the
withEnvironment
flag of
newEnv
's
EnvironmentRecord
to
true
Set the
running execution context
's LexicalEnvironment to
newEnv
Let
be the result of evaluating
Statement
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Completion
UpdateEmpty
undefined
)).
Note
No matter how control leaves the embedded
Statement
, whether normally or by some form of
abrupt completion
or exception, the LexicalEnvironment is always restored to its former state.
13.12
The
switch
Statement
Syntax
SwitchStatement
[Yield, Await, Return]
switch
Expression
[+In, ?Yield, ?Await]
CaseBlock
[?Yield, ?Await, ?Return]
CaseBlock
[Yield, Await, Return]
CaseClauses
[?Yield, ?Await, ?Return]
opt
CaseClauses
[?Yield, ?Await, ?Return]
opt
DefaultClause
[?Yield, ?Await, ?Return]
CaseClauses
[?Yield, ?Await, ?Return]
opt
CaseClauses
[Yield, Await, Return]
CaseClause
[?Yield, ?Await, ?Return]
CaseClauses
[?Yield, ?Await, ?Return]
CaseClause
[?Yield, ?Await, ?Return]
CaseClause
[Yield, Await, Return]
case
Expression
[+In, ?Yield, ?Await]
StatementList
[?Yield, ?Await, ?Return]
opt
DefaultClause
[Yield, Await, Return]
default
StatementList
[?Yield, ?Await, ?Return]
opt
13.12.1
Static Semantics: Early Errors
SwitchStatement
switch
Expression
CaseBlock
It is a Syntax Error if the LexicallyDeclaredNames of
CaseBlock
contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
CaseBlock
also occurs in the VarDeclaredNames of
CaseBlock
13.12.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
SwitchStatement
switch
Expression
CaseBlock
Return ContainsDuplicateLabels of
CaseBlock
with argument
labelSet
CaseBlock
Return
false
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, then
Let
hasDuplicates
be ContainsDuplicateLabels of the first
CaseClauses
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Let
hasDuplicates
be ContainsDuplicateLabels of
DefaultClause
with argument
labelSet
If
hasDuplicates
is
true
, return
true
If the second
CaseClauses
is not present, return
false
Return ContainsDuplicateLabels of the second
CaseClauses
with argument
labelSet
CaseClauses
CaseClauses
CaseClause
Let
hasDuplicates
be ContainsDuplicateLabels of
CaseClauses
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
CaseClause
with argument
labelSet
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return ContainsDuplicateLabels of
StatementList
with argument
labelSet
Return
false
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return ContainsDuplicateLabels of
StatementList
with argument
labelSet
Return
false
13.12.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
SwitchStatement
switch
Expression
CaseBlock
Return ContainsUndefinedBreakTarget of
CaseBlock
with argument
labelSet
CaseBlock
Return
false
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, then
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of the first
CaseClauses
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
DefaultClause
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
If the second
CaseClauses
is not present, return
false
Return ContainsUndefinedBreakTarget of the second
CaseClauses
with argument
labelSet
CaseClauses
CaseClauses
CaseClause
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
CaseClauses
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
CaseClause
with argument
labelSet
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return ContainsUndefinedBreakTarget of
StatementList
with argument
labelSet
Return
false
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return ContainsUndefinedBreakTarget of
StatementList
with argument
labelSet
Return
false
13.12.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
SwitchStatement
switch
Expression
CaseBlock
Return ContainsUndefinedContinueTarget of
CaseBlock
with arguments
iterationSet
and « ».
CaseBlock
Return
false
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, then
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of the first
CaseClauses
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
DefaultClause
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
If the second
CaseClauses
is not present, return
false
Return ContainsUndefinedContinueTarget of the second
CaseClauses
with arguments
iterationSet
and « ».
CaseClauses
CaseClauses
CaseClause
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
CaseClauses
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
CaseClause
with arguments
iterationSet
and « ».
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return ContainsUndefinedContinueTarget of
StatementList
with arguments
iterationSet
and « ».
Return
false
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return ContainsUndefinedContinueTarget of
StatementList
with arguments
iterationSet
and « ».
Return
false
13.12.5
Static Semantics: LexicallyDeclaredNames
CaseBlock
Return a new empty
List
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, let
names
be the LexicallyDeclaredNames of the first
CaseClauses
Else, let
names
be a new empty
List
Append to
names
the elements of the LexicallyDeclaredNames of the
DefaultClause
If the second
CaseClauses
is not present, return
names
Return the result of appending to
names
the elements of the LexicallyDeclaredNames of the second
CaseClauses
CaseClauses
CaseClauses
CaseClause
Let
names
be LexicallyDeclaredNames of
CaseClauses
Append to
names
the elements of the LexicallyDeclaredNames of
CaseClause
Return
names
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return the LexicallyDeclaredNames of
StatementList
Return a new empty
List
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return the LexicallyDeclaredNames of
StatementList
Return a new empty
List
13.12.6
Static Semantics: LexicallyScopedDeclarations
CaseBlock
Return a new empty
List
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, let
declarations
be the LexicallyScopedDeclarations of the first
CaseClauses
Else, let
declarations
be a new empty
List
Append to
declarations
the elements of the LexicallyScopedDeclarations of the
DefaultClause
If the second
CaseClauses
is not present, return
declarations
Return the result of appending to
declarations
the elements of the LexicallyScopedDeclarations of the second
CaseClauses
CaseClauses
CaseClauses
CaseClause
Let
declarations
be LexicallyScopedDeclarations of
CaseClauses
Append to
declarations
the elements of the LexicallyScopedDeclarations of
CaseClause
Return
declarations
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return the LexicallyScopedDeclarations of
StatementList
Return a new empty
List
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return the LexicallyScopedDeclarations of
StatementList
Return a new empty
List
13.12.7
Static Semantics: VarDeclaredNames
SwitchStatement
switch
Expression
CaseBlock
Return the VarDeclaredNames of
CaseBlock
CaseBlock
Return a new empty
List
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, let
names
be the VarDeclaredNames of the first
CaseClauses
Else, let
names
be a new empty
List
Append to
names
the elements of the VarDeclaredNames of the
DefaultClause
If the second
CaseClauses
is not present, return
names
Return the result of appending to
names
the elements of the VarDeclaredNames of the second
CaseClauses
CaseClauses
CaseClauses
CaseClause
Let
names
be VarDeclaredNames of
CaseClauses
Append to
names
the elements of the VarDeclaredNames of
CaseClause
Return
names
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return the VarDeclaredNames of
StatementList
Return a new empty
List
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return the VarDeclaredNames of
StatementList
Return a new empty
List
13.12.8
Static Semantics: VarScopedDeclarations
SwitchStatement
switch
Expression
CaseBlock
Return the VarScopedDeclarations of
CaseBlock
CaseBlock
Return a new empty
List
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
If the first
CaseClauses
is present, let
declarations
be the VarScopedDeclarations of the first
CaseClauses
Else, let
declarations
be a new empty
List
Append to
declarations
the elements of the VarScopedDeclarations of the
DefaultClause
If the second
CaseClauses
is not present, return
declarations
Return the result of appending to
declarations
the elements of the VarScopedDeclarations of the second
CaseClauses
CaseClauses
CaseClauses
CaseClause
Let
declarations
be VarScopedDeclarations of
CaseClauses
Append to
declarations
the elements of the VarScopedDeclarations of
CaseClause
Return
declarations
CaseClause
case
Expression
StatementList
opt
If the
StatementList
is present, return the VarScopedDeclarations of
StatementList
Return a new empty
List
DefaultClause
default
StatementList
opt
If the
StatementList
is present, return the VarScopedDeclarations of
StatementList
Return a new empty
List
13.12.9
Runtime Semantics: CaseBlockEvaluation
With parameter
input
CaseBlock
Return
NormalCompletion
undefined
).
CaseBlock
CaseClauses
Let
be
undefined
Let
be the
List
of
CaseClause
items in
CaseClauses
, in source text order.
Let
found
be
false
For each
CaseClause
in
, do
If
found
is
false
, then
Set
found
to ?
CaseClauseIsSelected
input
).
If
found
is
true
, then
Let
be the result of evaluating
If
.[[Value]] is not
empty
, set
to
.[[Value]].
If
is an
abrupt completion
, return
Completion
UpdateEmpty
)).
Return
NormalCompletion
).
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
Let
be
undefined
If the first
CaseClauses
is present, then
Let
be the
List
of
CaseClause
items in the first
CaseClauses
, in source text order.
Else,
Let
be « ».
Let
found
be
false
For each
CaseClause
in
, do
If
found
is
false
, then
Set
found
to ?
CaseClauseIsSelected
input
).
If
found
is
true
, then
Let
be the result of evaluating
If
.[[Value]] is not
empty
, set
to
.[[Value]].
If
is an
abrupt completion
, return
Completion
UpdateEmpty
)).
Let
foundInB
be
false
If the second
CaseClauses
is present, then
Let
be the
List
of
CaseClause
items in the second
CaseClauses
, in source text order.
Else,
Let
be « ».
If
found
is
false
, then
For each
CaseClause
in
, do
If
foundInB
is
false
, then
Set
foundInB
to ?
CaseClauseIsSelected
input
).
If
foundInB
is
true
, then
Let
be the result of evaluating
CaseClause
If
.[[Value]] is not
empty
, set
to
.[[Value]].
If
is an
abrupt completion
, return
Completion
UpdateEmpty
)).
If
foundInB
is
true
, return
NormalCompletion
).
Let
be the result of evaluating
DefaultClause
If
.[[Value]] is not
empty
, set
to
.[[Value]].
If
is an
abrupt completion
, return
Completion
UpdateEmpty
)).
For each
CaseClause
in
(NOTE: this is another complete iteration of the second
CaseClauses
), do
Let
be the result of evaluating
CaseClause
If
.[[Value]] is not
empty
, set
to
.[[Value]].
If
is an
abrupt completion
, return
Completion
UpdateEmpty
)).
Return
NormalCompletion
).
13.12.10
Runtime Semantics: CaseClauseIsSelected (
input
The abstract operation CaseClauseIsSelected, given
CaseClause
and value
input
, determines whether
matches
input
Assert
is an instance of the production
CaseClause
case
Expression
StatementList
opt
Let
exprRef
be the result of evaluating the
Expression
of
Let
clauseSelector
be ?
GetValue
exprRef
).
Return the result of performing
Strict Equality Comparison
input
===
clauseSelector
Note
This operation does not execute
's
StatementList
(if any). The
CaseBlock
algorithm uses its return value to determine which
StatementList
to start executing.
13.12.11
Runtime Semantics: Evaluation
SwitchStatement
switch
Expression
CaseBlock
Let
exprRef
be the result of evaluating
Expression
Let
switchValue
be ?
GetValue
exprRef
).
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
blockEnv
be
NewDeclarativeEnvironment
oldEnv
).
Perform
BlockDeclarationInstantiation
CaseBlock
blockEnv
).
Set the
running execution context
's LexicalEnvironment to
blockEnv
Let
be the result of performing CaseBlockEvaluation of
CaseBlock
with argument
switchValue
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Note
No matter how control leaves the
SwitchStatement
the LexicalEnvironment is always restored to its former state.
CaseClause
case
Expression
Return
NormalCompletion
empty
).
CaseClause
case
Expression
StatementList
Return the result of evaluating
StatementList
DefaultClause
default
Return
NormalCompletion
empty
).
DefaultClause
default
StatementList
Return the result of evaluating
StatementList
13.13
Labelled Statements
Syntax
LabelledStatement
[Yield, Await, Return]
LabelIdentifier
[?Yield, ?Await]
LabelledItem
[?Yield, ?Await, ?Return]
LabelledItem
[Yield, Await, Return]
Statement
[?Yield, ?Await, ?Return]
FunctionDeclaration
[?Yield, ?Await, ~Default]
Note
Statement
may be prefixed by a label. Labelled statements are only used in conjunction with labelled
break
and
continue
statements. ECMAScript has no
goto
statement. A
Statement
can be part of a
LabelledStatement
, which itself can be part of a
LabelledStatement
and so on. The labels introduced this way are collectively referred to
as the “current label set” when describing the semantics of individual
statements.
13.13.1
Static Semantics: Early Errors
LabelledItem
FunctionDeclaration
It is a Syntax Error if any source text matches this rule.
Note
An alternative definition for this rule is provided in
B.3.2
13.13.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
LabelledStatement
LabelIdentifier
LabelledItem
Let
label
be the StringValue of
LabelIdentifier
If
label
is an element of
labelSet
, return
true
Let
newLabelSet
be a copy of
labelSet
with
label
appended.
Return ContainsDuplicateLabels of
LabelledItem
with argument
newLabelSet
LabelledItem
FunctionDeclaration
Return
false
13.13.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
LabelledStatement
LabelIdentifier
LabelledItem
Let
label
be the StringValue of
LabelIdentifier
Let
newLabelSet
be a copy of
labelSet
with
label
appended.
Return ContainsUndefinedBreakTarget of
LabelledItem
with argument
newLabelSet
LabelledItem
FunctionDeclaration
Return
false
13.13.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
LabelledStatement
LabelIdentifier
LabelledItem
Let
label
be the StringValue of
LabelIdentifier
Let
newLabelSet
be a copy of
labelSet
with
label
appended.
Return ContainsUndefinedContinueTarget of
LabelledItem
with arguments
iterationSet
and
newLabelSet
LabelledItem
FunctionDeclaration
Return
false
13.13.5
Static Semantics: IsLabelledFunction (
stmt
The abstract operation IsLabelledFunction with argument
stmt
performs the following steps:
If
stmt
is not a
LabelledStatement
, return
false
Let
item
be the
LabelledItem
of
stmt
If
item
is
LabelledItem
FunctionDeclaration
, return
true
Let
subStmt
be the
Statement
of
item
Return
IsLabelledFunction
subStmt
).
13.13.6
Static Semantics: LexicallyDeclaredNames
LabelledStatement
LabelIdentifier
LabelledItem
Return the LexicallyDeclaredNames of
LabelledItem
LabelledItem
Statement
Return a new empty
List
LabelledItem
FunctionDeclaration
Return BoundNames of
FunctionDeclaration
13.13.7
Static Semantics: LexicallyScopedDeclarations
LabelledStatement
LabelIdentifier
LabelledItem
Return the LexicallyScopedDeclarations of
LabelledItem
LabelledItem
Statement
Return a new empty
List
LabelledItem
FunctionDeclaration
Return a new
List
containing
FunctionDeclaration
13.13.8
Static Semantics: TopLevelLexicallyDeclaredNames
LabelledStatement
LabelIdentifier
LabelledItem
Return a new empty
List
13.13.9
Static Semantics: TopLevelLexicallyScopedDeclarations
LabelledStatement
LabelIdentifier
LabelledItem
Return a new empty
List
13.13.10
Static Semantics: TopLevelVarDeclaredNames
LabelledStatement
LabelIdentifier
LabelledItem
Return the TopLevelVarDeclaredNames of
LabelledItem
LabelledItem
Statement
If
Statement
is
Statement
LabelledStatement
, return TopLevelVarDeclaredNames of
Statement
Return VarDeclaredNames of
Statement
LabelledItem
FunctionDeclaration
Return BoundNames of
FunctionDeclaration
13.13.11
Static Semantics: TopLevelVarScopedDeclarations
LabelledStatement
LabelIdentifier
LabelledItem
Return the TopLevelVarScopedDeclarations of
LabelledItem
LabelledItem
Statement
If
Statement
is
Statement
LabelledStatement
, return TopLevelVarScopedDeclarations of
Statement
Return VarScopedDeclarations of
Statement
LabelledItem
FunctionDeclaration
Return a new
List
containing
FunctionDeclaration
13.13.12
Static Semantics: VarDeclaredNames
LabelledStatement
LabelIdentifier
LabelledItem
Return the VarDeclaredNames of
LabelledItem
LabelledItem
FunctionDeclaration
Return a new empty
List
13.13.13
Static Semantics: VarScopedDeclarations
LabelledStatement
LabelIdentifier
LabelledItem
Return the VarScopedDeclarations of
LabelledItem
LabelledItem
FunctionDeclaration
Return a new empty
List
13.13.14
Runtime Semantics: LabelledEvaluation
With parameter
labelSet
LabelledStatement
LabelIdentifier
LabelledItem
Let
label
be the StringValue of
LabelIdentifier
Append
label
as an element of
labelSet
Let
stmtResult
be LabelledEvaluation of
LabelledItem
with argument
labelSet
If
stmtResult
.[[Type]] is
break
and
SameValue
stmtResult
.[[Target]],
label
) is
true
, then
Set
stmtResult
to
NormalCompletion
stmtResult
.[[Value]]).
Return
Completion
stmtResult
).
LabelledItem
Statement
If
Statement
is either a
LabelledStatement
or a
BreakableStatement
, then
Return LabelledEvaluation of
Statement
with argument
labelSet
Else,
Return the result of evaluating
Statement
LabelledItem
FunctionDeclaration
Return the result of evaluating
FunctionDeclaration
13.13.15
Runtime Semantics: Evaluation
LabelledStatement
LabelIdentifier
LabelledItem
Let
newLabelSet
be a new empty
List
Return LabelledEvaluation of this
LabelledStatement
with argument
newLabelSet
13.14
The
throw
Statement
Syntax
ThrowStatement
[Yield, Await]
throw
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
13.14.1
Runtime Semantics: Evaluation
ThrowStatement
throw
Expression
Let
exprRef
be the result of evaluating
Expression
Let
exprValue
be ?
GetValue
exprRef
).
Return
ThrowCompletion
exprValue
).
13.15
The
try
Statement
Syntax
TryStatement
[Yield, Await, Return]
try
Block
[?Yield, ?Await, ?Return]
Catch
[?Yield, ?Await, ?Return]
try
Block
[?Yield, ?Await, ?Return]
Finally
[?Yield, ?Await, ?Return]
try
Block
[?Yield, ?Await, ?Return]
Catch
[?Yield, ?Await, ?Return]
Finally
[?Yield, ?Await, ?Return]
Catch
[Yield, Await, Return]
catch
CatchParameter
[?Yield, ?Await]
Block
[?Yield, ?Await, ?Return]
catch
Block
[?Yield, ?Await, ?Return]
Finally
[Yield, Await, Return]
finally
Block
[?Yield, ?Await, ?Return]
CatchParameter
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
Note
The
try
statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a
throw
statement. The
catch
clause provides the exception-handling code. When a catch clause catches an exception, its
CatchParameter
is bound to that exception.
13.15.1
Static Semantics: Early Errors
Catch
catch
CatchParameter
Block
It is a Syntax Error if BoundNames of
CatchParameter
contains any duplicate elements.
It is a Syntax Error if any element of the BoundNames of
CatchParameter
also occurs in the LexicallyDeclaredNames of
Block
It is a Syntax Error if any element of the BoundNames of
CatchParameter
also occurs in the VarDeclaredNames of
Block
Note
An alternative
static semantics
for this production is given in
B.3.5
13.15.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
TryStatement
try
Block
Catch
Let
hasDuplicates
be ContainsDuplicateLabels of
Block
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
Catch
with argument
labelSet
TryStatement
try
Block
Finally
Let
hasDuplicates
be ContainsDuplicateLabels of
Block
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
Finally
with argument
labelSet
TryStatement
try
Block
Catch
Finally
Let
hasDuplicates
be ContainsDuplicateLabels of
Block
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Let
hasDuplicates
be ContainsDuplicateLabels of
Catch
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
Finally
with argument
labelSet
Catch
catch
CatchParameter
Block
Return ContainsDuplicateLabels of
Block
with argument
labelSet
13.15.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
TryStatement
try
Block
Catch
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
Block
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
Catch
with argument
labelSet
TryStatement
try
Block
Finally
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
Block
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
Finally
with argument
labelSet
TryStatement
try
Block
Catch
Finally
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
Block
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
Catch
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
Finally
with argument
labelSet
Catch
catch
CatchParameter
Block
Return ContainsUndefinedBreakTarget of
Block
with argument
labelSet
13.15.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
TryStatement
try
Block
Catch
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
Block
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
Catch
with arguments
iterationSet
and « ».
TryStatement
try
Block
Finally
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
Block
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
Finally
with arguments
iterationSet
and « ».
TryStatement
try
Block
Catch
Finally
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
Block
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
Catch
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
Finally
with arguments
iterationSet
and « ».
Catch
catch
CatchParameter
Block
Return ContainsUndefinedContinueTarget of
Block
with arguments
iterationSet
and « ».
13.15.5
Static Semantics: VarDeclaredNames
TryStatement
try
Block
Catch
Let
names
be VarDeclaredNames of
Block
Append to
names
the elements of the VarDeclaredNames of
Catch
Return
names
TryStatement
try
Block
Finally
Let
names
be VarDeclaredNames of
Block
Append to
names
the elements of the VarDeclaredNames of
Finally
Return
names
TryStatement
try
Block
Catch
Finally
Let
names
be VarDeclaredNames of
Block
Append to
names
the elements of the VarDeclaredNames of
Catch
Append to
names
the elements of the VarDeclaredNames of
Finally
Return
names
Catch
catch
CatchParameter
Block
Return the VarDeclaredNames of
Block
13.15.6
Static Semantics: VarScopedDeclarations
TryStatement
try
Block
Catch
Let
declarations
be VarScopedDeclarations of
Block
Append to
declarations
the elements of the VarScopedDeclarations of
Catch
Return
declarations
TryStatement
try
Block
Finally
Let
declarations
be VarScopedDeclarations of
Block
Append to
declarations
the elements of the VarScopedDeclarations of
Finally
Return
declarations
TryStatement
try
Block
Catch
Finally
Let
declarations
be VarScopedDeclarations of
Block
Append to
declarations
the elements of the VarScopedDeclarations of
Catch
Append to
declarations
the elements of the VarScopedDeclarations of
Finally
Return
declarations
Catch
catch
CatchParameter
Block
Return the VarScopedDeclarations of
Block
13.15.7
Runtime Semantics: CatchClauseEvaluation
With parameter
thrownValue
Catch
catch
CatchParameter
Block
Let
oldEnv
be the
running execution context
's LexicalEnvironment.
Let
catchEnv
be
NewDeclarativeEnvironment
oldEnv
).
Let
catchEnvRec
be
catchEnv
's
EnvironmentRecord
For each element
argName
of the BoundNames of
CatchParameter
, do
Perform !
catchEnvRec
.CreateMutableBinding(
argName
false
).
Set the
running execution context
's LexicalEnvironment to
catchEnv
Let
status
be the result of performing BindingInitialization for
CatchParameter
passing
thrownValue
and
catchEnv
as arguments.
If
status
is an
abrupt completion
, then
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Completion
status
).
Let
be the result of evaluating
Block
Set the
running execution context
's LexicalEnvironment to
oldEnv
Return
Completion
).
Catch
catch
Block
Return the result of evaluating
Block
Note
No matter how control leaves the
Block
the LexicalEnvironment is always restored to its former state.
13.15.8
Runtime Semantics: Evaluation
TryStatement
try
Block
Catch
Let
be the result of evaluating
Block
If
.[[Type]] is
throw
, let
be CatchClauseEvaluation of
Catch
with argument
.[[Value]].
Else, let
be
Return
Completion
UpdateEmpty
undefined
)).
TryStatement
try
Block
Finally
Let
be the result of evaluating
Block
Let
be the result of evaluating
Finally
If
.[[Type]] is
normal
, set
to
Return
Completion
UpdateEmpty
undefined
)).
TryStatement
try
Block
Catch
Finally
Let
be the result of evaluating
Block
If
.[[Type]] is
throw
, let
be CatchClauseEvaluation of
Catch
with argument
.[[Value]].
Else, let
be
Let
be the result of evaluating
Finally
If
.[[Type]] is
normal
, set
to
Return
Completion
UpdateEmpty
undefined
)).
13.16
The
debugger
Statement
Syntax
DebuggerStatement
debugger
13.16.1
Runtime Semantics: Evaluation
Note
Evaluating a
DebuggerStatement
may allow an implementation to cause a breakpoint when run under a
debugger. If a debugger is not present or active this statement has no
observable effect.
DebuggerStatement
debugger
If an implementation-defined debugging facility is available and enabled, then
Perform an implementation-defined debugging action.
Let
result
be an implementation-defined
Completion
value.
Else,
Let
result
be
NormalCompletion
empty
).
Return
result
14
ECMAScript Language: Functions and Classes
Note
Various ECMAScript language elements cause the creation of ECMAScript function objects (
9.2
). Evaluation of such functions starts with the execution of their [[Call]] internal method (
9.2.1
).
14.1
Function Definitions
Syntax
FunctionDeclaration
[Yield, Await, Default]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
[+Default]
function
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
FunctionExpression
function
BindingIdentifier
[~Yield, ~Await]
opt
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
UniqueFormalParameters
[Yield, Await]
FormalParameters
[?Yield, ?Await]
FormalParameters
[Yield, Await]
[empty]
FunctionRestParameter
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FunctionRestParameter
[?Yield, ?Await]
FormalParameterList
[Yield, Await]
FormalParameter
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameter
[?Yield, ?Await]
FunctionRestParameter
[Yield, Await]
BindingRestElement
[?Yield, ?Await]
FormalParameter
[Yield, Await]
BindingElement
[?Yield, ?Await]
FunctionBody
[Yield, Await]
FunctionStatementList
[?Yield, ?Await]
FunctionStatementList
[Yield, Await]
StatementList
[?Yield, ?Await, +Return]
opt
14.1.1
Directive Prologues and the Use Strict Directive
Directive Prologue
is the longest sequence of
ExpressionStatement
s occurring as the initial
StatementListItem
s or
ModuleItem
s of a
FunctionBody
, a
ScriptBody
, or a
ModuleBody
and where each
ExpressionStatement
in the sequence consists entirely of a
StringLiteral
token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A
Directive Prologue
may be an empty sequence.
Use Strict Directive
is an
ExpressionStatement
in a
Directive Prologue
whose
StringLiteral
is either the exact code unit sequences
"use strict"
or
'use strict'
. A
Use Strict Directive
may not contain an
EscapeSequence
or
LineContinuation
Directive Prologue
may contain more than one
Use Strict Directive
. However, an implementation may issue a warning if this occurs.
Note
The
ExpressionStatement
s of a
Directive Prologue
are evaluated normally during evaluation of the containing production.
Implementations may define implementation specific meanings for
ExpressionStatement
s which are not a
Use Strict Directive
and which occur in a
Directive Prologue
. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a
Directive Prologue
an
ExpressionStatement
that is not a
Use Strict Directive
and which does not have a meaning defined by the implementation.
14.1.2
Static Semantics: Early Errors
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
FunctionDeclaration
function
FormalParameters
FunctionBody
FunctionExpression
function
BindingIdentifier
opt
FormalParameters
FunctionBody
If the source code matching this production is
strict mode code
, the Early Error rules for
UniqueFormalParameters
FormalParameters
are applied.
If the source code matching this production is
strict mode code
, it is a Syntax Error if
BindingIdentifier
is present and the StringValue of
BindingIdentifier
is
"eval"
or
"arguments"
It is a Syntax Error if ContainsUseStrict of
FunctionBody
is
true
and IsSimpleParameterList of
FormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
FormalParameters
also occurs in the LexicallyDeclaredNames of
FunctionBody
It is a Syntax Error if
FormalParameters
Contains
SuperProperty
is
true
It is a Syntax Error if
FunctionBody
Contains
SuperProperty
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperCall
is
true
It is a Syntax Error if
FunctionBody
Contains
SuperCall
is
true
Note 1
The LexicallyDeclaredNames of a
FunctionBody
does not include identifiers bound using var or function declarations.
UniqueFormalParameters
FormalParameters
It is a Syntax Error if BoundNames of
FormalParameters
contains any duplicate elements.
FormalParameters
FormalParameterList
It is a Syntax Error if IsSimpleParameterList of
FormalParameterList
is
false
and BoundNames of
FormalParameterList
contains any duplicate elements.
Note 2
Multiple occurrences of the same
BindingIdentifier
in a
FormalParameterList
is only allowed for functions which have simple parameter lists and which are not defined in
strict mode code
FunctionBody
FunctionStatementList
It is a Syntax Error if the LexicallyDeclaredNames of
FunctionStatementList
contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
FunctionStatementList
also occurs in the VarDeclaredNames of
FunctionStatementList
It is a Syntax Error if ContainsDuplicateLabels of
FunctionStatementList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedBreakTarget of
FunctionStatementList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedContinueTarget of
FunctionStatementList
with arguments « » and « » is
true
14.1.3
Static Semantics: BoundNames
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
Return the BoundNames of
BindingIdentifier
FunctionDeclaration
function
FormalParameters
FunctionBody
Return «
"*default*"
».
Note
"*default*"
is used within this specification as
a synthetic name for hoistable anonymous functions that are defined
using export declarations.
FormalParameters
[empty]
Return a new empty
List
FormalParameters
FormalParameterList
FunctionRestParameter
Let
names
be BoundNames of
FormalParameterList
Append to
names
the BoundNames of
FunctionRestParameter
Return
names
FormalParameterList
FormalParameterList
FormalParameter
Let
names
be BoundNames of
FormalParameterList
Append to
names
the BoundNames of
FormalParameter
Return
names
14.1.4
Static Semantics: Contains
With parameter
symbol
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
FunctionDeclaration
function
FormalParameters
FunctionBody
FunctionExpression
function
BindingIdentifier
opt
FormalParameters
FunctionBody
Return
false
Note
Static semantic rules that depend upon substructure generally do not look into function definitions.
14.1.5
Static Semantics: ContainsExpression
FormalParameters
[empty]
Return
false
FormalParameters
FormalParameterList
FunctionRestParameter
If ContainsExpression of
FormalParameterList
is
true
, return
true
Return ContainsExpression of
FunctionRestParameter
FormalParameterList
FormalParameterList
FormalParameter
If ContainsExpression of
FormalParameterList
is
true
, return
true
Return ContainsExpression of
FormalParameter
14.1.6
Static Semantics: ContainsUseStrict
FunctionBody
FunctionStatementList
If the
Directive Prologue
of
FunctionStatementList
contains a
Use Strict Directive
, return
true
; otherwise, return
false
14.1.7
Static Semantics: ExpectedArgumentCount
FormalParameters
[empty]
Return 0.
FormalParameters
FormalParameterList
FunctionRestParameter
Return ExpectedArgumentCount of
FormalParameterList
Note
The ExpectedArgumentCount of a
FormalParameterList
is the number of
FormalParameters
to the left of either the rest parameter or the first
FormalParameter
with an Initializer. A
FormalParameter
without an initializer is allowed after the first parameter with an
initializer but such parameters are considered to be optional with
undefined
as their default value.
FormalParameterList
FormalParameterList
FormalParameter
Let
count
be ExpectedArgumentCount of
FormalParameterList
If HasInitializer of
FormalParameterList
is
true
or HasInitializer of
FormalParameter
is
true
, return
count
Return
count
+ 1.
14.1.8
Static Semantics: HasInitializer
FormalParameterList
FormalParameterList
FormalParameter
If HasInitializer of
FormalParameterList
is
true
, return
true
Return HasInitializer of
FormalParameter
14.1.9
Static Semantics: HasName
FunctionExpression
function
FormalParameters
FunctionBody
Return
false
FunctionExpression
function
BindingIdentifier
FormalParameters
FunctionBody
Return
true
14.1.10
Static Semantics: IsAnonymousFunctionDefinition (
expr
The abstract operation IsAnonymousFunctionDefinition determines
if its argument is a function definition that does not bind a name. The
argument
expr
is the result of parsing an
AssignmentExpression
or
Initializer
. The following steps are taken:
If IsFunctionDefinition of
expr
is
false
, return
false
Let
hasName
be the result of HasName of
expr
If
hasName
is
true
, return
false
Return
true
14.1.11
Static Semantics: IsConstantDeclaration
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
FunctionDeclaration
function
FormalParameters
FunctionBody
Return
false
14.1.12
Static Semantics: IsFunctionDefinition
FunctionExpression
function
BindingIdentifier
opt
FormalParameters
FunctionBody
Return
true
14.1.13
Static Semantics: IsSimpleParameterList
FormalParameters
[empty]
Return
true
FormalParameters
FormalParameterList
FunctionRestParameter
Return
false
FormalParameterList
FormalParameterList
FormalParameter
If IsSimpleParameterList of
FormalParameterList
is
false
, return
false
Return IsSimpleParameterList of
FormalParameter
FormalParameter
BindingElement
Return IsSimpleParameterList of
BindingElement
14.1.14
Static Semantics: LexicallyDeclaredNames
FunctionStatementList
[empty]
Return a new empty
List
FunctionStatementList
StatementList
Return TopLevelLexicallyDeclaredNames of
StatementList
14.1.15
Static Semantics: LexicallyScopedDeclarations
FunctionStatementList
[empty]
Return a new empty
List
FunctionStatementList
StatementList
Return the TopLevelLexicallyScopedDeclarations of
StatementList
14.1.16
Static Semantics: VarDeclaredNames
FunctionStatementList
[empty]
Return a new empty
List
FunctionStatementList
StatementList
Return TopLevelVarDeclaredNames of
StatementList
14.1.17
Static Semantics: VarScopedDeclarations
FunctionStatementList
[empty]
Return a new empty
List
FunctionStatementList
StatementList
Return the TopLevelVarScopedDeclarations of
StatementList
14.1.18
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
FunctionBody
FunctionStatementList
Perform ?
FunctionDeclarationInstantiation
functionObject
argumentsList
).
Return the result of evaluating
FunctionStatementList
14.1.19
Runtime Semantics: IteratorBindingInitialization
With parameters
iteratorRecord
and
environment
Note 1
When
undefined
is passed for
environment
it indicates that a
PutValue
operation should be used to assign the initialization value. This is
the case for formal parameter lists of non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal
with the possibility of multiple parameters with the same name.
FormalParameters
[empty]
Return
NormalCompletion
empty
).
FormalParameters
FormalParameterList
FunctionRestParameter
Perform ? IteratorBindingInitialization for
FormalParameterList
using
iteratorRecord
and
environment
as the arguments.
Return the result of performing IteratorBindingInitialization for
FunctionRestParameter
using
iteratorRecord
and
environment
as the arguments.
FormalParameterList
FormalParameterList
FormalParameter
Perform ? IteratorBindingInitialization for
FormalParameterList
using
iteratorRecord
and
environment
as the arguments.
Return the result of performing IteratorBindingInitialization for
FormalParameter
using
iteratorRecord
and
environment
as the arguments.
FormalParameter
BindingElement
If ContainsExpression of
BindingElement
is
false
, return the result of performing IteratorBindingInitialization for
BindingElement
using
iteratorRecord
and
environment
as the arguments.
Let
currentContext
be the
running execution context
Let
originalEnv
be the VariableEnvironment of
currentContext
Assert
: The VariableEnvironment and LexicalEnvironment of
currentContext
are the same.
Assert
environment
and
originalEnv
are the same.
Let
paramVarEnv
be
NewDeclarativeEnvironment
originalEnv
).
Set the VariableEnvironment of
currentContext
to
paramVarEnv
Set the LexicalEnvironment of
currentContext
to
paramVarEnv
Let
result
be the result of performing IteratorBindingInitialization for
BindingElement
using
iteratorRecord
and
environment
as the arguments.
Set the VariableEnvironment of
currentContext
to
originalEnv
Set the LexicalEnvironment of
currentContext
to
originalEnv
Return
result
Note 2
The new
Environment Record
created in step 6 is only used if the
BindingElement
contains a
direct eval
FunctionRestParameter
BindingRestElement
If ContainsExpression of
BindingRestElement
is
false
, return the result of performing IteratorBindingInitialization for
BindingRestElement
using
iteratorRecord
and
environment
as the arguments.
Let
currentContext
be the
running execution context
Let
originalEnv
be the VariableEnvironment of
currentContext
Assert
: The VariableEnvironment and LexicalEnvironment of
currentContext
are the same.
Assert
environment
and
originalEnv
are the same.
Let
paramVarEnv
be
NewDeclarativeEnvironment
originalEnv
).
Set the VariableEnvironment of
currentContext
to
paramVarEnv
Set the LexicalEnvironment of
currentContext
to
paramVarEnv
Let
result
be the result of performing IteratorBindingInitialization for
BindingRestElement
using
iteratorRecord
and
environment
as the arguments.
Set the VariableEnvironment of
currentContext
to
originalEnv
Set the LexicalEnvironment of
currentContext
to
originalEnv
Return
result
Note 3
The new
Environment Record
created in step 6 is only used if the
BindingRestElement
contains a
direct eval
14.1.20
Runtime Semantics: InstantiateFunctionObject
With parameter
scope
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
If the function code for
FunctionDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
name
be StringValue of
BindingIdentifier
Let
be
FunctionCreate
Normal
FormalParameters
FunctionBody
scope
strict
).
Perform
MakeConstructor
).
Perform
SetFunctionName
name
).
Set
.[[SourceText]] to the source text matched by
FunctionDeclaration
Return
FunctionDeclaration
function
FormalParameters
FunctionBody
Let
be
FunctionCreate
Normal
FormalParameters
FunctionBody
scope
true
).
Perform
MakeConstructor
).
Perform
SetFunctionName
"default"
).
Set
.[[SourceText]] to the source text matched by
FunctionDeclaration
Return
Note
An anonymous
FunctionDeclaration
can only occur as part of an
export default
declaration, and its function code is therefore always
strict mode code
14.1.21
Runtime Semantics: NamedEvaluation
With parameter
name
FunctionExpression
function
FormalParameters
FunctionBody
Let
closure
be the result of evaluating this
FunctionExpression
Perform
SetFunctionName
closure
name
).
Return
closure
14.1.22
Runtime Semantics: Evaluation
FunctionDeclaration
function
BindingIdentifier
FormalParameters
FunctionBody
Return
NormalCompletion
empty
).
Note 1
An alternative semantics is provided in
B.3.3
FunctionDeclaration
function
FormalParameters
FunctionBody
Return
NormalCompletion
empty
).
FunctionExpression
function
FormalParameters
FunctionBody
If the function code for
FunctionExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
closure
be
FunctionCreate
Normal
FormalParameters
FunctionBody
scope
strict
).
Perform
MakeConstructor
closure
).
Set
closure
.[[SourceText]] to the source text matched by
FunctionExpression
Return
closure
FunctionExpression
function
BindingIdentifier
FormalParameters
FunctionBody
If the function code for
FunctionExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
funcEnv
be
NewDeclarativeEnvironment
scope
).
Let
envRec
be
funcEnv
's
EnvironmentRecord
Let
name
be StringValue of
BindingIdentifier
Perform
envRec
.CreateImmutableBinding(
name
false
).
Let
closure
be
FunctionCreate
Normal
FormalParameters
FunctionBody
funcEnv
strict
).
Perform
MakeConstructor
closure
).
Perform
SetFunctionName
closure
name
).
Set
closure
.[[SourceText]] to the source text matched by
FunctionExpression
Perform
envRec
.InitializeBinding(
name
closure
).
Return
closure
Note 2
The
BindingIdentifier
in a
FunctionExpression
can be referenced from inside the
FunctionExpression
's
FunctionBody
to allow the function to call itself recursively. However, unlike in a
FunctionDeclaration
, the
BindingIdentifier
in a
FunctionExpression
cannot be referenced from and does not affect the scope enclosing the
FunctionExpression
Note 3
prototype
property is automatically created for every function defined using a
FunctionDeclaration
or
FunctionExpression
, to allow for the possibility that the function will be used as a
constructor
FunctionStatementList
[empty]
Return
NormalCompletion
undefined
).
14.2
Arrow Function Definitions
Syntax
ArrowFunction
[In, Yield, Await]
ArrowParameters
[?Yield, ?Await]
[no
LineTerminator
here]
=>
ConciseBody
[?In]
ArrowParameters
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
ConciseBody
[In]
[lookahead ≠
AssignmentExpression
[?In, ~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
Supplemental Syntax
When the production
ArrowParameters
[Yield, Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
is recognized the following grammar is used to refine the interpretation of
CoverParenthesizedExpressionAndArrowParameterList
ArrowFormalParameters
[Yield, Await]
UniqueFormalParameters
[?Yield, ?Await]
14.2.1
Static Semantics: Early Errors
ArrowFunction
ArrowParameters
=>
ConciseBody
It is a Syntax Error if
ArrowParameters
Contains
YieldExpression
is
true
It is a Syntax Error if
ArrowParameters
Contains
AwaitExpression
is
true
It is a Syntax Error if ContainsUseStrict of
ConciseBody
is
true
and IsSimpleParameterList of
ArrowParameters
is
false
It is a Syntax Error if any element of the BoundNames of
ArrowParameters
also occurs in the LexicallyDeclaredNames of
ConciseBody
ArrowParameters
CoverParenthesizedExpressionAndArrowParameterList
It is a Syntax Error if
CoverParenthesizedExpressionAndArrowParameterList
is not
covering
an
ArrowFormalParameters
All
early error
rules for
ArrowFormalParameters
and its derived productions also apply to CoveredFormalsList of
CoverParenthesizedExpressionAndArrowParameterList
14.2.2
Static Semantics: BoundNames
ArrowParameters
CoverParenthesizedExpressionAndArrowParameterList
Let
formals
be CoveredFormalsList of
CoverParenthesizedExpressionAndArrowParameterList
Return the BoundNames of
formals
14.2.3
Static Semantics: Contains
With parameter
symbol
ArrowFunction
ArrowParameters
=>
ConciseBody
If
symbol
is not one of
NewTarget
SuperProperty
SuperCall
super
or
this
, return
false
If
ArrowParameters
Contains
symbol
is
true
, return
true
Return
ConciseBody
Contains
symbol
Note
Normally, Contains does not look inside most function forms. However, Contains is used to detect
new.target
this
, and
super
usage within an
ArrowFunction
ArrowParameters
CoverParenthesizedExpressionAndArrowParameterList
Let
formals
be CoveredFormalsList of
CoverParenthesizedExpressionAndArrowParameterList
Return
formals
Contains
symbol
14.2.4
Static Semantics: ContainsExpression
ArrowParameters
BindingIdentifier
Return
false
14.2.5
Static Semantics: ContainsUseStrict
ConciseBody
AssignmentExpression
Return
false
14.2.6
Static Semantics: ExpectedArgumentCount
ArrowParameters
BindingIdentifier
Return 1.
14.2.7
Static Semantics: HasName
ArrowFunction
ArrowParameters
=>
ConciseBody
Return
false
14.2.8
Static Semantics: IsSimpleParameterList
ArrowParameters
BindingIdentifier
Return
true
ArrowParameters
CoverParenthesizedExpressionAndArrowParameterList
Let
formals
be CoveredFormalsList of
CoverParenthesizedExpressionAndArrowParameterList
Return IsSimpleParameterList of
formals
14.2.9
Static Semantics: CoveredFormalsList
ArrowParameters
BindingIdentifier
Return this
ArrowParameters
CoverParenthesizedExpressionAndArrowParameterList
Expression
...
BindingIdentifier
...
BindingPattern
Expression
...
BindingIdentifier
Expression
...
BindingPattern
Return the
ArrowFormalParameters
that is
covered
by
CoverParenthesizedExpressionAndArrowParameterList
14.2.10
Static Semantics: LexicallyDeclaredNames
ConciseBody
AssignmentExpression
Return a new empty
List
14.2.11
Static Semantics: LexicallyScopedDeclarations
ConciseBody
AssignmentExpression
Return a new empty
List
14.2.12
Static Semantics: VarDeclaredNames
ConciseBody
AssignmentExpression
Return a new empty
List
14.2.13
Static Semantics: VarScopedDeclarations
ConciseBody
AssignmentExpression
Return a new empty
List
14.2.14
Runtime Semantics: IteratorBindingInitialization
With parameters
iteratorRecord
and
environment
Note
When
undefined
is passed for
environment
it indicates that a
PutValue
operation should be used to assign the initialization value. This is
the case for formal parameter lists of non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal
with the possibility of multiple parameters with the same name.
ArrowParameters
BindingIdentifier
Assert
iteratorRecord
.[[Done]] is
false
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
be
IteratorValue
next
).
If
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
).
If
iteratorRecord
.[[Done]] is
true
, let
be
undefined
Return the result of performing BindingInitialization for
BindingIdentifier
using
and
environment
as the arguments.
14.2.15
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
ConciseBody
AssignmentExpression
Perform ?
FunctionDeclarationInstantiation
functionObject
argumentsList
).
Let
exprRef
be the result of evaluating
AssignmentExpression
Let
exprValue
be ?
GetValue
exprRef
).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
exprValue
, [[Target]]:
empty
}.
14.2.16
Runtime Semantics: NamedEvaluation
With parameter
name
ArrowFunction
ArrowParameters
=>
ConciseBody
Let
closure
be the result of evaluating this
ArrowFunction
Perform
SetFunctionName
closure
name
).
Return
closure
14.2.17
Runtime Semantics: Evaluation
ArrowFunction
ArrowParameters
=>
ConciseBody
If the function code for this
ArrowFunction
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
parameters
be CoveredFormalsList of
ArrowParameters
Let
closure
be
FunctionCreate
Arrow
parameters
ConciseBody
scope
strict
).
Set
closure
.[[SourceText]] to the source text matched by
ArrowFunction
Return
closure
Note
An
ArrowFunction
does not define local bindings for
arguments
super
this
, or
new.target
. Any reference to
arguments
super
this
, or
new.target
within an
ArrowFunction
must resolve to a binding in a lexically enclosing environment.
Typically this will be the Function Environment of an immediately
enclosing function. Even though an
ArrowFunction
may contain references to
super
, the
function object
created in step 4 is not made into a method by performing
MakeMethod
. An
ArrowFunction
that references
super
is always contained within a non-
ArrowFunction
and the necessary state to implement
super
is accessible via the
scope
that is captured by the
function object
of the
ArrowFunction
14.3
Method Definitions
Syntax
MethodDefinition
[Yield, Await]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
GeneratorMethod
[?Yield, ?Await]
AsyncMethod
[?Yield, ?Await]
AsyncGeneratorMethod
[?Yield, ?Await]
get
PropertyName
[?Yield, ?Await]
FunctionBody
[~Yield, ~Await]
set
PropertyName
[?Yield, ?Await]
PropertySetParameterList
FunctionBody
[~Yield, ~Await]
PropertySetParameterList
FormalParameter
[~Yield, ~Await]
14.3.1
Static Semantics: Early Errors
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
It is a Syntax Error if ContainsUseStrict of
FunctionBody
is
true
and IsSimpleParameterList of
UniqueFormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters
also occurs in the LexicallyDeclaredNames of
FunctionBody
MethodDefinition
set
PropertyName
PropertySetParameterList
FunctionBody
It is a Syntax Error if BoundNames of
PropertySetParameterList
contains any duplicate elements.
It is a Syntax Error if ContainsUseStrict of
FunctionBody
is
true
and IsSimpleParameterList of
PropertySetParameterList
is
false
It is a Syntax Error if any element of the BoundNames of
PropertySetParameterList
also occurs in the LexicallyDeclaredNames of
FunctionBody
14.3.2
Static Semantics: ComputedPropertyContains
With parameter
symbol
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
get
PropertyName
FunctionBody
set
PropertyName
PropertySetParameterList
FunctionBody
Return the result of ComputedPropertyContains for
PropertyName
with argument
symbol
14.3.3
Static Semantics: ExpectedArgumentCount
PropertySetParameterList
FormalParameter
If HasInitializer of
FormalParameter
is
true
, return 0.
Return 1.
14.3.4
Static Semantics: HasDirectSuper
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
If
UniqueFormalParameters
Contains
SuperCall
is
true
, return
true
Return
FunctionBody
Contains
SuperCall
MethodDefinition
get
PropertyName
FunctionBody
Return
FunctionBody
Contains
SuperCall
MethodDefinition
set
PropertyName
PropertySetParameterList
FunctionBody
If
PropertySetParameterList
Contains
SuperCall
is
true
, return
true
Return
FunctionBody
Contains
SuperCall
14.3.5
Static Semantics: PropName
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
get
PropertyName
FunctionBody
set
PropertyName
PropertySetParameterList
FunctionBody
Return PropName of
PropertyName
14.3.6
Static Semantics: SpecialMethod
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
Return
false
MethodDefinition
GeneratorMethod
AsyncMethod
AsyncGeneratorMethod
get
PropertyName
FunctionBody
set
PropertyName
PropertySetParameterList
FunctionBody
Return
true
14.3.7
Runtime Semantics: DefineMethod
With parameters
object
and optional parameter
functionPrototype
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
MethodDefinition
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
If
functionPrototype
is present as a parameter, then
Let
kind
be
Normal
Let
prototype
be
functionPrototype
Else,
Let
kind
be
Method
Let
prototype
be the intrinsic object
%FunctionPrototype%
Let
closure
be
FunctionCreate
kind
UniqueFormalParameters
FunctionBody
scope
strict
prototype
).
Perform
MakeMethod
closure
object
).
Set
closure
.[[SourceText]] to the source text matched by
MethodDefinition
Return the
Record
{ [[Key]]:
propKey
, [[Closure]]:
closure
}.
14.3.8
Runtime Semantics: PropertyDefinitionEvaluation
With parameters
object
and
enumerable
MethodDefinition
PropertyName
UniqueFormalParameters
FunctionBody
Let
methodDef
be DefineMethod of
MethodDefinition
with argument
object
ReturnIfAbrupt
methodDef
).
Perform
SetFunctionName
methodDef
.[[Closure]],
methodDef
.[[Key]]).
Let
desc
be the PropertyDescriptor { [[Value]]:
methodDef
.[[Closure]], [[Writable]]:
true
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
methodDef
.[[Key]],
desc
).
MethodDefinition
get
PropertyName
FunctionBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
MethodDefinition
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
formalParameterList
be an instance of the production
FormalParameters
[empty]
Let
closure
be
FunctionCreate
Method
formalParameterList
FunctionBody
scope
strict
).
Perform
MakeMethod
closure
object
).
Perform
SetFunctionName
closure
propKey
"get"
).
Set
closure
.[[SourceText]] to the source text matched by
MethodDefinition
Let
desc
be the PropertyDescriptor { [[Get]]:
closure
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
propKey
desc
).
MethodDefinition
set
PropertyName
PropertySetParameterList
FunctionBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
MethodDefinition
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
closure
be
FunctionCreate
Method
PropertySetParameterList
FunctionBody
scope
strict
).
Perform
MakeMethod
closure
object
).
Perform
SetFunctionName
closure
propKey
"set"
).
Set
closure
.[[SourceText]] to the source text matched by
MethodDefinition
Let
desc
be the PropertyDescriptor { [[Set]]:
closure
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
propKey
desc
).
14.4
Generator Function Definitions
Syntax
GeneratorMethod
[Yield, Await]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorDeclaration
[Yield, Await, Default]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[+Yield, ~Await]
GeneratorBody
[+Default]
function
FormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorExpression
function
BindingIdentifier
[+Yield, ~Await]
opt
FormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorBody
FunctionBody
[+Yield, ~Await]
YieldExpression
[In, Await]
yield
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
Note 1
The syntactic context immediately following
yield
requires use of the
InputElementRegExpOrTemplateTail
lexical goal.
Note 2
YieldExpression
cannot be used within the
FormalParameters
of a generator function because any expressions that are part of
FormalParameters
are evaluated before the resulting generator object is in a resumable state.
Note 3
Abstract operations
relating to generator objects are defined in
25.4.3
14.4.1
Static Semantics: Early Errors
GeneratorMethod
PropertyName
UniqueFormalParameters
GeneratorBody
It is a Syntax Error if HasDirectSuper of
GeneratorMethod
is
true
It is a Syntax Error if
UniqueFormalParameters
Contains
YieldExpression
is
true
It is a Syntax Error if ContainsUseStrict of
GeneratorBody
is
true
and IsSimpleParameterList of
UniqueFormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters
also occurs in the LexicallyDeclaredNames of
GeneratorBody
GeneratorDeclaration
function
BindingIdentifier
FormalParameters
GeneratorBody
GeneratorDeclaration
function
FormalParameters
GeneratorBody
GeneratorExpression
function
BindingIdentifier
opt
FormalParameters
GeneratorBody
If the source code matching this production is
strict mode code
, the Early Error rules for
UniqueFormalParameters
FormalParameters
are applied.
If the source code matching this production is
strict mode code
, it is a Syntax Error if
BindingIdentifier
is present and the StringValue of
BindingIdentifier
is
"eval"
or
"arguments"
It is a Syntax Error if ContainsUseStrict of
GeneratorBody
is
true
and IsSimpleParameterList of
FormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
FormalParameters
also occurs in the LexicallyDeclaredNames of
GeneratorBody
It is a Syntax Error if
FormalParameters
Contains
YieldExpression
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperProperty
is
true
It is a Syntax Error if
GeneratorBody
Contains
SuperProperty
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperCall
is
true
It is a Syntax Error if
GeneratorBody
Contains
SuperCall
is
true
14.4.2
Static Semantics: BoundNames
GeneratorDeclaration
function
BindingIdentifier
FormalParameters
GeneratorBody
Return the BoundNames of
BindingIdentifier
GeneratorDeclaration
function
FormalParameters
GeneratorBody
Return «
"*default*"
».
Note
"*default*"
is used within this specification as
a synthetic name for hoistable anonymous functions that are defined
using export declarations.
14.4.3
Static Semantics: ComputedPropertyContains
With parameter
symbol
GeneratorMethod
PropertyName
UniqueFormalParameters
GeneratorBody
Return the result of ComputedPropertyContains for
PropertyName
with argument
symbol
14.4.4
Static Semantics: Contains
With parameter
symbol
GeneratorDeclaration
function
BindingIdentifier
FormalParameters
GeneratorBody
GeneratorDeclaration
function
FormalParameters
GeneratorBody
GeneratorExpression
function
BindingIdentifier
opt
FormalParameters
GeneratorBody
Return
false
Note
Static semantic rules that depend upon substructure generally do not look into function definitions.
14.4.5
Static Semantics: HasDirectSuper
GeneratorMethod
PropertyName
UniqueFormalParameters
GeneratorBody
If
UniqueFormalParameters
Contains
SuperCall
is
true
, return
true
Return
GeneratorBody
Contains
SuperCall
14.4.6
Static Semantics: HasName
GeneratorExpression
function
FormalParameters
GeneratorBody
Return
false
GeneratorExpression
function
BindingIdentifier
FormalParameters
GeneratorBody
Return
true
14.4.7
Static Semantics: IsConstantDeclaration
GeneratorDeclaration
function
BindingIdentifier
FormalParameters
GeneratorBody
GeneratorDeclaration
function
FormalParameters
GeneratorBody
Return
false
14.4.8
Static Semantics: IsFunctionDefinition
GeneratorExpression
function
BindingIdentifier
opt
FormalParameters
GeneratorBody
Return
true
14.4.9
Static Semantics: PropName
GeneratorMethod
PropertyName
UniqueFormalParameters
GeneratorBody
Return PropName of
PropertyName
14.4.10
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
GeneratorBody
FunctionBody
Perform ?
FunctionDeclarationInstantiation
functionObject
argumentsList
).
Let
be ?
OrdinaryCreateFromConstructor
functionObject
"%GeneratorPrototype%"
, « [[GeneratorState]], [[GeneratorContext]] »).
Perform
GeneratorStart
FunctionBody
).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
, [[Target]]:
empty
}.
14.4.11
Runtime Semantics: InstantiateFunctionObject
With parameter
scope
GeneratorDeclaration
function
BindingIdentifier
FormalParameters
GeneratorBody
If the function code for
GeneratorDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
name
be StringValue of
BindingIdentifier
Let
be
GeneratorFunctionCreate
Normal
FormalParameters
GeneratorBody
scope
strict
).
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetFunctionName
name
).
Set
.[[SourceText]] to the source text matched by
GeneratorDeclaration
Return
GeneratorDeclaration
function
FormalParameters
GeneratorBody
Let
be
GeneratorFunctionCreate
Normal
FormalParameters
GeneratorBody
scope
true
).
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetFunctionName
"default"
).
Set
.[[SourceText]] to the source text matched by
GeneratorDeclaration
Return
Note
An anonymous
GeneratorDeclaration
can only occur as part of an
export default
declaration, and its function code is therefore always
strict mode code
14.4.12
Runtime Semantics: PropertyDefinitionEvaluation
With parameters
object
and
enumerable
GeneratorMethod
PropertyName
UniqueFormalParameters
GeneratorBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
GeneratorMethod
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
closure
be
GeneratorFunctionCreate
Method
UniqueFormalParameters
GeneratorBody
scope
strict
).
Perform
MakeMethod
closure
object
).
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetFunctionName
closure
propKey
).
Set
closure
.[[SourceText]] to the source text matched by
GeneratorMethod
Let
desc
be the PropertyDescriptor { [[Value]]:
closure
, [[Writable]]:
true
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
propKey
desc
).
14.4.13
Runtime Semantics: NamedEvaluation
With parameter
name
GeneratorExpression
function
FormalParameters
GeneratorBody
Let
closure
be the result of evaluating this
GeneratorExpression
Perform
SetFunctionName
closure
name
).
Return
closure
14.4.14
Runtime Semantics: Evaluation
GeneratorExpression
function
FormalParameters
GeneratorBody
If the function code for this
GeneratorExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
closure
be
GeneratorFunctionCreate
Normal
FormalParameters
GeneratorBody
scope
strict
).
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Set
closure
.[[SourceText]] to the source text matched by
GeneratorExpression
Return
closure
GeneratorExpression
function
BindingIdentifier
FormalParameters
GeneratorBody
If the function code for this
GeneratorExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
funcEnv
be
NewDeclarativeEnvironment
scope
).
Let
envRec
be
funcEnv
's
EnvironmentRecord
Let
name
be StringValue of
BindingIdentifier
Perform
envRec
.CreateImmutableBinding(
name
false
).
Let
closure
be
GeneratorFunctionCreate
Normal
FormalParameters
GeneratorBody
funcEnv
strict
).
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetFunctionName
closure
name
).
Perform
envRec
.InitializeBinding(
name
closure
).
Set
closure
.[[SourceText]] to the source text matched by
GeneratorExpression
Return
closure
Note
The
BindingIdentifier
in a
GeneratorExpression
can be referenced from inside the
GeneratorExpression
's
FunctionBody
to allow the generator code to call itself recursively. However, unlike in a
GeneratorDeclaration
, the
BindingIdentifier
in a
GeneratorExpression
cannot be referenced from and does not affect the scope enclosing the
GeneratorExpression
YieldExpression
yield
Let
generatorKind
be !
GetGeneratorKind
().
If
generatorKind
is
async
, then return ?
AsyncGeneratorYield
undefined
).
Otherwise, return ?
GeneratorYield
CreateIterResultObject
undefined
false
)).
YieldExpression
yield
AssignmentExpression
Let
generatorKind
be !
GetGeneratorKind
().
Let
exprRef
be the result of evaluating
AssignmentExpression
Let
value
be ?
GetValue
exprRef
).
If
generatorKind
is
async
, then return ?
AsyncGeneratorYield
value
).
Otherwise, return ?
GeneratorYield
CreateIterResultObject
value
false
)).
YieldExpression
yield
AssignmentExpression
Let
generatorKind
be !
GetGeneratorKind
().
Let
exprRef
be the result of evaluating
AssignmentExpression
Let
value
be ?
GetValue
exprRef
).
Let
iteratorRecord
be ?
GetIterator
value
generatorKind
).
Let
iterator
be
iteratorRecord
.[[Iterator]].
Let
received
be
NormalCompletion
undefined
).
Repeat,
If
received
.[[Type]] is
normal
, then
Let
innerResult
be ?
Call
iteratorRecord
.[[NextMethod]],
iteratorRecord
.[[Iterator]], «
received
.[[Value]] »).
If
generatorKind
is
async
, then set
innerResult
to ?
Await
innerResult
).
If
Type
innerResult
) is not Object, throw a
TypeError
exception.
Let
done
be ?
IteratorComplete
innerResult
).
If
done
is
true
, then
Return ?
IteratorValue
innerResult
).
If
generatorKind
is
async
, then set
received
to
AsyncGeneratorYield
(?
IteratorValue
innerResult
)).
Else, set
received
to
GeneratorYield
innerResult
).
Else if
received
.[[Type]] is
throw
, then
Let
throw
be ?
GetMethod
iterator
"throw"
).
If
throw
is not
undefined
, then
Let
innerResult
be ?
Call
throw
iterator
, «
received
.[[Value]] »).
If
generatorKind
is
async
, then set
innerResult
to ?
Await
innerResult
).
NOTE: Exceptions from the inner iterator
throw
method are propagated. Normal completions from an inner
throw
method are processed similarly to an inner
next
If
Type
innerResult
) is not Object, throw a
TypeError
exception.
Let
done
be ?
IteratorComplete
innerResult
).
If
done
is
true
, then
Return ?
IteratorValue
innerResult
).
If
generatorKind
is
async
, then set
received
to
AsyncGeneratorYield
(?
IteratorValue
innerResult
)).
Else, set
received
to
GeneratorYield
innerResult
).
Else,
NOTE: If
iterator
does not have a
throw
method, this throw is going to terminate the
yield*
loop. But first we need to give
iterator
a chance to clean up.
Let
closeCompletion
be
Completion
{ [[Type]]:
normal
, [[Value]]:
empty
, [[Target]]:
empty
}.
If
generatorKind
is
async
, perform ?
AsyncIteratorClose
iteratorRecord
closeCompletion
).
Else, perform ?
IteratorClose
iteratorRecord
closeCompletion
).
NOTE: The next step throws a
TypeError
to indicate that there was a
yield*
protocol violation:
iterator
does not have a
throw
method.
Throw a
TypeError
exception.
Else,
Assert
received
.[[Type]] is
return
Let
return
be ?
GetMethod
iterator
"return"
).
If
return
is
undefined
, then
If
generatorKind
is
async
, then set
received
.[[Value]] to ?
Await
received
.[[Value]]).
Return
Completion
received
).
Let
innerReturnResult
be ?
Call
return
iterator
, «
received
.[[Value]] »).
If
generatorKind
is
async
, then set
innerReturnResult
to ?
Await
innerReturnResult
).
If
Type
innerReturnResult
) is not Object, throw a
TypeError
exception.
Let
done
be ?
IteratorComplete
innerReturnResult
).
If
done
is
true
, then
Let
value
be ?
IteratorValue
innerReturnResult
).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
value
, [[Target]]:
empty
}.
If
generatorKind
is
async
, then set
received
to
AsyncGeneratorYield
(?
IteratorValue
innerReturnResult
)).
Else, set
received
to
GeneratorYield
innerReturnResult
).
14.5
Async Generator Function Definitions
Syntax
AsyncGeneratorMethod
[Yield, Await]
async
[no
LineTerminator
here]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorDeclaration
[Yield, Await, Default]
async
[no
LineTerminator
here]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
[+Default]
async
[no
LineTerminator
here]
function
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorExpression
async
[no
LineTerminator
here]
function
BindingIdentifier
[+Yield, +Await]
opt
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorBody
FunctionBody
[+Yield, +Await]
Note 1
YieldExpression
and
AwaitExpression
cannot be used within the
FormalParameters
of an async generator function because any expressions that are part of
FormalParameters
are evaluated before the resulting async generator object is in a resumable state.
Note 2
Abstract operations
relating to async generator objects are defined in
25.5.3
14.5.1
Static Semantics: Early Errors
AsyncGeneratorMethod
async
PropertyName
UniqueFormalParameters
AsyncGeneratorBody
It is a Syntax Error if HasDirectSuper of
AsyncGeneratorMethod
is
true
It is a Syntax Error if
UniqueFormalParameters
Contains
YieldExpression
is
true
It is a Syntax Error if
UniqueFormalParameters
Contains
AwaitExpression
is
true
It is a Syntax Error if ContainsUseStrict of
AsyncGeneratorBody
is
true
and IsSimpleParameterList of
UniqueFormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters
also occurs in the LexicallyDeclaredNames of
AsyncGeneratorBody
AsyncGeneratorDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
AsyncGeneratorDeclaration
async
function
FormalParameters
AsyncGeneratorBody
AsyncGeneratorExpression
async
function
BindingIdentifier
opt
FormalParameters
AsyncGeneratorBody
If the source code matching this production is
strict mode code
, the Early Error rules for
UniqueFormalParameters
FormalParameters
are applied.
If the source code matching this production is
strict mode code
, it is a Syntax Error if
BindingIdentifier
is the
IdentifierName
eval
or the
IdentifierName
arguments
It is a Syntax Error if ContainsUseStrict of
AsyncGeneratorBody
is
true
and IsSimpleParameterList of
FormalParameters
is
false
It is a Syntax Error if any element of the BoundNames of
FormalParameters
also occurs in the LexicallyDeclaredNames of
AsyncGeneratorBody
It is a Syntax Error if
FormalParameters
Contains
YieldExpression
is
true
It is a Syntax Error if
FormalParameters
Contains
AwaitExpression
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperProperty
is
true
It is a Syntax Error if
AsyncGeneratorBody
Contains
SuperProperty
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperCall
is
true
It is a Syntax Error if
AsyncGeneratorBody
Contains
SuperCall
is
true
14.5.2
Static Semantics: BoundNames
AsyncGeneratorDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
Return the BoundNames of
BindingIdentifier
AsyncGeneratorDeclaration
async
function
FormalParameters
AsyncGeneratorBody
Return «
"*default*"
».
Note
"*default*"
is used within this specification as
a synthetic name for hoistable anonymous functions that are defined
using export declarations.
14.5.3
Static Semantics: ComputedPropertyContains
With parameter
symbol
AsyncGeneratorMethod
async
PropertyName
UniqueFormalParameters
AsyncGeneratorBody
Return the result of ComputedPropertyContains for
PropertyName
with argument
symbol
14.5.4
Static Semantics: Contains
With parameter
symbol
AsyncGeneratorDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
AsyncGeneratorDeclaration
async
function
FormalParameters
AsyncGeneratorBody
AsyncGeneratorExpression
async
function
BindingIdentifier
opt
FormalParameters
AsyncGeneratorBody
Return
false
Note
Static semantic rules that depend upon substructure generally do not look into function definitions.
14.5.5
Static Semantics: HasDirectSuper
AsyncGeneratorMethod
async
PropertyName
UniqueFormalParameters
AsyncGeneratorBody
If
UniqueFormalParameters
Contains
SuperCall
is
true
, return
true
Return
AsyncGeneratorBody
Contains
SuperCall
14.5.6
Static Semantics: HasName
AsyncGeneratorExpression
async
function
FormalParameters
AsyncGeneratorBody
Return
false
AsyncGeneratorExpression
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
Return
true
14.5.7
Static Semantics: IsConstantDeclaration
AsyncGeneratorDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
AsyncGeneratorDeclaration
async
function
FormalParameters
AsyncGeneratorBody
Return
false
14.5.8
Static Semantics: IsFunctionDefinition
AsyncGeneratorExpression
async
function
BindingIdentifier
opt
FormalParameters
AsyncGeneratorBody
Return
true
14.5.9
Static Semantics: PropName
AsyncGeneratorMethod
async
PropertyName
UniqueFormalParameters
AsyncGeneratorBody
Return PropName of
PropertyName
14.5.10
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
AsyncGeneratorBody
FunctionBody
Perform ?
FunctionDeclarationInstantiation
functionObject
argumentsList
).
Let
generator
be ?
OrdinaryCreateFromConstructor
functionObject
"%AsyncGeneratorPrototype%"
, « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]] »).
Perform !
AsyncGeneratorStart
generator
FunctionBody
).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
generator
, [[Target]]:
empty
}.
14.5.11
Runtime Semantics: InstantiateFunctionObject
With parameter
scope
AsyncGeneratorDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
If the function code for
AsyncGeneratorDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
name
be StringValue of
BindingIdentifier
Let
be !
AsyncGeneratorFunctionCreate
Normal
FormalParameters
AsyncGeneratorBody
scope
strict
).
Let
prototype
be !
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform !
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform !
SetFunctionName
name
).
Set
.[[SourceText]] to the source text matched by
AsyncGeneratorDeclaration
Return
AsyncGeneratorDeclaration
async
function
FormalParameters
AsyncGeneratorBody
If the function code for
AsyncGeneratorDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
be
AsyncGeneratorFunctionCreate
Normal
FormalParameters
AsyncGeneratorBody
scope
strict
).
Let
prototype
be
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform
SetFunctionName
"default"
).
Set
.[[SourceText]] to the source text matched by
AsyncGeneratorDeclaration
Return
Note
An anonymous
AsyncGeneratorDeclaration
can only occur as part of an
export default
declaration.
14.5.12
Runtime Semantics: PropertyDefinitionEvaluation
With parameter
object
and
enumerable
AsyncGeneratorMethod
async
PropertyName
UniqueFormalParameters
AsyncGeneratorBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
AsyncGeneratorMethod
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
closure
be !
AsyncGeneratorFunctionCreate
Method
UniqueFormalParameters
AsyncGeneratorBody
scope
strict
).
Perform !
MakeMethod
closure
object
).
Let
prototype
be !
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform !
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform !
SetFunctionName
closure
propKey
).
Set
closure
.[[SourceText]] to the source text matched by
AsyncGeneratorMethod
Let
desc
be PropertyDescriptor { [[Value]]:
closure
, [[Writable]]:
true
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
propKey
desc
).
14.5.13
Runtime Semantics: NamedEvaluation
With parameter
name
AsyncGeneratorExpression
async
function
FormalParameters
AsyncGeneratorBody
Let
closure
be the result of evaluating this
AsyncGeneratorExpression
Perform
SetFunctionName
closure
name
).
Return
closure
14.5.14
Runtime Semantics: Evaluation
AsyncGeneratorExpression
async
function
FormalParameters
AsyncGeneratorBody
If the function code for this
AsyncGeneratorExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
closure
be !
AsyncGeneratorFunctionCreate
Normal
FormalParameters
AsyncGeneratorBody
scope
strict
).
Let
prototype
be !
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform !
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Set
closure
.[[SourceText]] to the source text matched by
AsyncGeneratorExpression
Return
closure
AsyncGeneratorExpression
async
function
BindingIdentifier
FormalParameters
AsyncGeneratorBody
If the function code for this
AsyncGeneratorExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the
running execution context
's LexicalEnvironment.
Let
funcEnv
be !
NewDeclarativeEnvironment
scope
).
Let
envRec
be
funcEnv
's
EnvironmentRecord
Let
name
be StringValue of
BindingIdentifier
Perform !
envRec
.CreateImmutableBinding(
name
).
Let
closure
be !
AsyncGeneratorFunctionCreate
Normal
FormalParameters
AsyncGeneratorBody
funcEnv
strict
).
Let
prototype
be !
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform !
DefinePropertyOrThrow
closure
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Perform !
SetFunctionName
closure
name
).
Perform !
envRec
.InitializeBinding(
name
closure
).
Set
closure
.[[SourceText]] to the source text matched by
AsyncGeneratorExpression
Return
closure
Note
The
BindingIdentifier
in an
AsyncGeneratorExpression
can be referenced from inside the
AsyncGeneratorExpression
's
AsyncGeneratorBody
to allow the generator code to call itself recursively. However, unlike in an
AsyncGeneratorDeclaration
, the
BindingIdentifier
in an
AsyncGeneratorExpression
cannot be referenced from and does not affect the scope enclosing the
AsyncGeneratorExpression
14.6
Class Definitions
Syntax
ClassDeclaration
[Yield, Await, Default]
class
BindingIdentifier
[?Yield, ?Await]
ClassTail
[?Yield, ?Await]
[+Default]
class
ClassTail
[?Yield, ?Await]
ClassExpression
[Yield, Await]
class
BindingIdentifier
[?Yield, ?Await]
opt
ClassTail
[?Yield, ?Await]
ClassTail
[Yield, Await]
ClassHeritage
[?Yield, ?Await]
opt
ClassBody
[?Yield, ?Await]
opt
ClassHeritage
[Yield, Await]
extends
LeftHandSideExpression
[?Yield, ?Await]
ClassBody
[Yield, Await]
ClassElementList
[?Yield, ?Await]
ClassElementList
[Yield, Await]
ClassElement
[?Yield, ?Await]
ClassElementList
[?Yield, ?Await]
ClassElement
[?Yield, ?Await]
ClassElement
[Yield, Await]
MethodDefinition
[?Yield, ?Await]
static
MethodDefinition
[?Yield, ?Await]
Note
A class definition is always
strict mode code
14.6.1
Static Semantics: Early Errors
ClassTail
ClassHeritage
opt
ClassBody
It is a Syntax Error if
ClassHeritage
is not present and the following algorithm evaluates to
true
Let
constructor
be ConstructorMethod of
ClassBody
If
constructor
is
empty
, return
false
Return HasDirectSuper of
constructor
ClassBody
ClassElementList
It is a Syntax Error if PrototypePropertyNameList of
ClassElementList
contains more than one occurrence of
"constructor"
ClassElement
MethodDefinition
It is a Syntax Error if PropName of
MethodDefinition
is not
"constructor"
and HasDirectSuper of
MethodDefinition
is
true
It is a Syntax Error if PropName of
MethodDefinition
is
"constructor"
and SpecialMethod of
MethodDefinition
is
true
ClassElement
static
MethodDefinition
It is a Syntax Error if HasDirectSuper of
MethodDefinition
is
true
It is a Syntax Error if PropName of
MethodDefinition
is
"prototype"
14.6.2
Static Semantics: BoundNames
ClassDeclaration
class
BindingIdentifier
ClassTail
Return the BoundNames of
BindingIdentifier
ClassDeclaration
class
ClassTail
Return «
"*default*"
».
14.6.3
Static Semantics: ConstructorMethod
ClassElementList
ClassElement
If
ClassElement
is
ClassElement
, return
empty
If IsStatic of
ClassElement
is
true
, return
empty
If PropName of
ClassElement
is not
"constructor"
, return
empty
Return
ClassElement
ClassElementList
ClassElementList
ClassElement
Let
head
be ConstructorMethod of
ClassElementList
If
head
is not
empty
, return
head
If
ClassElement
is
ClassElement
, return
empty
If IsStatic of
ClassElement
is
true
, return
empty
If PropName of
ClassElement
is not
"constructor"
, return
empty
Return
ClassElement
Note
Early Error rules ensure that there is only one method definition named
"constructor"
and that it is not an
accessor property
or generator definition.
14.6.4
Static Semantics: Contains
With parameter
symbol
ClassTail
ClassHeritage
opt
ClassBody
If
symbol
is
ClassBody
, return
true
If
symbol
is
ClassHeritage
, then
If
ClassHeritage
is present, return
true
; otherwise return
false
Let
inHeritage
be
ClassHeritage
Contains
symbol
If
inHeritage
is
true
, return
true
Return the result of ComputedPropertyContains for
ClassBody
with argument
symbol
Note
Static semantic rules that depend upon substructure generally do not look into class bodies except for
PropertyName
s.
14.6.5
Static Semantics: ComputedPropertyContains
With parameter
symbol
ClassElementList
ClassElementList
ClassElement
Let
inList
be the result of ComputedPropertyContains for
ClassElementList
with argument
symbol
If
inList
is
true
, return
true
Return the result of ComputedPropertyContains for
ClassElement
with argument
symbol
ClassElement
MethodDefinition
Return the result of ComputedPropertyContains for
MethodDefinition
with argument
symbol
ClassElement
static
MethodDefinition
Return the result of ComputedPropertyContains for
MethodDefinition
with argument
symbol
ClassElement
Return
false
14.6.6
Static Semantics: HasName
ClassExpression
class
ClassTail
Return
false
ClassExpression
class
BindingIdentifier
ClassTail
Return
true
14.6.7
Static Semantics: IsConstantDeclaration
ClassDeclaration
class
BindingIdentifier
ClassTail
ClassDeclaration
class
ClassTail
Return
false
14.6.8
Static Semantics: IsFunctionDefinition
ClassExpression
class
BindingIdentifier
opt
ClassTail
Return
true
14.6.9
Static Semantics: IsStatic
ClassElement
MethodDefinition
Return
false
ClassElement
static
MethodDefinition
Return
true
ClassElement
Return
false
14.6.10
Static Semantics: NonConstructorMethodDefinitions
ClassElementList
ClassElement
If
ClassElement
is
ClassElement
, return a new empty
List
If IsStatic of
ClassElement
is
false
and PropName of
ClassElement
is
"constructor"
, return a new empty
List
Return a
List
containing
ClassElement
ClassElementList
ClassElementList
ClassElement
Let
list
be NonConstructorMethodDefinitions of
ClassElementList
If
ClassElement
is
ClassElement
, return
list
If IsStatic of
ClassElement
is
false
and PropName of
ClassElement
is
"constructor"
, return
list
Append
ClassElement
to the end of
list
Return
list
14.6.11
Static Semantics: PrototypePropertyNameList
ClassElementList
ClassElement
If PropName of
ClassElement
is
empty
, return a new empty
List
If IsStatic of
ClassElement
is
true
, return a new empty
List
Return a
List
containing PropName of
ClassElement
ClassElementList
ClassElementList
ClassElement
Let
list
be PrototypePropertyNameList of
ClassElementList
If PropName of
ClassElement
is
empty
, return
list
If IsStatic of
ClassElement
is
true
, return
list
Append PropName of
ClassElement
to the end of
list
Return
list
14.6.12
Static Semantics: PropName
ClassElement
Return
empty
14.6.13
Runtime Semantics: ClassDefinitionEvaluation
With parameters
classBinding
and
className
ClassTail
ClassHeritage
opt
ClassBody
opt
Let
lex
be the LexicalEnvironment of the
running execution context
Let
classScope
be
NewDeclarativeEnvironment
lex
).
Let
classScopeEnvRec
be
classScope
's
EnvironmentRecord
If
classBinding
is not
undefined
, then
Perform
classScopeEnvRec
.CreateImmutableBinding(
classBinding
true
).
If
ClassHeritage
opt
is not present, then
Let
protoParent
be the intrinsic object
%ObjectPrototype%
Let
constructorParent
be the intrinsic object
%FunctionPrototype%
Else,
Set the
running execution context
's LexicalEnvironment to
classScope
Let
superclassRef
be the result of evaluating
ClassHeritage
Set the
running execution context
's LexicalEnvironment to
lex
Let
superclass
be ?
GetValue
superclassRef
).
If
superclass
is
null
, then
Let
protoParent
be
null
Let
constructorParent
be the intrinsic object
%FunctionPrototype%
Else if
IsConstructor
superclass
) is
false
, throw a
TypeError
exception.
Else,
Let
protoParent
be ?
Get
superclass
"prototype"
).
If
Type
protoParent
) is neither Object nor Null, throw a
TypeError
exception.
Let
constructorParent
be
superclass
Let
proto
be
ObjectCreate
protoParent
).
If
ClassBody
opt
is not present, let
constructor
be
empty
Else, let
constructor
be ConstructorMethod of
ClassBody
If
constructor
is
empty
, then
If
ClassHeritage
opt
is present, then
Set
constructor
to the result of parsing the source text
constructor
(... args){
super
(...args);}
using the syntactic grammar with the
goal symbol
MethodDefinition
[~Yield, ~Await]
Else,
Set
constructor
to the result of parsing the source text
constructor
(){ }
using the syntactic grammar with the
goal symbol
MethodDefinition
[~Yield, ~Await]
Set the
running execution context
's LexicalEnvironment to
classScope
Let
constructorInfo
be the result of performing DefineMethod for
constructor
with arguments
proto
and
constructorParent
as the optional
functionPrototype
argument.
Assert
constructorInfo
is not an
abrupt completion
Let
be
constructorInfo
.[[Closure]].
If
ClassHeritage
opt
is present, set
.[[ConstructorKind]] to
"derived"
Perform
MakeConstructor
false
proto
).
Perform
MakeClassConstructor
).
If
className
is not
undefined
, then
Perform
SetFunctionName
className
).
Perform
CreateMethodProperty
proto
"constructor"
).
If
ClassBody
opt
is not present, let
methods
be a new empty
List
Else, let
methods
be NonConstructorMethodDefinitions of
ClassBody
For each
ClassElement
in order from
methods
, do
If IsStatic of
is
false
, then
Let
status
be the result of performing PropertyDefinitionEvaluation for
with arguments
proto
and
false
Else,
Let
status
be the result of performing PropertyDefinitionEvaluation for
with arguments
and
false
If
status
is an
abrupt completion
, then
Set the
running execution context
's LexicalEnvironment to
lex
Return
Completion
status
).
Set the
running execution context
's LexicalEnvironment to
lex
If
classBinding
is not
undefined
, then
Perform
classScopeEnvRec
.InitializeBinding(
classBinding
).
Return
14.6.14
Runtime Semantics: BindingClassDeclarationEvaluation
ClassDeclaration
class
BindingIdentifier
ClassTail
Let
className
be StringValue of
BindingIdentifier
Let
value
be the result of ClassDefinitionEvaluation of
ClassTail
with arguments
className
and
className
ReturnIfAbrupt
value
).
Set
value
.[[SourceText]] to the source text matched by
ClassDeclaration
Let
env
be the
running execution context
's LexicalEnvironment.
Perform ?
InitializeBoundName
className
value
env
).
Return
value
ClassDeclaration
class
ClassTail
Let
value
be the result of ClassDefinitionEvaluation of
ClassTail
with arguments
undefined
and
"default"
ReturnIfAbrupt
value
).
Set
value
.[[SourceText]] to the source text matched by
ClassDeclaration
Return
value
Note
ClassDeclaration
class
ClassTail
only occurs as part of an
ExportDeclaration
and establishing its binding is handled as part of the evaluation action for that production. See
15.2.3.11
14.6.15
Runtime Semantics: NamedEvaluation
With parameter
name
ClassExpression
class
ClassTail
Return the result of ClassDefinitionEvaluation of
ClassTail
with arguments
undefined
and
name
14.6.16
Runtime Semantics: Evaluation
ClassDeclaration
class
BindingIdentifier
ClassTail
Perform ? BindingClassDeclarationEvaluation of this
ClassDeclaration
Return
NormalCompletion
empty
).
Note
ClassDeclaration
class
ClassTail
only occurs as part of an
ExportDeclaration
and is never directly evaluated.
ClassExpression
class
BindingIdentifier
opt
ClassTail
If
BindingIdentifier
opt
is not present, let
className
be
undefined
Else, let
className
be StringValue of
BindingIdentifier
Let
value
be the result of ClassDefinitionEvaluation of
ClassTail
with arguments
className
and
className
ReturnIfAbrupt
value
).
Set
value
.[[SourceText]] to the source text matched by
ClassExpression
Return
value
14.7
Async Function Definitions
Syntax
AsyncFunctionDeclaration
[Yield, Await, Default]
async
[no
LineTerminator
here]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
[+Default]
async
[no
LineTerminator
here]
function
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncFunctionExpression
async
[no
LineTerminator
here]
function
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
async
[no
LineTerminator
here]
function
BindingIdentifier
[~Yield, +Await]
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncMethod
[Yield, Await]
async
[no
LineTerminator
here]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncFunctionBody
FunctionBody
[~Yield, +Await]
AwaitExpression
[Yield]
await
UnaryExpression
[?Yield, +Await]
Note 1
await
is parsed as an
AwaitExpression
when the
Await
parameter is present. The
Await
parameter is present in the following contexts:
In an
AsyncFunctionBody
In the
FormalParameters
of an
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
, or
AsyncGeneratorExpression
AwaitExpression
in this position is a Syntax error via
static semantics
When
Module
is the syntactic
goal symbol
and the
Await
parameter is absent,
await
is parsed as a keyword and will be a Syntax error. When
Script
is the syntactic
goal symbol
await
may be parsed as an identifier when the
Await
parameter is absent. This includes the following contexts:
Anywhere outside of an
AsyncFunctionBody
or
FormalParameters
of an
AsyncFunctionDeclaration
AsyncFunctionExpression
AsyncGeneratorDeclaration
, or
AsyncGeneratorExpression
In the
BindingIdentifier
of a
FunctionExpression
GeneratorExpression
, or
AsyncGeneratorExpression
Note 2
Unlike
YieldExpression
, it is a Syntax Error to omit the operand of an
AwaitExpression
. You must await something.
14.7.1
Static Semantics: Early Errors
AsyncMethod
async
PropertyName
UniqueFormalParameters
AsyncFunctionBody
It is a Syntax Error if ContainsUseStrict of
AsyncFunctionBody
is
true
and IsSimpleParameterList of
UniqueFormalParameters
is
false
It is a Syntax Error if HasDirectSuper of
AsyncMethod
is
true
It is a Syntax Error if
UniqueFormalParameters
Contains
AwaitExpression
is
true
It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters
also occurs in the LexicallyDeclaredNames of
AsyncFunctionBody
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
AsyncFunctionExpression
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
It is a Syntax Error if ContainsUseStrict of
AsyncFunctionBody
is
true
and IsSimpleParameterList of
FormalParameters
is
false
It is a Syntax Error if
FormalParameters
Contains
AwaitExpression
is
true
If the source code matching this production is strict code, the Early Error rules for
UniqueFormalParameters
FormalParameters
are applied.
If the source code matching this production is strict code, it is a Syntax Error if
BindingIdentifier
is present and the StringValue of
BindingIdentifier
is
"eval"
or
"arguments"
It is a Syntax Error if any element of the BoundNames of
FormalParameters
also occurs in the LexicallyDeclaredNames of
AsyncFunctionBody
It is a Syntax Error if
FormalParameters
Contains
SuperProperty
is
true
It is a Syntax Error if
AsyncFunctionBody
Contains
SuperProperty
is
true
It is a Syntax Error if
FormalParameters
Contains
SuperCall
is
true
It is a Syntax Error if
AsyncFunctionBody
Contains
SuperCall
is
true
14.7.2
Static Semantics: BoundNames
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
Return the BoundNames of
BindingIdentifier
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
Return «
"*default*"
».
Note
*default*
is used within this specification as a synthetic name for hoistable
anonymous functions that are defined using export declarations.
14.7.3
Static Semantics: ComputedPropertyContains
With parameter
symbol
AsyncMethod
async
PropertyName
UniqueFormalParameters
AsyncFunctionBody
Return the result of ComputedPropertyContains for
PropertyName
with argument
symbol
14.7.4
Static Semantics: Contains
With parameter
symbol
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
AsyncFunctionExpression
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
Return
false
14.7.5
Static Semantics: HasDirectSuper
AsyncMethod
async
PropertyName
UniqueFormalParameters
AsyncFunctionBody
If
UniqueFormalParameters
Contains
SuperCall
is
true
, return
true
Return
AsyncFunctionBody
Contains
SuperCall
14.7.6
Static Semantics: HasName
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
Return
false
AsyncFunctionExpression
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
Return
true
14.7.7
Static Semantics: IsConstantDeclaration
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
Return
false
14.7.8
Static Semantics: IsFunctionDefinition
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
AsyncFunctionExpression
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
Return
true
14.7.9
Static Semantics: PropName
AsyncMethod
async
PropertyName
UniqueFormalParameters
AsyncFunctionBody
Return PropName of
PropertyName
14.7.10
Runtime Semantics: InstantiateFunctionObject
With parameter
scope
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
If the function code for
AsyncFunctionDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise, let
strict
be
false
Let
name
be StringValue of
BindingIdentifier
Let
be !
AsyncFunctionCreate
Normal
FormalParameters
AsyncFunctionBody
scope
strict
).
Perform !
SetFunctionName
name
).
Set
.[[SourceText]] to the source text matched by
AsyncFunctionDeclaration
Return
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
If the function code for
AsyncFunctionDeclaration
is
strict mode code
, let
strict
be
true
. Otherwise, let
strict
be
false
Let
be !
AsyncFunctionCreate
Normal
FormalParameters
AsyncFunctionBody
scope
strict
).
Perform !
SetFunctionName
"default"
).
Set
.[[SourceText]] to the source text matched by
AsyncFunctionDeclaration
Return
14.7.11
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
AsyncFunctionBody
FunctionBody
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
Let
declResult
be
FunctionDeclarationInstantiation
functionObject
argumentsList
).
If
declResult
is not an
abrupt completion
, then
Perform !
AsyncFunctionStart
promiseCapability
FunctionBody
).
Else
declResult
is an
abrupt completion
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
declResult
.[[Value]] »).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
promiseCapability
.[[Promise]], [[Target]]:
empty
}.
14.7.12
Runtime Semantics: PropertyDefinitionEvaluation
With parameters
object
and
enumerable
AsyncMethod
async
PropertyName
UniqueFormalParameters
AsyncFunctionBody
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If the function code for this
AsyncMethod
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
closure
be !
AsyncFunctionCreate
Method
UniqueFormalParameters
AsyncFunctionBody
scope
strict
).
Perform !
MakeMethod
closure
object
).
Perform !
SetFunctionName
closure
propKey
).
Set
closure
.[[SourceText]] to the source text matched by
AsyncMethod
Let
desc
be the PropertyDescriptor { [[Value]]:
closure
, [[Writable]]:
true
, [[Enumerable]]:
enumerable
, [[Configurable]]:
true
}.
Return ?
DefinePropertyOrThrow
object
propKey
desc
).
14.7.13
Runtime Semantics: NamedEvaluation
With parameter
name
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
Let
closure
be the result of evaluating this
AsyncFunctionExpression
Perform
SetFunctionName
closure
name
).
Return
closure
14.7.14
Runtime Semantics: Evaluation
AsyncFunctionDeclaration
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
Return
NormalCompletion
empty
).
AsyncFunctionDeclaration
async
function
FormalParameters
AsyncFunctionBody
Return
NormalCompletion
empty
).
AsyncFunctionExpression
async
function
FormalParameters
AsyncFunctionBody
If the function code for
AsyncFunctionExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
closure
be !
AsyncFunctionCreate
Normal
FormalParameters
AsyncFunctionBody
scope
strict
).
Set
closure
.[[SourceText]] to the source text matched by
AsyncFunctionExpression
Return
closure
AsyncFunctionExpression
async
function
BindingIdentifier
FormalParameters
AsyncFunctionBody
If the function code for
AsyncFunctionExpression
is
strict mode code
, let
strict
be
true
. Otherwise let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
funcEnv
be !
NewDeclarativeEnvironment
scope
).
Let
envRec
be
funcEnv
's
EnvironmentRecord
Let
name
be StringValue of
BindingIdentifier
Perform !
envRec
.CreateImmutableBinding(
name
).
Let
closure
be !
AsyncFunctionCreate
Normal
FormalParameters
AsyncFunctionBody
funcEnv
strict
).
Perform !
SetFunctionName
closure
name
).
Perform !
envRec
.InitializeBinding(
name
closure
).
Set
closure
.[[SourceText]] to the source text matched by
AsyncFunctionExpression
Return
closure
AwaitExpression
await
UnaryExpression
Let
exprRef
be the result of evaluating
UnaryExpression
Let
value
be ?
GetValue
exprRef
).
Return ?
Await
value
).
14.8
Async Arrow Function Definitions
Syntax
AsyncArrowFunction
[In, Yield, Await]
async
[no
LineTerminator
here]
AsyncArrowBindingIdentifier
[?Yield]
[no
LineTerminator
here]
=>
AsyncConciseBody
[?In]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
[no
LineTerminator
here]
=>
AsyncConciseBody
[?In]
AsyncConciseBody
[In]
[lookahead ≠
AssignmentExpression
[?In, ~Yield, +Await]
AsyncFunctionBody
AsyncArrowBindingIdentifier
[Yield]
BindingIdentifier
[?Yield, +Await]
CoverCallExpressionAndAsyncArrowHead
[Yield, Await]
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
Supplemental Syntax
When processing an instance of the production
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
the interpretation of
CoverCallExpressionAndAsyncArrowHead
is refined using the following grammar:
AsyncArrowHead
async
[no
LineTerminator
here]
ArrowFormalParameters
[~Yield, +Await]
14.8.1
Static Semantics: Early Errors
AsyncArrowFunction
async
AsyncArrowBindingIdentifier
=>
AsyncConciseBody
It is a Syntax Error if any element of the BoundNames of
AsyncArrowBindingIdentifier
also occurs in the LexicallyDeclaredNames of
AsyncConciseBody
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead
Contains
YieldExpression
is
true
It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead
Contains
AwaitExpression
is
true
It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead
is not
covering
an
AsyncArrowHead
It is a Syntax Error if any element of the BoundNames of
CoverCallExpressionAndAsyncArrowHead
also occurs in the LexicallyDeclaredNames of
AsyncConciseBody
It is a Syntax Error if ContainsUseStrict of
AsyncConciseBody
is
true
and IsSimpleParameterList of
CoverCallExpressionAndAsyncArrowHead
is
false
All Early Error rules for
AsyncArrowHead
and its derived productions apply to CoveredAsyncArrowHead of
CoverCallExpressionAndAsyncArrowHead
14.8.2
Static Semantics: CoveredAsyncArrowHead
CoverCallExpressionAndAsyncArrowHead
MemberExpression
Arguments
Return the
AsyncArrowHead
that is
covered
by
CoverCallExpressionAndAsyncArrowHead
14.8.3
Static Semantics: BoundNames
CoverCallExpressionAndAsyncArrowHead
MemberExpression
Arguments
Let
head
be CoveredAsyncArrowHead of
CoverCallExpressionAndAsyncArrowHead
Return the BoundNames of
head
14.8.4
Static Semantics: Contains
With parameter
symbol
AsyncArrowFunction
async
AsyncArrowBindingIdentifier
=>
AsyncConciseBody
If
symbol
is not one of
NewTarget
SuperProperty
SuperCall
super
, or
this
, return
false
Return
AsyncConciseBody
Contains
symbol
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
If
symbol
is not one of
NewTarget
SuperProperty
SuperCall
super
, or
this
, return
false
Let
head
be CoveredAsyncArrowHead of
CoverCallExpressionAndAsyncArrowHead
If
head
Contains
symbol
is
true
, return
true
Return
AsyncConciseBody
Contains
symbol
Note
Normally, Contains does not look inside most function forms. However, Contains is used to detect
new.target
this
, and
super
usage within an AsyncArrowFunction.
14.8.5
Static Semantics: ContainsExpression
AsyncArrowBindingIdentifier
BindingIdentifier
Return
false
14.8.6
Static Semantics: ExpectedArgumentCount
AsyncArrowBindingIdentifier
BindingIdentifier
Return 1.
14.8.7
Static Semantics: HasName
AsyncArrowFunction
async
AsyncArrowBindingIdentifier
=>
AsyncConciseBody
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
Return
false
14.8.8
Static Semantics: IsSimpleParameterList
AsyncArrowBindingIdentifier
[Yield]
BindingIdentifier
[?Yield, +Await]
Return
true
CoverCallExpressionAndAsyncArrowHead
MemberExpression
Arguments
Let
head
be CoveredAsyncArrowHead of
CoverCallExpressionAndAsyncArrowHead
Return IsSimpleParameterList of
head
14.8.9
Static Semantics: LexicallyDeclaredNames
AsyncConciseBody
AssignmentExpression
Return a new empty
List
14.8.10
Static Semantics: LexicallyScopedDeclarations
AsyncConciseBody
AssignmentExpression
Return a new empty
List
14.8.11
Static Semantics: VarDeclaredNames
AsyncConciseBody
AssignmentExpression
Return a new empty
List
14.8.12
Static Semantics: VarScopedDeclarations
AsyncConciseBody
AssignmentExpression
Return a new empty
List
14.8.13
Runtime Semantics: IteratorBindingInitialization
With parameters
iteratorRecord
and
environment
AsyncArrowBindingIdentifier
BindingIdentifier
Assert
iteratorRecord
.[[Done]] is
false
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, set
iteratorRecord
.[[Done]] to
true
Else,
Let
be
IteratorValue
next
).
If
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
).
If
iteratorRecord
.[[Done]] is
true
, let
be
undefined
Return the result of performing BindingInitialization for
BindingIdentifier
using
and
environment
as the arguments.
14.8.14
Runtime Semantics: EvaluateBody
With parameters
functionObject
and
List
argumentsList
AsyncConciseBody
AssignmentExpression
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
Let
declResult
be
FunctionDeclarationInstantiation
functionObject
argumentsList
).
If
declResult
is not an
abrupt completion
, then
Perform !
AsyncFunctionStart
promiseCapability
AssignmentExpression
).
Else
declResult
is an
abrupt completion
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
declResult
.[[Value]] »).
Return
Completion
{ [[Type]]:
return
, [[Value]]:
promiseCapability
.[[Promise]], [[Target]]:
empty
}.
AsyncConciseBody
AsyncFunctionBody
Return the result of EvaluateBody of
AsyncFunctionBody
passing
functionObject
and
argumentsList
as the arguments.
14.8.15
Runtime Semantics: NamedEvaluation
With parameter
name
AsyncArrowFunction
async
AsyncArrowBindingIdentifier
=>
AsyncConciseBody
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
Let
closure
be the result of evaluating this
AsyncArrowFunction
Perform
SetFunctionName
closure
name
).
Return
closure
14.8.16
Runtime Semantics: Evaluation
AsyncArrowFunction
async
AsyncArrowBindingIdentifier
=>
AsyncConciseBody
If the function code for this
AsyncArrowFunction
is
strict mode code
, let
strict
be
true
. Otherwise, let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
parameters
be
AsyncArrowBindingIdentifier
Let
closure
be !
AsyncFunctionCreate
Arrow
parameters
AsyncConciseBody
scope
strict
).
Return
closure
AsyncArrowFunction
CoverCallExpressionAndAsyncArrowHead
=>
AsyncConciseBody
If the function code for this
AsyncArrowFunction
is
strict mode code
, let
strict
be
true
. Otherwise, let
strict
be
false
Let
scope
be the LexicalEnvironment of the
running execution context
Let
head
be CoveredAsyncArrowHead of
CoverCallExpressionAndAsyncArrowHead
Let
parameters
be the
ArrowFormalParameters
of
head
Let
closure
be !
AsyncFunctionCreate
Arrow
parameters
AsyncConciseBody
scope
strict
).
Return
closure
14.9
Tail Position Calls
14.9.1
Static Semantics: IsInTailPosition (
call
The abstract operation IsInTailPosition with argument
call
performs the following steps:
Assert
call
is a
Parse Node
If the source code matching
call
is
non-strict code
, return
false
If
call
is not contained within a
FunctionBody
ConciseBody
, or
AsyncConciseBody
, return
false
Let
body
be the
FunctionBody
ConciseBody
, or
AsyncConciseBody
that most closely contains
call
If
body
is the
FunctionBody
of a
GeneratorBody
, return
false
If
body
is the
FunctionBody
of an
AsyncFunctionBody
, return
false
If
body
is the
FunctionBody
of an
AsyncGeneratorBody
, return
false
If
body
is an
AsyncConciseBody
, return
false
Return the result of HasCallInTailPosition of
body
with argument
call
Note
Tail Position calls are only defined in
strict mode code
because of a common non-standard language extension (see
9.2.9
) that enables observation of the chain of caller contexts.
14.9.2
Static Semantics: HasCallInTailPosition
With parameter
call
Note
call
is a
Parse Node
that represents a specific range of source text. When the following algorithms compare
call
to another
Parse Node
, it is a test of whether they represent the same source text.
14.9.2.1
Statement Rules
ConciseBody
AssignmentExpression
Return HasCallInTailPosition of
AssignmentExpression
with argument
call
StatementList
StatementList
StatementListItem
Let
has
be HasCallInTailPosition of
StatementList
with argument
call
If
has
is
true
, return
true
Return HasCallInTailPosition of
StatementListItem
with argument
call
FunctionStatementList
[empty]
StatementListItem
Declaration
Statement
VariableStatement
EmptyStatement
ExpressionStatement
ContinueStatement
BreakStatement
ThrowStatement
DebuggerStatement
Block
ReturnStatement
return
LabelledItem
FunctionDeclaration
IterationStatement
for
LeftHandSideExpression
of
AssignmentExpression
Statement
for
var
ForBinding
of
AssignmentExpression
Statement
for
ForDeclaration
of
AssignmentExpression
Statement
CaseBlock
Return
false
IfStatement
if
Expression
Statement
else
Statement
Let
has
be HasCallInTailPosition of the first
Statement
with argument
call
If
has
is
true
, return
true
Return HasCallInTailPosition of the second
Statement
with argument
call
IfStatement
if
Expression
Statement
IterationStatement
do
Statement
while
Expression
while
Expression
Statement
for
Expression
opt
Expression
opt
Expression
opt
Statement
for
var
VariableDeclarationList
Expression
opt
Expression
opt
Statement
for
LexicalDeclaration
Expression
opt
Expression
opt
Statement
for
LeftHandSideExpression
in
Expression
Statement
for
var
ForBinding
in
Expression
Statement
for
ForDeclaration
in
Expression
Statement
WithStatement
with
Expression
Statement
Return HasCallInTailPosition of
Statement
with argument
call
LabelledStatement
LabelIdentifier
LabelledItem
Return HasCallInTailPosition of
LabelledItem
with argument
call
ReturnStatement
return
Expression
Return HasCallInTailPosition of
Expression
with argument
call
SwitchStatement
switch
Expression
CaseBlock
Return HasCallInTailPosition of
CaseBlock
with argument
call
CaseBlock
CaseClauses
opt
DefaultClause
CaseClauses
opt
Let
has
be
false
If the first
CaseClauses
is present, let
has
be HasCallInTailPosition of the first
CaseClauses
with argument
call
If
has
is
true
, return
true
Let
has
be HasCallInTailPosition of the
DefaultClause
with argument
call
If
has
is
true
, return
true
If the second
CaseClauses
is present, let
has
be HasCallInTailPosition of the second
CaseClauses
with argument
call
Return
has
CaseClauses
CaseClauses
CaseClause
Let
has
be HasCallInTailPosition of
CaseClauses
with argument
call
If
has
is
true
, return
true
Return HasCallInTailPosition of
CaseClause
with argument
call
CaseClause
case
Expression
StatementList
opt
DefaultClause
default
StatementList
opt
If
StatementList
is present, return HasCallInTailPosition of
StatementList
with argument
call
Return
false
TryStatement
try
Block
Catch
Return HasCallInTailPosition of
Catch
with argument
call
TryStatement
try
Block
Finally
TryStatement
try
Block
Catch
Finally
Return HasCallInTailPosition of
Finally
with argument
call
Catch
catch
CatchParameter
Block
Return HasCallInTailPosition of
Block
with argument
call
14.9.2.2
Expression Rules
Note
A potential tail position call that is immediately followed by return
GetValue
of the call result is also a possible tail position call. Function calls cannot return reference values, so such a
GetValue
operation will always return the same value as the actual function call result.
AssignmentExpression
YieldExpression
ArrowFunction
AsyncArrowFunction
LeftHandSideExpression
AssignmentExpression
LeftHandSideExpression
AssignmentOperator
AssignmentExpression
BitwiseANDExpression
BitwiseANDExpression
EqualityExpression
BitwiseXORExpression
BitwiseXORExpression
BitwiseANDExpression
BitwiseORExpression
BitwiseORExpression
BitwiseXORExpression
EqualityExpression
EqualityExpression
==
RelationalExpression
EqualityExpression
!=
RelationalExpression
EqualityExpression
===
RelationalExpression
EqualityExpression
!==
RelationalExpression
RelationalExpression
RelationalExpression
ShiftExpression
RelationalExpression
ShiftExpression
RelationalExpression
<=
ShiftExpression
RelationalExpression
>=
ShiftExpression
RelationalExpression
instanceof
ShiftExpression
RelationalExpression
in
ShiftExpression
ShiftExpression
ShiftExpression
<<
AdditiveExpression
ShiftExpression
>>
AdditiveExpression
ShiftExpression
>>>
AdditiveExpression
AdditiveExpression
AdditiveExpression
MultiplicativeExpression
AdditiveExpression
MultiplicativeExpression
MultiplicativeExpression
MultiplicativeExpression
MultiplicativeOperator
ExponentiationExpression
ExponentiationExpression
UpdateExpression
**
ExponentiationExpression
UpdateExpression
LeftHandSideExpression
++
LeftHandSideExpression
--
++
UnaryExpression
--
UnaryExpression
UnaryExpression
delete
UnaryExpression
void
UnaryExpression
typeof
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
UnaryExpression
AwaitExpression
CallExpression
SuperCall
CallExpression
Expression
CallExpression
IdentifierName
NewExpression
new
NewExpression
MemberExpression
MemberExpression
Expression
MemberExpression
IdentifierName
SuperProperty
MetaProperty
new
MemberExpression
Arguments
PrimaryExpression
this
IdentifierReference
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
AsyncGeneratorExpression
RegularExpressionLiteral
TemplateLiteral
Return
false
Expression
AssignmentExpression
Expression
AssignmentExpression
Return HasCallInTailPosition of
AssignmentExpression
with argument
call
ConditionalExpression
LogicalORExpression
AssignmentExpression
AssignmentExpression
Let
has
be HasCallInTailPosition of the first
AssignmentExpression
with argument
call
If
has
is
true
, return
true
Return HasCallInTailPosition of the second
AssignmentExpression
with argument
call
LogicalANDExpression
LogicalANDExpression
&&
BitwiseORExpression
Return HasCallInTailPosition of
BitwiseORExpression
with argument
call
LogicalORExpression
LogicalORExpression
||
LogicalANDExpression
Return HasCallInTailPosition of
LogicalANDExpression
with argument
call
CallExpression
CoverCallExpressionAndAsyncArrowHead
CallExpression
Arguments
CallExpression
TemplateLiteral
If this
CallExpression
is
call
, return
true
Return
false
MemberExpression
MemberExpression
TemplateLiteral
If this
MemberExpression
is
call
, return
true
Return
false
PrimaryExpression
CoverParenthesizedExpressionAndArrowParameterList
Let
expr
be CoveredParenthesizedExpression of
CoverParenthesizedExpressionAndArrowParameterList
Return HasCallInTailPosition of
expr
with argument
call
ParenthesizedExpression
Expression
Return HasCallInTailPosition of
Expression
with argument
call
14.9.3
Runtime Semantics: PrepareForTailCall ( )
The abstract operation PrepareForTailCall performs the following steps:
Let
leafContext
be the
running execution context
Suspend
leafContext
Pop
leafContext
from the
execution context stack
. The
execution context
now on the top of the stack becomes the
running execution context
Assert
leafContext
has no further use. It will never be activated as the
running execution context
A tail position call must either release any transient internal resources associated with the currently executing function
execution context
before invoking the target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an
implementation's activation record stack by the amount that the size of
the target function's activation record exceeds the size of the calling
function's activation record. If the target function's activation record
is smaller, then the total size of the stack should decrease.
15
ECMAScript Language: Scripts and Modules
15.1
Scripts
Syntax
Script
ScriptBody
opt
ScriptBody
StatementList
[~Yield, ~Await, ~Return]
15.1.1
Static Semantics: Early Errors
Script
ScriptBody
It is a Syntax Error if the LexicallyDeclaredNames of
ScriptBody
contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
ScriptBody
also occurs in the VarDeclaredNames of
ScriptBody
ScriptBody
StatementList
It is a Syntax Error if
StatementList
Contains
super
unless the source code containing
super
is eval code that is being processed by a
direct eval
. Additional
early error
rules for
super
within
direct eval
are defined in
18.2.1.1
It is a Syntax Error if
StatementList
Contains
NewTarget
unless the source code containing
NewTarget
is eval code that is being processed by a
direct eval
. Additional
early error
rules for
NewTarget
in
direct eval
are defined in
18.2.1.1
It is a Syntax Error if ContainsDuplicateLabels of
StatementList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedBreakTarget of
StatementList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedContinueTarget of
StatementList
with arguments « » and « » is
true
15.1.2
Static Semantics: IsStrict
ScriptBody
StatementList
If the
Directive Prologue
of
StatementList
contains a
Use Strict Directive
, return
true
; otherwise, return
false
15.1.3
Static Semantics: LexicallyDeclaredNames
ScriptBody
StatementList
Return TopLevelLexicallyDeclaredNames of
StatementList
Note
At the top level of a
Script
, function declarations are treated like var declarations rather than like lexical declarations.
15.1.4
Static Semantics: LexicallyScopedDeclarations
ScriptBody
StatementList
Return TopLevelLexicallyScopedDeclarations of
StatementList
15.1.5
Static Semantics: VarDeclaredNames
ScriptBody
StatementList
Return TopLevelVarDeclaredNames of
StatementList
15.1.6
Static Semantics: VarScopedDeclarations
ScriptBody
StatementList
Return TopLevelVarScopedDeclarations of
StatementList
15.1.7
Runtime Semantics: Evaluation
Script
[empty]
Return
NormalCompletion
undefined
).
15.1.8
Script Records
Script Record
encapsulates information about a script being evaluated. Each script record contains the fields listed in
Table 36
Table 36:
Script Record
Fields
Field Name
Value Type
Meaning
[[Realm]]
Realm Record
undefined
The
realm
within which this script was created.
undefined
if not yet assigned.
[[Environment]]
Lexical Environment
undefined
The
Lexical Environment
containing the top level bindings for this script. This field is set when the script is instantiated.
[[ECMAScriptCode]]
Parse Node
The result of parsing the source text of this module using
Script
as the
goal symbol
[[HostDefined]]
Any, default value is
undefined
Field reserved for use by host environments that need to associate additional information with a script.
15.1.9
ParseScript (
sourceText
realm
hostDefined
The abstract operation ParseScript with arguments
sourceText
realm
, and
hostDefined
creates a
Script Record
based upon the result of parsing
sourceText
as a
Script
. ParseScript performs the following steps:
Assert
sourceText
is an ECMAScript source text (see clause
10
).
Parse
sourceText
using
Script
as the
goal symbol
and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let
body
be the resulting parse tree. Otherwise, let
body
be a
List
of one or more
SyntaxError
or
ReferenceError
objects representing the parsing errors and/or early errors. Parsing and
early error
detection may be interweaved in an implementation-dependent manner. If more than one parsing error or
early error
is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If
body
is a
List
of errors, return
body
Return
Script Record
{ [[Realm]]:
realm
, [[Environment]]:
undefined
, [[ECMAScriptCode]]:
body
, [[HostDefined]]:
hostDefined
}.
Note
An implementation may parse script source text and analyse it
for Early Error conditions prior to evaluation of ParseScript for that
script source text. However, the reporting of any errors must be
deferred until the point where this specification actually performs
ParseScript upon that source text.
15.1.10
ScriptEvaluation (
scriptRecord
Let
globalEnv
be
scriptRecord
.[[Realm]].[[GlobalEnv]].
Let
scriptCxt
be a new ECMAScript code
execution context
Set the Function of
scriptCxt
to
null
Set the
Realm
of
scriptCxt
to
scriptRecord
.[[Realm]].
Set the ScriptOrModule of
scriptCxt
to
scriptRecord
Set the VariableEnvironment of
scriptCxt
to
globalEnv
Set the LexicalEnvironment of
scriptCxt
to
globalEnv
Suspend
the currently
running execution context
Push
scriptCxt
on to the
execution context stack
scriptCxt
is now the
running execution context
Let
scriptBody
be
scriptRecord
.[[ECMAScriptCode]].
Let
result
be
GlobalDeclarationInstantiation
scriptBody
globalEnv
).
If
result
.[[Type]] is
normal
, then
Set
result
to the result of evaluating
scriptBody
If
result
.[[Type]] is
normal
and
result
.[[Value]] is
empty
, then
Set
result
to
NormalCompletion
undefined
).
Suspend
scriptCxt
and remove it from the
execution context stack
Assert
: The
execution context stack
is not empty.
Resume the context that is now on the top of the
execution context stack
as the
running execution context
Return
Completion
result
).
15.1.11
Runtime Semantics: GlobalDeclarationInstantiation (
script
env
Note 1
When an
execution context
is established for evaluating scripts, declarations are instantiated in the current
global environment
. Each global binding declared in the code is instantiated.
GlobalDeclarationInstantiation is performed as follows using arguments
script
and
env
script
is the
ScriptBody
for which the
execution context
is being established.
env
is the global lexical environment in which bindings are to be created.
Let
envRec
be
env
's
EnvironmentRecord
Assert
envRec
is a global
Environment Record
Let
lexNames
be the LexicallyDeclaredNames of
script
Let
varNames
be the VarDeclaredNames of
script
For each
name
in
lexNames
, do
If
envRec
.HasVarDeclaration(
name
) is
true
, throw a
SyntaxError
exception.
If
envRec
.HasLexicalDeclaration(
name
) is
true
, throw a
SyntaxError
exception.
Let
hasRestrictedGlobal
be ?
envRec
.HasRestrictedGlobalProperty(
name
).
If
hasRestrictedGlobal
is
true
, throw a
SyntaxError
exception.
For each
name
in
varNames
, do
If
envRec
.HasLexicalDeclaration(
name
) is
true
, throw a
SyntaxError
exception.
Let
varDeclarations
be the VarScopedDeclarations of
script
Let
functionsToInitialize
be a new empty
List
Let
declaredFunctionNames
be a new empty
List
For each
in
varDeclarations
, in reverse list order, do
If
is neither a
VariableDeclaration
nor a
ForBinding
nor a
BindingIdentifier
, then
Assert
is either a
FunctionDeclaration
, a
GeneratorDeclaration
, an
AsyncFunctionDeclaration
, or an
AsyncGeneratorDeclaration
NOTE: If there are multiple function declarations for the same name, the last declaration is used.
Let
fn
be the sole element of the BoundNames of
If
fn
is not an element of
declaredFunctionNames
, then
Let
fnDefinable
be ?
envRec
.CanDeclareGlobalFunction(
fn
).
If
fnDefinable
is
false
, throw a
TypeError
exception.
Append
fn
to
declaredFunctionNames
Insert
as the first element of
functionsToInitialize
Let
declaredVarNames
be a new empty
List
For each
in
varDeclarations
, do
If
is a
VariableDeclaration
, a
ForBinding
, or a
BindingIdentifier
, then
For each String
vn
in the BoundNames of
, do
If
vn
is not an element of
declaredFunctionNames
, then
Let
vnDefinable
be ?
envRec
.CanDeclareGlobalVar(
vn
).
If
vnDefinable
is
false
, throw a
TypeError
exception.
If
vn
is not an element of
declaredVarNames
, then
Append
vn
to
declaredVarNames
NOTE: No abnormal terminations occur after this algorithm step if the
global object
is an ordinary object. However, if the
global object
is a Proxy
exotic object
it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex
B.3.3.2
adds additional steps at this point.
Let
lexDeclarations
be the LexicallyScopedDeclarations of
script
For each element
in
lexDeclarations
, do
NOTE: Lexically declared names are only instantiated here but not initialized.
For each element
dn
of the BoundNames of
, do
If IsConstantDeclaration of
is
true
, then
Perform ?
envRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform ?
envRec
.CreateMutableBinding(
dn
false
).
For each
Parse Node
in
functionsToInitialize
, do
Let
fn
be the sole element of the BoundNames of
Let
fo
be the result of performing InstantiateFunctionObject for
with argument
env
Perform ?
envRec
.CreateGlobalFunctionBinding(
fn
fo
false
).
For each String
vn
in
declaredVarNames
, in list order, do
Perform ?
envRec
.CreateGlobalVarBinding(
vn
false
).
Return
NormalCompletion
empty
).
Note 2
Early errors specified in
15.1.1
prevent name conflicts between function/var declarations and
let/const/class declarations as well as redeclaration of let/const/class
bindings for declaration contained within a single
Script
. However, such conflicts and redeclarations that span more than one
Script
are detected as runtime errors during GlobalDeclarationInstantiation.
If any such errors are detected, no bindings are instantiated for the
script. However, if the
global object
is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an
abrupt completion
and some global declarations not being instantiated. If this occurs, the code for the
Script
is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the
global object
result in global bindings that may be shadowed by let/const/class declarations.
15.1.12
Runtime Semantics: ScriptEvaluationJob (
sourceText
hostDefined
The job ScriptEvaluationJob with parameters
sourceText
and
hostDefined
parses, validates, and evaluates
sourceText
as a
Script
Assert
sourceText
is an ECMAScript source text (see clause
10
).
Let
realm
be
the current Realm Record
Let
be
ParseScript
sourceText
realm
hostDefined
).
If
is a
List
of errors, then
Perform
HostReportErrors
).
Return
NormalCompletion
undefined
).
Return ?
ScriptEvaluation
).
15.2
Modules
Syntax
Module
ModuleBody
opt
ModuleBody
ModuleItemList
ModuleItemList
ModuleItem
ModuleItemList
ModuleItem
ModuleItem
ImportDeclaration
ExportDeclaration
StatementListItem
[~Yield, ~Await, ~Return]
15.2.1
Module Semantics
15.2.1.1
Static Semantics: Early Errors
ModuleBody
ModuleItemList
It is a Syntax Error if the LexicallyDeclaredNames of
ModuleItemList
contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of
ModuleItemList
also occurs in the VarDeclaredNames of
ModuleItemList
It is a Syntax Error if the ExportedNames of
ModuleItemList
contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of
ModuleItemList
does not also occur in either the VarDeclaredNames of
ModuleItemList
, or the LexicallyDeclaredNames of
ModuleItemList
It is a Syntax Error if
ModuleItemList
Contains
super
It is a Syntax Error if
ModuleItemList
Contains
NewTarget
It is a Syntax Error if ContainsDuplicateLabels of
ModuleItemList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedBreakTarget of
ModuleItemList
with argument « » is
true
It is a Syntax Error if ContainsUndefinedContinueTarget of
ModuleItemList
with arguments « » and « » is
true
Note
The duplicate ExportedNames rule implies that multiple
export default
ExportDeclaration
items within a
ModuleBody
is a Syntax Error. Additional error conditions relating to conflicting
or duplicate declarations are checked during module linking prior to
evaluation of a
Module
. If any such errors are detected the
Module
is not evaluated.
15.2.1.2
Static Semantics: ContainsDuplicateLabels
With parameter
labelSet
ModuleItemList
ModuleItemList
ModuleItem
Let
hasDuplicates
be ContainsDuplicateLabels of
ModuleItemList
with argument
labelSet
If
hasDuplicates
is
true
, return
true
Return ContainsDuplicateLabels of
ModuleItem
with argument
labelSet
ModuleItem
ImportDeclaration
ExportDeclaration
Return
false
15.2.1.3
Static Semantics: ContainsUndefinedBreakTarget
With parameter
labelSet
ModuleItemList
ModuleItemList
ModuleItem
Let
hasUndefinedLabels
be ContainsUndefinedBreakTarget of
ModuleItemList
with argument
labelSet
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedBreakTarget of
ModuleItem
with argument
labelSet
ModuleItem
ImportDeclaration
ExportDeclaration
Return
false
15.2.1.4
Static Semantics: ContainsUndefinedContinueTarget
With parameters
iterationSet
and
labelSet
ModuleItemList
ModuleItemList
ModuleItem
Let
hasUndefinedLabels
be ContainsUndefinedContinueTarget of
ModuleItemList
with arguments
iterationSet
and « ».
If
hasUndefinedLabels
is
true
, return
true
Return ContainsUndefinedContinueTarget of
ModuleItem
with arguments
iterationSet
and « ».
ModuleItem
ImportDeclaration
ExportDeclaration
Return
false
15.2.1.5
Static Semantics: ExportedBindings
Note
ExportedBindings are the locally bound names that are explicitly associated with a
Module
's ExportedNames.
ModuleItemList
ModuleItemList
ModuleItem
Let
names
be ExportedBindings of
ModuleItemList
Append to
names
the elements of the ExportedBindings of
ModuleItem
Return
names
ModuleItem
ImportDeclaration
StatementListItem
Return a new empty
List
15.2.1.6
Static Semantics: ExportedNames
Note
ExportedNames are the externally visible names that a
Module
explicitly maps to one of its local name bindings.
ModuleItemList
ModuleItemList
ModuleItem
Let
names
be ExportedNames of
ModuleItemList
Append to
names
the elements of the ExportedNames of
ModuleItem
Return
names
ModuleItem
ExportDeclaration
Return the ExportedNames of
ExportDeclaration
ModuleItem
ImportDeclaration
StatementListItem
Return a new empty
List
15.2.1.7
Static Semantics: ExportEntries
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItemList
ModuleItem
Let
entries
be ExportEntries of
ModuleItemList
Append to
entries
the elements of the ExportEntries of
ModuleItem
Return
entries
ModuleItem
ImportDeclaration
StatementListItem
Return a new empty
List
15.2.1.8
Static Semantics: ImportEntries
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItemList
ModuleItem
Let
entries
be ImportEntries of
ModuleItemList
Append to
entries
the elements of the ImportEntries of
ModuleItem
Return
entries
ModuleItem
ExportDeclaration
StatementListItem
Return a new empty
List
15.2.1.9
Static Semantics: ImportedLocalNames (
importEntries
The abstract operation ImportedLocalNames with argument
importEntries
creates a
List
of all of the local name bindings defined by a
List
of ImportEntry Records (see
Table 42
). ImportedLocalNames performs the following steps:
Let
localNames
be a new empty
List
For each
ImportEntry Record
in
importEntries
, do
Append
.[[LocalName]] to
localNames
Return
localNames
15.2.1.10
Static Semantics: ModuleRequests
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItem
Return ModuleRequests of
ModuleItem
ModuleItemList
ModuleItemList
ModuleItem
Let
moduleNames
be ModuleRequests of
ModuleItemList
Let
additionalNames
be ModuleRequests of
ModuleItem
Append to
moduleNames
each element of
additionalNames
that is not already an element of
moduleNames
Return
moduleNames
ModuleItem
StatementListItem
Return a new empty
List
15.2.1.11
Static Semantics: LexicallyDeclaredNames
Note 1
The LexicallyDeclaredNames of a
Module
includes the names of all of its imported bindings.
ModuleItemList
ModuleItemList
ModuleItem
Let
names
be LexicallyDeclaredNames of
ModuleItemList
Append to
names
the elements of the LexicallyDeclaredNames of
ModuleItem
Return
names
ModuleItem
ImportDeclaration
Return the BoundNames of
ImportDeclaration
ModuleItem
ExportDeclaration
If
ExportDeclaration
is
export
VariableStatement
, return a new empty
List
Return the BoundNames of
ExportDeclaration
ModuleItem
StatementListItem
Return LexicallyDeclaredNames of
StatementListItem
Note 2
At the top level of a
Module
, function declarations are treated like lexical declarations rather than like var declarations.
15.2.1.12
Static Semantics: LexicallyScopedDeclarations
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItemList
ModuleItem
Let
declarations
be LexicallyScopedDeclarations of
ModuleItemList
Append to
declarations
the elements of the LexicallyScopedDeclarations of
ModuleItem
Return
declarations
ModuleItem
ImportDeclaration
Return a new empty
List
15.2.1.13
Static Semantics: VarDeclaredNames
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItemList
ModuleItem
Let
names
be VarDeclaredNames of
ModuleItemList
Append to
names
the elements of the VarDeclaredNames of
ModuleItem
Return
names
ModuleItem
ImportDeclaration
Return a new empty
List
ModuleItem
ExportDeclaration
If
ExportDeclaration
is
export
VariableStatement
, return BoundNames of
ExportDeclaration
Return a new empty
List
15.2.1.14
Static Semantics: VarScopedDeclarations
Module
[empty]
Return a new empty
List
ModuleItemList
ModuleItemList
ModuleItem
Let
declarations
be VarScopedDeclarations of
ModuleItemList
Append to
declarations
the elements of the VarScopedDeclarations of
ModuleItem
Return
declarations
ModuleItem
ImportDeclaration
Return a new empty
List
ModuleItem
ExportDeclaration
If
ExportDeclaration
is
export
VariableStatement
, return VarScopedDeclarations of
VariableStatement
Return a new empty
List
15.2.1.15
Abstract Module Records
Module Record
encapsulates structural
information about the imports and exports of a single module. This
information is used to link the imports and exports of sets of connected
modules. A Module Record includes four fields that are only used when
evaluating a module.
For specification purposes Module Record values are values of the
Record
specification type and can be thought of as existing in a simple
object-oriented hierarchy where Module Record is an abstract class with
both abstract and concrete subclasses. This specification defines the
abstract subclass named
Cyclic Module Record
and its concrete subclass named
Source Text Module Record
Other specifications and implementations may define additional Module
Record subclasses corresponding to alternative module definition
facilities that they defined.
Module Record defines the fields listed in
Table 37
. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in
Table 38
. All Module definition subclasses must provide concrete implementations of these abstract methods.
Table 37:
Module Record
Fields
Field Name
Value Type
Meaning
[[Realm]]
Realm Record
undefined
The
Realm
within which this module was created.
undefined
if not yet assigned.
[[Environment]]
Lexical Environment
undefined
The
Lexical Environment
containing the top level bindings for this module. This field is set when the module is instantiated.
[[Namespace]]
Object |
undefined
The Module Namespace Object (
26.3
) if one has been created for this module. Otherwise
undefined
[[HostDefined]]
Any, default value is
undefined
Field reserved for use by host environments that need to associate additional information with a module.
Table 38: Abstract Methods of Module Records
Method
Purpose
GetExportedNames(
exportStarSet
Return a list of all names that are either directly or indirectly exported from this module.
ResolveExport(
exportName
resolveSet
Return the binding of a name exported by this module. Bindings are represented by a
ResolvedBinding Record
, of the form { [[Module]]:
Module Record
, [[BindingName]]: String }. Return
null
if the name cannot be resolved, or
"ambiguous"
if multiple bindings were found.
This operation must be idempotent if it completes normally. Each time it is called with a specific
exportName
resolveSet
pair as arguments it must return the same result.
Instantiate()
Prepare the module for evaluation by transitively resolving all module dependencies and creating a module
Environment Record
Evaluate()
If this module has already been evaluated successfully, return
undefined
if it has already been evaluated unsuccessfully, throw the exception
that was produced. Otherwise, transitively evaluate all module
dependencies of this module and then evaluate this module.
Instantiate must have completed successfully prior to invoking this method.
15.2.1.16
Cyclic Module Records
Cyclic Module Record
is used to represent information about a module that can participate in
dependency cycles with other modules that are subclasses of the
Cyclic Module Record
type. Module Records that are not subclasses of the
Cyclic Module Record
type must not participate in dependency cycles with Source Text Module Records.
In addition to the fields defined in
Table 37
Cyclic Module Records have the additional fields listed in
Table 39
Table 39: Additional Fields of Cyclic Module Records
Field Name
Value Type
Meaning
[[Status]]
String
Initially
"uninstantiated"
. Transitions to
"instantiating"
"instantiated"
"evaluating"
"evaluated"
(in that order) as the module progresses throughout its lifecycle.
[[EvaluationError]]
An
abrupt completion
undefined
A completion of type
throw
representing the exception that occurred during evaluation.
undefined
if no exception occurred or if [[Status]] is not
"evaluated"
[[DFSIndex]]
Integer |
undefined
Auxiliary field used during Instantiate and Evaluate only.
If [[Status]] is
"instantiating"
or
"evaluating"
this nonnegative number records the point at which the module was first
visited during the ongoing depth-first traversal of the dependency
graph.
[[DFSAncestorIndex]]
Integer |
undefined
Auxiliary field used during Instantiate and Evaluate only. If [[Status]] is
"instantiating"
or
"evaluating"
, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
[[RequestedModules]]
List
of String
List
of all the
ModuleSpecifier
strings used by the module represented by this record to request the importation of a module. The
List
is source code occurrence ordered.
In addition to the methods defined in
Table 38
Cyclic Module Records have the additional methods listed in
Table 40
Table 40: Additional Abstract Methods of Cyclic Module Records
Method
Purpose
InitializeEnvironment
()
Initialize the
Lexical Environment
of the module, including resolving all imported bindings.
ExecuteModule
()
Initialize the
execution context
of the module and evaluate the module's code within it.
15.2.1.16.1
Instantiate ( ) Concrete Method
The Instantiate concrete method of a
Cyclic Module Record
implements the corresponding
Module Record
abstract method.
On success, Instantiate transitions this module's [[Status]] from
"uninstantiated"
to
"instantiated"
. On failure, an exception is thrown and this module's [[Status]] remains
"uninstantiated"
This abstract method performs the following steps (most of the work is done by the auxiliary function
InnerModuleInstantiation
):
Let
module
be this
Cyclic Module Record
Assert
module
.[[Status]] is not
"instantiating"
or
"evaluating"
Let
stack
be a new empty
List
Let
result
be
InnerModuleInstantiation
module
stack
, 0).
If
result
is an
abrupt completion
, then
For each module
in
stack
, do
Assert
.[[Status]] is
"instantiating"
Set
.[[Status]] to
"uninstantiated"
Set
.[[Environment]] to
undefined
Set
.[[DFSIndex]] to
undefined
Set
.[[DFSAncestorIndex]] to
undefined
Assert
module
.[[Status]] is
"uninstantiated"
Return
result
Assert
module
.[[Status]] is
"instantiated"
or
"evaluated"
Assert
stack
is empty.
Return
undefined
15.2.1.16.1.1
InnerModuleInstantiation (
module
stack
index
The InnerModuleInstantiation abstract operation is used by Instantiate to perform the actual instantiation process for the
Cyclic Module Record
module
, as well as recursively on all other modules in the dependency graph. The
stack
and
index
parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]]
fields, keep track of the depth-first search (DFS) traversal. In
particular, [[DFSAncestorIndex]] is used to discover strongly connected
components (SCCs), such that all modules in an SCC transition to
"instantiated"
together.
This abstract operation performs the following steps:
If
module
is not a
Cyclic Module Record
, then
Perform ?
module
.Instantiate().
Return
index
If
module
.[[Status]] is
"instantiating"
"instantiated"
, or
"evaluated"
, then
Return
index
Assert
module
.[[Status]] is
"uninstantiated"
Set
module
.[[Status]] to
"instantiating"
Set
module
.[[DFSIndex]] to
index
Set
module
.[[DFSAncestorIndex]] to
index
Increase
index
by 1.
Append
module
to
stack
For each String
required
that is an element of
module
.[[RequestedModules]], do
Let
requiredModule
be ?
HostResolveImportedModule
module
required
).
Set
index
to ?
InnerModuleInstantiation
requiredModule
stack
index
).
Assert
requiredModule
.[[Status]] is either
"instantiating"
"instantiated"
, or
"evaluated"
Assert
requiredModule
.[[Status]] is
"instantiating"
if and only if
requiredModule
is in
stack
If
requiredModule
.[[Status]] is
"instantiating"
, then
Assert
requiredModule
is a
Cyclic Module Record
Set
module
.[[DFSAncestorIndex]] to
min
module
.[[DFSAncestorIndex]],
requiredModule
.[[DFSAncestorIndex]]).
Perform ?
module
InitializeEnvironment
().
Assert
module
occurs exactly once in
stack
Assert
module
.[[DFSAncestorIndex]] is less than or equal to
module
.[[DFSIndex]].
If
module
.[[DFSAncestorIndex]] equals
module
.[[DFSIndex]], then
Let
done
be
false
Repeat, while
done
is
false
Let
requiredModule
be the last element in
stack
Remove the last element of
stack
Set
requiredModule
.[[Status]] to
"instantiated"
If
requiredModule
and
module
are the same
Module Record
, set
done
to
true
Return
index
15.2.1.16.2
Evaluate ( ) Concrete Method
The Evaluate concrete method of a
Cyclic Module Record
implements the corresponding
Module Record
abstract method.
Evaluate transitions this module's [[Status]] from
"instantiated"
to
"evaluated"
If execution results in an exception, that exception is
recorded in the [[EvaluationError]] field and rethrown by future
invocations of Evaluate.
This abstract method performs the following steps (most of the work is done by the auxiliary function
InnerModuleEvaluation
):
Let
module
be this
Cyclic Module Record
Assert
module
.[[Status]] is
"instantiated"
or
"evaluated"
Let
stack
be a new empty
List
Let
result
be
InnerModuleEvaluation
module
stack
, 0).
If
result
is an
abrupt completion
, then
For each module
in
stack
, do
Assert
.[[Status]] is
"evaluating"
Set
.[[Status]] to
"evaluated"
Set
.[[EvaluationError]] to
result
Assert
module
.[[Status]] is
"evaluated"
and
module
.[[EvaluationError]] is
result
Return
result
Assert
module
.[[Status]] is
"evaluated"
and
module
.[[EvaluationError]] is
undefined
Assert
stack
is empty.
Return
undefined
15.2.1.16.2.1
InnerModuleEvaluation (
module
stack
index
The InnerModuleEvaluation abstract operation is used by Evaluate to perform the actual evaluation process for the
Source Text Module Record
module
, as well as recursively on all other modules in the dependency graph. The
stack
and
index
parameters, as well as
module
's [[DFSIndex]] and [[DFSAncestoreIndex]] fields, are used the same way as in
InnerModuleInstantiation
This abstract operation performs the following steps:
If
module
is not a
Cyclic Module Record
, then
Perform ?
module
.Evaluate().
Return
index
If
module
.[[Status]] is
"evaluated"
, then
If
module
.[[EvaluationError]] is
undefined
, return
index
Otherwise return
module
.[[EvaluationError]].
If
module
.[[Status]] is
"evaluating"
, return
index
Assert
module
.[[Status]] is
"instantiated"
Set
module
.[[Status]] to
"evaluating"
Set
module
.[[DFSIndex]] to
index
Set
module
.[[DFSAncestorIndex]] to
index
Increase
index
by 1.
Append
module
to
stack
For each String
required
that is an element of
module
.[[RequestedModules]], do
Let
requiredModule
be !
HostResolveImportedModule
module
required
).
NOTE:
Instantiate must be completed successfully prior to invoking this
method, so every requested module is guaranteed to resolve successfully.
Set
index
to ?
InnerModuleEvaluation
requiredModule
stack
index
).
Assert
requiredModule
.[[Status]] is either
"evaluating"
or
"evaluated"
Assert
requiredModule
.[[Status]] is
"evaluating"
if and only if
requiredModule
is in
stack
If
requiredModule
.[[Status]] is
"evaluating"
, then
Assert
requiredModule
is a
Cyclic Module Record
Set
module
.[[DFSAncestorIndex]] to
min
module
.[[DFSAncestorIndex]],
requiredModule
.[[DFSAncestorIndex]]).
Perform ?
module
ExecuteModule
().
Assert
module
occurs exactly once in
stack
Assert
module
.[[DFSAncestorIndex]] is less than or equal to
module
.[[DFSIndex]].
If
module
.[[DFSAncestorIndex]] equals
module
.[[DFSIndex]], then
Let
done
be
false
Repeat, while
done
is
false
Let
requiredModule
be the last element in
stack
Remove the last element of
stack
Set
requiredModule
.[[Status]] to
"evaluated"
If
requiredModule
and
module
are the same
Module Record
, set
done
to
true
Return
index
15.2.1.16.3
Example Cyclic Module Record Graphs
This non-normative section gives a series of examples of
the instantiation and evaluation of a few common module graphs, with a
specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls
.Instantiate(), this will complete successfully by assumption, and recursively instantiate modules
and
as well, such that
.[[Status]] =
.[[Status]] =
.[[Status]] =
"instantiated"
This preparatory step can be performed at any time. Later, when the
host is ready to incur any possible side effects of the modules, it can
call
.Evaluate(), which will complete successfully (again by assumption), recursively having evaluated first
and then
. Each module's [[Status]] at this point will be
"evaluated
".
Consider then cases involving instantiation errors. If
InnerModuleInstantiation
of
succeeds but, thereafter, fails for
, for example because it imports something that
does not provide, then the original
.Instantiate() will fail, and both
and
's [[Status]] remain
"uninstantiated"
's [[Status]] has become
"instantiated"
, though.
Finally, consider a case involving evaluation errors. If
InnerModuleEvaluation
of
succeeds but, thereafter, fails for
, for example because
contains code that throws an exception, then the original
.Evaluate() will fail. The resulting exception will be recorded in both
and
's [[EvaluationError]] fields, and their [[Status]] will become
"evaluated"
will also become
"evaluated"
but, in contrast to
and
will remain without an [[EvaluationError]], as it successfully
completed evaluation. Storing the exception ensures that any time a host
tries to reuse
or
by calling their Evaluate()
method, it will encounter the same exception. (Hosts are not required
to reuse Cyclic Module Records; similarly, hosts are not required to
expose the exception objects thrown by these methods. However, the
specification enables such uses.)
The difference here between instantiation and evaluation
errors is due to how evaluation must be only performed once, as it can
cause side effects; it is thus important to remember whether evaluation
has already been performed, even if unsuccessfully. (In the error case,
it makes sense to also remember the exception because otherwise
subsequent Evaluate() calls would have to synthesize a new one.)
Instantiation, on the other hand, is side-effect-free, and thus even if
it fails, it can be retried at a later time with no issues.
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module
declares a dependency on some other module, but no
Module Record
exists for that module, i.e.
HostResolveImportedModule
throws an exception when asked for it. This could occur for a variety
of reasons, such as the corresponding resource not existing, or the
resource existing but
ParseModule
throwing an exception when trying to parse the resulting source text.
Hosts can choose to expose the cause of failure via the exception they
throw from
HostResolveImportedModule
. In any case, this exception causes an instantiation failure, which as before results in
's [[Status]] remaining
"uninstantiated"
Lastly, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module
, so that the host proceeds by calling
.Instantiate(), which performs
InnerModuleInstantiation
on
. This in turn calls
InnerModuleInstantiation
on
. Because of the cycle, this again triggers
InnerModuleInstantiation
on
, but at this point it is a no-op since
.[[Status]] is already
"instantiating"
.[[Status]] itself remains
"instantiating"
when control gets back to
and
InnerModuleInstantiation
is triggered on
. After this returns with
.[[Status]] being
"instantiated"
, both
and
transition from
"instantiating"
to
"instantiated"
together; this is by design, since they form a strongly connected component.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where
has an instantiation error; for example, it tries to import a binding from
that does not exist. In that case, the above steps still occur, including the early return from the second call to
InnerModuleInstantiation
on
. However, once we unwind back to the original
InnerModuleInstantiation
on
, it fails during
InitializeEnvironment
, namely right after
.ResolveExport(). The thrown
SyntaxError
exception propagates up to
.Instantiate, which resets all modules that are currently on its
stack
(these are always exactly the modules that are still
"instantiating"
). Hence both
and
become
"uninstantiated"
. Note that
is left as
"instantiated"
Finally, consider a case where
has an
evaluation error; for example, its source code throws an exception. In
that case, the evaluation-time analog of the above steps still occurs,
including the early return from the second call to
InnerModuleEvaluation
on
. However, once we unwind back to the original
InnerModuleEvaluation
on
, it fails by assumption. The exception thrown propagates up to
.Evaluate(), which records the error in all modules that are currently on its
stack
(i.e., the modules that are still
"evaluating"
). Hence both
and
become
"evaluated"
and the exception is recorded in both
and
's [[EvaluationError]] fields, while
is left as
"evaluated"
with no [[EvaluationError]].
15.2.1.17
Source Text Module Records
Source Text Module Record
is used to represent information about a module that was defined from ECMAScript source text (
10
) that was parsed using the
goal symbol
Module
Its fields contain digested information about the names that are
imported by the module and its concrete methods use this digest to link,
instantiate, and evaluate the module.
Source Text Module Record
can exist in a module graph with other subclasses of the abstract
Module Record
type, and can participate in cycles with other subclasses of the
Cyclic Module Record
type.
In addition to the fields defined in
Table 39
, Source Text Module Records have the additional fields listed in
Table 41
. Each of these fields is initially set in
ParseModule
Table 41: Additional Fields of Source Text Module Records
Field Name
Value Type
Meaning
[[ECMAScriptCode]]
Parse Node
The result of parsing the source text of this module using
Module
as the
goal symbol
[[ImportEntries]]
List
of ImportEntry Records
List
of ImportEntry records derived from the code of this module.
[[LocalExportEntries]]
List
of ExportEntry Records
List
of ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
[[IndirectExportEntries]]
List
of ExportEntry Records
List
of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module.
[[StarExportEntries]]
List
of ExportEntry Records
List
of ExportEntry records derived from the code of this module that
correspond to export * declarations that occur within the module.
An
ImportEntry Record
is a
Record
that digests information about a single declarative import. Each
ImportEntry Record
has the fields defined in
Table 42
Table 42:
ImportEntry Record
Fields
Field Name
Value Type
Meaning
[[ModuleRequest]]
String
String value of the
ModuleSpecifier
of the
ImportDeclaration
[[ImportName]]
String
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value
"*"
indicates that the import request is for the target module's namespace object.
[[LocalName]]
String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 43
gives examples of ImportEntry records fields used to represent the syntactic import forms:
Table 43 (Informative): Import Forms Mappings to ImportEntry Records
Import Statement Form
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
import v from "mod";
"mod"
"default"
"v"
import * as ns from "mod";
"mod"
"*"
"ns"
import {x} from "mod";
"mod"
"x"
"x"
import {x as v} from "mod";
"mod"
"x"
"v"
import "mod";
An
ImportEntry Record
is not created.
An
ExportEntry Record
is a
Record
that digests information about a single declarative export. Each
ExportEntry Record
has the fields defined in
Table 44
Table 44:
ExportEntry Record
Fields
Field Name
Value Type
Meaning
[[ExportName]]
String | null
The name used to export this binding by this module.
[[ModuleRequest]]
String | null
The String value of the
ModuleSpecifier
of the
ExportDeclaration
null
if the
ExportDeclaration
does not have a
ModuleSpecifier
[[ImportName]]
String | null
The name under which the desired binding is exported by the module identified by [[ModuleRequest]].
null
if the
ExportDeclaration
does not have a
ModuleSpecifier
"*"
indicates that the export request is for all exported bindings.
[[LocalName]]
String | null
The name that is used to locally access the exported value from within the importing module.
null
if the exported value is not locally accessible from within the module.
Note 2
Table 45
gives examples of the ExportEntry record fields used to represent the syntactic export forms:
Table 45 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f(){}
"default"
null
null
"f"
export default function(){}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
The following definitions specify the required concrete methods and other
abstract operations
for Source Text Module Records
15.2.1.17.1
ParseModule (
sourceText
realm
hostDefined
The abstract operation ParseModule with arguments
sourceText
realm
, and
hostDefined
creates a
Source Text Module Record
based upon the result of parsing
sourceText
as a
Module
. ParseModule performs the following steps:
Assert
sourceText
is an ECMAScript source text (see clause
10
).
Parse
sourceText
using
Module
as the
goal symbol
and analyse the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let
body
be the resulting parse tree. Otherwise, let
body
be a
List
of one or more
SyntaxError
or
ReferenceError
objects representing the parsing errors and/or early errors. Parsing and
early error
detection may be interweaved in an implementation-dependent manner. If more than one parsing error or
early error
is present, the number and ordering of error objects in the list is implementation-dependent, but at least one must be present.
If
body
is a
List
of errors, return
body
Let
requestedModules
be the ModuleRequests of
body
Let
importEntries
be ImportEntries of
body
Let
importedBoundNames
be
ImportedLocalNames
importEntries
).
Let
indirectExportEntries
be a new empty
List
Let
localExportEntries
be a new empty
List
Let
starExportEntries
be a new empty
List
Let
exportEntries
be ExportEntries of
body
For each
ExportEntry Record
ee
in
exportEntries
, do
If
ee
.[[ModuleRequest]] is
null
, then
If
ee
.[[LocalName]] is not an element of
importedBoundNames
, then
Append
ee
to
localExportEntries
Else,
Let
ie
be the element of
importEntries
whose [[LocalName]] is the same as
ee
.[[LocalName]].
If
ie
.[[ImportName]] is
"*"
, then
Assert
: This is a re-export of an imported module namespace object.
Append
ee
to
localExportEntries
Else this is a re-export of a single name,
Append the
ExportEntry Record
{ [[ModuleRequest]]:
ie
.[[ModuleRequest]], [[ImportName]]:
ie
.[[ImportName]], [[LocalName]]:
null
, [[ExportName]]:
ee
.[[ExportName]] } to
indirectExportEntries
Else if
ee
.[[ImportName]] is
"*"
, then
Append
ee
to
starExportEntries
Else,
Append
ee
to
indirectExportEntries
Return
Source Text Module Record
{ [[Realm]]:
realm
, [[Environment]]:
undefined
, [[Namespace]]:
undefined
, [[Status]]:
"uninstantiated"
, [[EvaluationError]]:
undefined
, [[HostDefined]]:
hostDefined
, [[ECMAScriptCode]]:
body
, [[RequestedModules]]:
requestedModules
, [[ImportEntries]]:
importEntries
, [[LocalExportEntries]]:
localExportEntries
, [[IndirectExportEntries]]:
indirectExportEntries
, [[StarExportEntries]]:
starExportEntries
, [[DFSIndex]]:
undefined
, [[DFSAncestorIndex]]:
undefined
}.
Note
An implementation may parse module source text and
analyse it for Early Error conditions prior to the evaluation of
ParseModule for that module source text. However, the reporting of any
errors must be deferred until the point where this specification
actually performs ParseModule upon that source text.
15.2.1.17.2
GetExportedNames (
exportStarSet
) Concrete Method
The GetExportedNames concrete method of a
Source Text Module Record
implements the corresponding
Module Record
abstract method.
It performs the following steps:
Let
module
be this
Source Text Module Record
If
exportStarSet
contains
module
, then
Assert
: We've reached the starting point of an
import *
circularity.
Return a new empty
List
Append
module
to
exportStarSet
Let
exportedNames
be a new empty
List
For each
ExportEntry Record
in
module
.[[LocalExportEntries]], do
Assert
module
provides the direct binding for this export.
Append
.[[ExportName]] to
exportedNames
For each
ExportEntry Record
in
module
.[[IndirectExportEntries]], do
Assert
module
imports a specific binding for this export.
Append
.[[ExportName]] to
exportedNames
For each
ExportEntry Record
in
module
.[[StarExportEntries]], do
Let
requestedModule
be ?
HostResolveImportedModule
module
.[[ModuleRequest]]).
Let
starNames
be ?
requestedModule
.GetExportedNames(
exportStarSet
).
For each element
of
starNames
, do
If
SameValue
"default"
) is
false
, then
If
is not an element of
exportedNames
, then
Append
to
exportedNames
Return
exportedNames
Note
GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.
15.2.1.17.3
ResolveExport (
exportName
resolveSet
) Concrete Method
The ResolveExport concrete method of a
Source Text Module Record
implements the corresponding
Module Record
abstract method.
ResolveExport attempts to resolve an imported binding to
the actual defining module and local binding name. The defining module
may be the module represented by the
Module Record
this method was invoked on or some other module that is imported by that module. The parameter
resolveSet
is used to detect unresolved circular import/export paths. If a pair consisting of specific
Module Record
and
exportName
is reached that is already in
resolveSet
, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of
module
and
exportName
is added to
resolveSet
If a defining module is found, a
ResolvedBinding Record
{ [[Module]], [[BindingName]] } is returned. This record identifies the
resolved binding of the originally requested export. If no definition
was found or the request is found to be circular,
null
is returned. If the request is found to be ambiguous, the string
"ambiguous"
is returned.
This abstract method performs the following steps:
Let
module
be this
Source Text Module Record
For each
Record
{ [[Module]], [[ExportName]] }
in
resolveSet
, do
If
module
and
.[[Module]] are the same
Module Record
and
SameValue
exportName
.[[ExportName]]) is
true
, then
Assert
: This is a circular import request.
Return
null
Append the
Record
{ [[Module]]:
module
, [[ExportName]]:
exportName
} to
resolveSet
For each
ExportEntry Record
in
module
.[[LocalExportEntries]], do
If
SameValue
exportName
.[[ExportName]]) is
true
, then
Assert
module
provides the direct binding for this export.
Return
ResolvedBinding Record
{ [[Module]]:
module
, [[BindingName]]:
.[[LocalName]] }.
For each
ExportEntry Record
in
module
.[[IndirectExportEntries]], do
If
SameValue
exportName
.[[ExportName]]) is
true
, then
Assert
module
imports a specific binding for this export.
Let
importedModule
be ?
HostResolveImportedModule
module
.[[ModuleRequest]]).
Return
importedModule
.ResolveExport(
.[[ImportName]],
resolveSet
).
If
SameValue
exportName
"default"
) is
true
, then
Assert
: A
default
export was not explicitly defined by this module.
Return
null
NOTE: A
default
export cannot be provided by an
export *
Let
starResolution
be
null
For each
ExportEntry Record
in
module
.[[StarExportEntries]], do
Let
importedModule
be ?
HostResolveImportedModule
module
.[[ModuleRequest]]).
Let
resolution
be ?
importedModule
.ResolveExport(
exportName
resolveSet
).
If
resolution
is
"ambiguous"
, return
"ambiguous"
If
resolution
is not
null
, then
Assert
resolution
is a
ResolvedBinding Record
If
starResolution
is
null
, set
starResolution
to
resolution
Else,
Assert
: There is more than one
import that includes the requested name.
If
resolution
.[[Module]] and
starResolution
.[[Module]] are not the same
Module Record
or
SameValue
resolution
.[[BindingName]],
starResolution
.[[BindingName]]) is
false
, return
"ambiguous"
Return
starResolution
15.2.1.17.4
InitializeEnvironment ( ) Concrete Method
The InitializeEnvironment concrete method of a
Source Text Module Record
implements the corresponding
Cyclic Module Record
abstract method.
This abstract method performs the following steps:
Let
module
be this
Source Text Module Record
For each
ExportEntry Record
in
module
.[[IndirectExportEntries]], do
Let
resolution
be ?
module
.ResolveExport(
.[[ExportName]], « »).
If
resolution
is
null
or
"ambiguous"
, throw a
SyntaxError
exception.
Assert
resolution
is a
ResolvedBinding Record
Assert
: All named exports from
module
are resolvable.
Let
realm
be
module
.[[Realm]].
Assert
realm
is not
undefined
Let
env
be
NewModuleEnvironment
realm
.[[GlobalEnv]]).
Set
module
.[[Environment]] to
env
Let
envRec
be
env
's
EnvironmentRecord
For each
ImportEntry Record
in
in
module
.[[ImportEntries]], do
Let
importedModule
be !
HostResolveImportedModule
module
in
.[[ModuleRequest]]).
NOTE: The above call cannot fail because imported module requests are a subset of
module
.[[RequestedModules]], and these have been resolved earlier in this algorithm.
If
in
.[[ImportName]] is
"*"
, then
Let
namespace
be ?
GetModuleNamespace
importedModule
).
Perform !
envRec
.CreateImmutableBinding(
in
.[[LocalName]],
true
).
Call
envRec
.InitializeBinding(
in
.[[LocalName]],
namespace
).
Else,
Let
resolution
be ?
importedModule
.ResolveExport(
in
.[[ImportName]], « »).
If
resolution
is
null
or
"ambiguous"
, throw a
SyntaxError
exception.
Call
envRec
.CreateImportBinding(
in
.[[LocalName]],
resolution
.[[Module]],
resolution
.[[BindingName]]).
Let
code
be
module
.[[ECMAScriptCode]].
Let
varDeclarations
be the VarScopedDeclarations of
code
Let
declaredVarNames
be a new empty
List
For each element
in
varDeclarations
, do
For each element
dn
of the BoundNames of
, do
If
dn
is not an element of
declaredVarNames
, then
Perform !
envRec
.CreateMutableBinding(
dn
false
).
Call
envRec
.InitializeBinding(
dn
undefined
).
Append
dn
to
declaredVarNames
Let
lexDeclarations
be the LexicallyScopedDeclarations of
code
For each element
in
lexDeclarations
, do
For each element
dn
of the BoundNames of
, do
If IsConstantDeclaration of
is
true
, then
Perform !
envRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform !
envRec
.CreateMutableBinding(
dn
false
).
If
is a
FunctionDeclaration
, a
GeneratorDeclaration
, an
AsyncFunctionDeclaration
, or an
AsyncGeneratorDeclaration
, then
Let
fo
be the result of performing InstantiateFunctionObject for
with argument
env
Call
envRec
.InitializeBinding(
dn
fo
).
Return
NormalCompletion
empty
).
15.2.1.17.5
ExecuteModule ( ) Concrete Method
The ExecuteModule concrete method of a
Source Text Module Record
implements the corresponding
Cyclic Module Record
abstract method.
This abstract method performs the following steps:
Let
module
be this
Source Text Module Record
Let
moduleCxt
be a new ECMAScript code
execution context
Set the Function of
moduleCxt
to
null
Assert
module
.[[Realm]] is not
undefined
Set the
Realm
of
moduleCxt
to
module
.[[Realm]].
Set the ScriptOrModule of
moduleCxt
to
module
Assert
module
has been linked and declarations in its
module environment
have been instantiated.
Set the VariableEnvironment of
moduleCxt
to
module
.[[Environment]].
Set the LexicalEnvironment of
moduleCxt
to
module
.[[Environment]].
Suspend
the currently
running execution context
Push
moduleCxt
on to the
execution context stack
moduleCxt
is now the
running execution context
Let
result
be the result of evaluating
module
.[[ECMAScriptCode]].
Suspend
moduleCxt
and remove it from the
execution context stack
Resume the context that is now on the top of the
execution context stack
as the
running execution context
Return
Completion
result
).
15.2.1.18
Runtime Semantics: HostResolveImportedModule (
referencingModule
specifier
HostResolveImportedModule is an implementation-defined abstract operation that provides the concrete
Module Record
subclass instance that corresponds to the
ModuleSpecifier
String,
specifier
, occurring within the context of the module represented by the
Module Record
referencingModule
The implementation of HostResolveImportedModule must conform to the following requirements:
The normal return value must be an instance of a concrete subclass of
Module Record
If a
Module Record
corresponding to the pair
referencingModule
specifier
does not exist or cannot be created, an exception must be thrown.
This operation must be idempotent if it completes normally. Each time it is called with a specific
referencingModule
specifier
pair as arguments it must return the same
Module Record
instance.
Multiple different
referencingModule
specifier
pairs may map to the same
Module Record
instance. The actual mapping semantic is implementation-defined but typically a normalization process is applied to
specifier
as part of the mapping process. A typical normalization process would
include actions such as alphabetic case folding and expansion of
relative and abbreviated path specifiers.
15.2.1.19
Runtime Semantics: GetModuleNamespace (
module
The GetModuleNamespace abstract operation retrieves the Module Namespace
Exotic object
representing
module
's exports, lazily creating it the first time it was requested, and storing it in
module
.[[Namespace]] for future retrieval.
This abstract operation performs the following steps:
Assert
module
is an instance of a concrete subclass of
Module Record
Assert
module
.[[Status]] is not
"uninstantiated"
Let
namespace
be
module
.[[Namespace]].
If
namespace
is
undefined
, then
Let
exportedNames
be ?
module
.GetExportedNames(« »).
Let
unambiguousNames
be a new empty
List
For each
name
that is an element of
exportedNames
, do
Let
resolution
be ?
module
.ResolveExport(
name
, « »).
If
resolution
is a
ResolvedBinding Record
, append
name
to
unambiguousNames
Set
namespace
to
ModuleNamespaceCreate
module
unambiguousNames
).
Return
namespace
Note
The only way GetModuleNamespace can throw is via one of the triggered
HostResolveImportedModule
calls. Unresolvable names are simply excluded from the namespace at
this point. They will lead to a real instantiation error later unless
they are all ambiguous star exports that are not explicitly requested
anywhere.
15.2.1.20
Runtime Semantics: TopLevelModuleEvaluationJob (
sourceText
hostDefined
A TopLevelModuleEvaluationJob with parameters
sourceText
and
hostDefined
is a job that parses, validates, and evaluates
sourceText
as a
Module
Assert
sourceText
is an ECMAScript source text (see clause
10
).
Let
realm
be
the current Realm Record
Let
be
ParseModule
sourceText
realm
hostDefined
).
If
is a
List
of errors, then
Perform
HostReportErrors
).
Return
NormalCompletion
undefined
).
Perform ?
.Instantiate().
Assert
: All dependencies of
have been transitively resolved and
is ready for evaluation.
Return ?
.Evaluate().
Note
An implementation may parse a
sourceText
as a
Module
, analyse it for Early Error conditions, and instantiate it prior to the execution of the TopLevelModuleEvaluationJob for that
sourceText
. An implementation may also resolve, pre-parse and pre-analyse, and pre-instantiate module dependencies of
sourceText
However, the reporting of any errors detected by these actions must be
deferred until the TopLevelModuleEvaluationJob is actually executed.
15.2.1.21
Runtime Semantics: Evaluation
Module
[empty]
Return
NormalCompletion
undefined
).
ModuleBody
ModuleItemList
Let
result
be the result of evaluating
ModuleItemList
If
result
.[[Type]] is
normal
and
result
.[[Value]] is
empty
, then
Return
NormalCompletion
undefined
).
Return
Completion
result
).
ModuleItemList
ModuleItemList
ModuleItem
Let
sl
be the result of evaluating
ModuleItemList
ReturnIfAbrupt
sl
).
Let
be the result of evaluating
ModuleItem
Return
Completion
UpdateEmpty
sl
)).
Note
The value of a
ModuleItemList
is the value of the last value-producing item in the
ModuleItemList
ModuleItem
ImportDeclaration
Return
NormalCompletion
empty
).
15.2.2
Imports
Syntax
ImportDeclaration
import
ImportClause
FromClause
import
ModuleSpecifier
ImportClause
ImportedDefaultBinding
NameSpaceImport
NamedImports
ImportedDefaultBinding
NameSpaceImport
ImportedDefaultBinding
NamedImports
ImportedDefaultBinding
ImportedBinding
NameSpaceImport
as
ImportedBinding
NamedImports
ImportsList
ImportsList
FromClause
from
ModuleSpecifier
ImportsList
ImportSpecifier
ImportsList
ImportSpecifier
ImportSpecifier
ImportedBinding
IdentifierName
as
ImportedBinding
ModuleSpecifier
StringLiteral
ImportedBinding
BindingIdentifier
[~Yield, ~Await]
15.2.2.1
Static Semantics: Early Errors
ModuleItem
ImportDeclaration
It is a Syntax Error if the BoundNames of
ImportDeclaration
contains any duplicate entries.
15.2.2.2
Static Semantics: BoundNames
ImportDeclaration
import
ImportClause
FromClause
Return the BoundNames of
ImportClause
ImportDeclaration
import
ModuleSpecifier
Return a new empty
List
ImportClause
ImportedDefaultBinding
NameSpaceImport
Let
names
be the BoundNames of
ImportedDefaultBinding
Append to
names
the elements of the BoundNames of
NameSpaceImport
Return
names
ImportClause
ImportedDefaultBinding
NamedImports
Let
names
be the BoundNames of
ImportedDefaultBinding
Append to
names
the elements of the BoundNames of
NamedImports
Return
names
NamedImports
Return a new empty
List
ImportsList
ImportsList
ImportSpecifier
Let
names
be the BoundNames of
ImportsList
Append to
names
the elements of the BoundNames of
ImportSpecifier
Return
names
ImportSpecifier
IdentifierName
as
ImportedBinding
Return the BoundNames of
ImportedBinding
15.2.2.3
Static Semantics: ImportEntries
ImportDeclaration
import
ImportClause
FromClause
Let
module
be the sole element of ModuleRequests of
FromClause
Return ImportEntriesForModule of
ImportClause
with argument
module
ImportDeclaration
import
ModuleSpecifier
Return a new empty
List
15.2.2.4
Static Semantics: ImportEntriesForModule
With parameter
module
ImportClause
ImportedDefaultBinding
NameSpaceImport
Let
entries
be ImportEntriesForModule of
ImportedDefaultBinding
with argument
module
Append to
entries
the elements of the ImportEntriesForModule of
NameSpaceImport
with argument
module
Return
entries
ImportClause
ImportedDefaultBinding
NamedImports
Let
entries
be ImportEntriesForModule of
ImportedDefaultBinding
with argument
module
Append to
entries
the elements of the ImportEntriesForModule of
NamedImports
with argument
module
Return
entries
ImportedDefaultBinding
ImportedBinding
Let
localName
be the sole element of BoundNames of
ImportedBinding
Let
defaultEntry
be the
ImportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
"default"
, [[LocalName]]:
localName
}.
Return a new
List
containing
defaultEntry
NameSpaceImport
as
ImportedBinding
Let
localName
be the StringValue of
ImportedBinding
Let
entry
be the
ImportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
"*"
, [[LocalName]]:
localName
}.
Return a new
List
containing
entry
NamedImports
Return a new empty
List
ImportsList
ImportsList
ImportSpecifier
Let
specs
be the ImportEntriesForModule of
ImportsList
with argument
module
Append to
specs
the elements of the ImportEntriesForModule of
ImportSpecifier
with argument
module
Return
specs
ImportSpecifier
ImportedBinding
Let
localName
be the sole element of BoundNames of
ImportedBinding
Let
entry
be the
ImportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
localName
, [[LocalName]]:
localName
}.
Return a new
List
containing
entry
ImportSpecifier
IdentifierName
as
ImportedBinding
Let
importName
be the StringValue of
IdentifierName
Let
localName
be the StringValue of
ImportedBinding
Let
entry
be the
ImportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
importName
, [[LocalName]]:
localName
}.
Return a new
List
containing
entry
15.2.2.5
Static Semantics: ModuleRequests
ImportDeclaration
import
ImportClause
FromClause
Return ModuleRequests of
FromClause
ModuleSpecifier
StringLiteral
Return a
List
containing the StringValue of
StringLiteral
15.2.3
Exports
Syntax
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
export
VariableStatement
[~Yield, ~Await]
export
Declaration
[~Yield, ~Await]
export
default
HoistableDeclaration
[~Yield, ~Await, +Default]
export
default
ClassDeclaration
[~Yield, ~Await, +Default]
export
default
[lookahead ∉ {
function
async
[no
LineTerminator
here]
function
class
}]
AssignmentExpression
[+In, ~Yield, ~Await]
ExportClause
ExportsList
ExportsList
ExportsList
ExportSpecifier
ExportsList
ExportSpecifier
ExportSpecifier
IdentifierName
IdentifierName
as
IdentifierName
15.2.3.1
Static Semantics: Early Errors
ExportDeclaration
export
ExportClause
For each
IdentifierName
in ReferencedBindings of
ExportClause
: It is a Syntax Error if StringValue of
is a
ReservedWord
or if the StringValue of
is one of:
"implements"
"interface"
"let"
"package"
"private"
"protected"
"public"
, or
"static"
Note
The above rule means that each ReferencedBindings of
ExportClause
is treated as an
IdentifierReference
15.2.3.2
Static Semantics: BoundNames
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
Return a new empty
List
ExportDeclaration
export
VariableStatement
Return the BoundNames of
VariableStatement
ExportDeclaration
export
Declaration
Return the BoundNames of
Declaration
ExportDeclaration
export
default
HoistableDeclaration
Let
declarationNames
be the BoundNames of
HoistableDeclaration
If
declarationNames
does not include the element
"*default*"
, append
"*default*"
to
declarationNames
Return
declarationNames
ExportDeclaration
export
default
ClassDeclaration
Let
declarationNames
be the BoundNames of
ClassDeclaration
If
declarationNames
does not include the element
"*default*"
, append
"*default*"
to
declarationNames
Return
declarationNames
ExportDeclaration
export
default
AssignmentExpression
Return «
"*default*"
».
15.2.3.3
Static Semantics: ExportedBindings
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
Return a new empty
List
ExportDeclaration
export
ExportClause
Return the ExportedBindings of
ExportClause
ExportDeclaration
export
VariableStatement
Return the BoundNames of
VariableStatement
ExportDeclaration
export
Declaration
Return the BoundNames of
Declaration
ExportDeclaration
export
default
HoistableDeclaration
ExportDeclaration
export
default
ClassDeclaration
ExportDeclaration
export
default
AssignmentExpression
Return the BoundNames of this
ExportDeclaration
ExportClause
Return a new empty
List
ExportsList
ExportsList
ExportSpecifier
Let
names
be the ExportedBindings of
ExportsList
Append to
names
the elements of the ExportedBindings of
ExportSpecifier
Return
names
ExportSpecifier
IdentifierName
Return a
List
containing the StringValue of
IdentifierName
ExportSpecifier
IdentifierName
as
IdentifierName
Return a
List
containing the StringValue of the first
IdentifierName
15.2.3.4
Static Semantics: ExportedNames
ExportDeclaration
export
FromClause
Return a new empty
List
ExportDeclaration
export
ExportClause
FromClause
ExportDeclaration
export
ExportClause
Return the ExportedNames of
ExportClause
ExportDeclaration
export
VariableStatement
Return the BoundNames of
VariableStatement
ExportDeclaration
export
Declaration
Return the BoundNames of
Declaration
ExportDeclaration
export
default
HoistableDeclaration
ExportDeclaration
export
default
ClassDeclaration
ExportDeclaration
export
default
AssignmentExpression
Return «
"default"
».
ExportClause
Return a new empty
List
ExportsList
ExportsList
ExportSpecifier
Let
names
be the ExportedNames of
ExportsList
Append to
names
the elements of the ExportedNames of
ExportSpecifier
Return
names
ExportSpecifier
IdentifierName
Return a
List
containing the StringValue of
IdentifierName
ExportSpecifier
IdentifierName
as
IdentifierName
Return a
List
containing the StringValue of the second
IdentifierName
15.2.3.5
Static Semantics: ExportEntries
ExportDeclaration
export
FromClause
Let
module
be the sole element of ModuleRequests of
FromClause
Let
entry
be the
ExportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
"*"
, [[LocalName]]:
null
, [[ExportName]]:
null
}.
Return a new
List
containing
entry
ExportDeclaration
export
ExportClause
FromClause
Let
module
be the sole element of ModuleRequests of
FromClause
Return ExportEntriesForModule of
ExportClause
with argument
module
ExportDeclaration
export
ExportClause
Return ExportEntriesForModule of
ExportClause
with argument
null
ExportDeclaration
export
VariableStatement
Let
entries
be a new empty
List
Let
names
be the BoundNames of
VariableStatement
For each
name
in
names
, do
Append the
ExportEntry Record
{ [[ModuleRequest]]:
null
, [[ImportName]]:
null
, [[LocalName]]:
name
, [[ExportName]]:
name
} to
entries
Return
entries
ExportDeclaration
export
Declaration
Let
entries
be a new empty
List
Let
names
be the BoundNames of
Declaration
For each
name
in
names
, do
Append the
ExportEntry Record
{ [[ModuleRequest]]:
null
, [[ImportName]]:
null
, [[LocalName]]:
name
, [[ExportName]]:
name
} to
entries
Return
entries
ExportDeclaration
export
default
HoistableDeclaration
Let
names
be BoundNames of
HoistableDeclaration
Let
localName
be the sole element of
names
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]:
null
, [[ImportName]]:
null
, [[LocalName]]:
localName
, [[ExportName]]:
"default"
}.
ExportDeclaration
export
default
ClassDeclaration
Let
names
be BoundNames of
ClassDeclaration
Let
localName
be the sole element of
names
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]:
null
, [[ImportName]]:
null
, [[LocalName]]:
localName
, [[ExportName]]:
"default"
}.
ExportDeclaration
export
default
AssignmentExpression
Let
entry
be the
ExportEntry Record
{ [[ModuleRequest]]:
null
, [[ImportName]]:
null
, [[LocalName]]:
"*default*"
, [[ExportName]]:
"default"
}.
Return a new
List
containing
entry
Note
"*default*"
is used within this specification as a synthetic name for anonymous default export values.
15.2.3.6
Static Semantics: ExportEntriesForModule
With parameter
module
ExportClause
Return a new empty
List
ExportsList
ExportsList
ExportSpecifier
Let
specs
be the ExportEntriesForModule of
ExportsList
with argument
module
Append to
specs
the elements of the ExportEntriesForModule of
ExportSpecifier
with argument
module
Return
specs
ExportSpecifier
IdentifierName
Let
sourceName
be the StringValue of
IdentifierName
If
module
is
null
, then
Let
localName
be
sourceName
Let
importName
be
null
Else,
Let
localName
be
null
Let
importName
be
sourceName
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
importName
, [[LocalName]]:
localName
, [[ExportName]]:
sourceName
}.
ExportSpecifier
IdentifierName
as
IdentifierName
Let
sourceName
be the StringValue of the first
IdentifierName
Let
exportName
be the StringValue of the second
IdentifierName
If
module
is
null
, then
Let
localName
be
sourceName
Let
importName
be
null
Else,
Let
localName
be
null
Let
importName
be
sourceName
Return a new
List
containing the
ExportEntry Record
{ [[ModuleRequest]]:
module
, [[ImportName]]:
importName
, [[LocalName]]:
localName
, [[ExportName]]:
exportName
}.
15.2.3.7
Static Semantics: IsConstantDeclaration
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
export
default
AssignmentExpression
Return
false
Note
It is not necessary to treat
export default
AssignmentExpression
as a constant declaration because there is no syntax that permits
assignment to the internal bound name used to reference a module's
default object.
15.2.3.8
Static Semantics: LexicallyScopedDeclarations
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
export
VariableStatement
Return a new empty
List
ExportDeclaration
export
Declaration
Return a new
List
containing DeclarationPart of
Declaration
ExportDeclaration
export
default
HoistableDeclaration
Return a new
List
containing DeclarationPart of
HoistableDeclaration
ExportDeclaration
export
default
ClassDeclaration
Return a new
List
containing
ClassDeclaration
ExportDeclaration
export
default
AssignmentExpression
Return a new
List
containing this
ExportDeclaration
15.2.3.9
Static Semantics: ModuleRequests
ExportDeclaration
export
FromClause
ExportDeclaration
export
ExportClause
FromClause
Return the ModuleRequests of
FromClause
ExportDeclaration
export
ExportClause
export
VariableStatement
export
Declaration
export
default
HoistableDeclaration
export
default
ClassDeclaration
export
default
AssignmentExpression
Return a new empty
List
15.2.3.10
Static Semantics: ReferencedBindings
ExportClause
Return a new empty
List
ExportsList
ExportsList
ExportSpecifier
Let
names
be the ReferencedBindings of
ExportsList
Append to
names
the elements of the ReferencedBindings of
ExportSpecifier
Return
names
ExportSpecifier
IdentifierName
Return a
List
containing the
IdentifierName
ExportSpecifier
IdentifierName
as
IdentifierName
Return a
List
containing the first
IdentifierName
15.2.3.11
Runtime Semantics: Evaluation
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
Return
NormalCompletion
empty
).
ExportDeclaration
export
VariableStatement
Return the result of evaluating
VariableStatement
ExportDeclaration
export
Declaration
Return the result of evaluating
Declaration
ExportDeclaration
export
default
HoistableDeclaration
Return the result of evaluating
HoistableDeclaration
ExportDeclaration
export
default
ClassDeclaration
Let
value
be the result of BindingClassDeclarationEvaluation of
ClassDeclaration
ReturnIfAbrupt
value
).
Let
className
be the sole element of BoundNames of
ClassDeclaration
If
className
is
"*default*"
, then
Let
env
be the
running execution context
's LexicalEnvironment.
Perform ?
InitializeBoundName
"*default*"
value
env
).
Return
NormalCompletion
empty
).
ExportDeclaration
export
default
AssignmentExpression
If
IsAnonymousFunctionDefinition
AssignmentExpression
) is
true
, then
Let
value
be the result of performing NamedEvaluation for
AssignmentExpression
with argument
"default"
Else,
Let
rhs
be the result of evaluating
AssignmentExpression
Let
value
be ?
GetValue
rhs
).
Let
env
be the
running execution context
's LexicalEnvironment.
Perform ?
InitializeBoundName
"*default*"
value
env
).
Return
NormalCompletion
empty
).
16
Error Handling and Language Extensions
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An
early error
is an error that can be detected and reported prior to the evaluation of any construct in the
Script
containing the error. The presence of an
early error
prevents the evaluation of the construct. An implementation must report early errors in a
Script
as part of parsing that
Script
in
ParseScript
. Early errors in a
Module
are reported at the point when the
Module
would be evaluated and the
Module
is never initialized. Early errors in
eval
code are reported at the time
eval
is called and prevent evaluation of the
eval
code. All errors that are not early errors are runtime errors.
An implementation must report as an
early error
any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early
errors even if the compiler can prove that a construct cannot execute
without error under any circumstances. An implementation may issue an
early warning in such a case, but it should not report the error until
the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in
16.2
, an implementation may extend
Script
syntax,
Module
syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling
eval
, using a regular expression literal, or using the
Function
or
RegExp
constructor
) that are allowed to throw
SyntaxError
are permitted to exhibit implementation-defined behaviour instead of throwing
SyntaxError
when they encounter an implementation-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in
16.2
an implementation may provide additional types, values, objects,
properties, and functions beyond those described in this specification.
This may cause constructs (such as looking up a variable in the global
scope) to have implementation-defined behaviour instead of throwing an
error (such as
ReferenceError
).
16.1
HostReportErrors (
errorList
HostReportErrors is an implementation-defined abstract operation
that allows host environments to report parsing errors, early errors,
and runtime errors.
An implementation of HostReportErrors must complete normally in
all cases. The default implementation of HostReportErrors is to
unconditionally return an empty normal completion.
Note
errorList
will be a
List
of ECMAScript language values. If the errors are parsing errors or early errors, these will always be
SyntaxError
or
ReferenceError
objects. Runtime errors, however, can be any ECMAScript value.
16.2
Forbidden Extensions
An implementation must not extend this specification in the following ways:
ECMAScript function objects defined using syntactic constructors in
strict mode code
must not be created with own properties named
"caller"
or
"arguments"
. Such own properties also must not be created for function objects defined using an
ArrowFunction
MethodDefinition
GeneratorDeclaration
GeneratorExpression
AsyncGeneratorDeclaration
AsyncGeneratorExpression
ClassDeclaration
ClassExpression
AsyncFunctionDeclaration
AsyncFunctionExpression
, or
AsyncArrowFunction
regardless of whether the definition is contained in
strict mode code
. Built-in functions, strict functions created using the
Function
constructor
, generator functions created using the
Generator
constructor
, async functions created using the
AsyncFunction
constructor
, and functions created using the
bind
method also must not be created with such own properties.
If an implementation extends any
function object
with an own property named
"caller"
the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a
strict function
object. If it is an
accessor property
, the function that is the value of the property's [[Get]] attribute must never return a
strict function
when called.
Neither mapped nor unmapped arguments objects may be created with an own property named
"caller"
The behaviour of the following methods must not be extended except as specified in ECMA-402:
Object.prototype.toLocaleString
Array.prototype.toLocaleString
Number.prototype.toLocaleString
Date.prototype.toLocaleDateString
Date.prototype.toLocaleString
Date.prototype.toLocaleTimeString
String.prototype.localeCompare
%TypedArray%
.prototype.toLocaleString
The RegExp pattern grammars in
21.2.1
and
B.1.4
must not be extended to recognize any of the source characters A-Z or a-z as
IdentityEscape
[+U]
when the
[U]
grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token
to immediately follow source text that matches the
BindingIdentifier
nonterminal symbol.
When processing
strict mode code
, the syntax of
NumericLiteral
must not be extended to include
LegacyOctalIntegerLiteral
and the syntax of
DecimalIntegerLiteral
must not be extended to include
NonOctalDecimalIntegerLiteral
as described in
B.1.1
TemplateCharacter
must not be extended to include
LegacyOctalEscapeSequence
as defined in
B.1.2
When processing
strict mode code
, the extensions defined in
B.3.2
B.3.3
B.3.4
, and
B.3.6
must not be supported.
When parsing for the
Module
goal symbol
, the lexical grammar extensions defined in
B.1.3
must not be supported.
17
ECMAScript Standard Built-in Objects
There are certain built-in objects available whenever an ECMAScript
Script
or
Module
begins execution. One, the
global object
, is part of the lexical environment of the executing program. Others are accessible as initial properties of the
global object
or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in
function object
with the characteristics described in
9.3
. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value
true
. Every built-in
function object
has a [[Realm]] internal slot whose value is the
Realm Record
of the
realm
for which the object was initially created.
Many built-in objects are functions: they can be invoked with
arguments. Some of them furthermore are constructors: they are functions
intended for use with the
new
operator. For each built-in
function, this specification describes the arguments required by that
function and the properties of that
function object
. For each built-in
constructor
, this specification furthermore describes properties of the prototype object of that
constructor
and properties of specific object instances returned by a
new
expression that invokes that
constructor
Unless otherwise specified in the description of a particular function, if a built-in function or
constructor
is given fewer arguments than the function is specified to require, the function or
constructor
shall behave exactly as if it had been given sufficient additional arguments, each such argument being the
undefined
value. Such missing arguments are considered to be “not present” and
may be identified in that manner by specification algorithms. In the
description of a particular function, the terms “
this
value” and “NewTarget” have the meanings given in
9.3
Unless otherwise specified in the description of a particular function, if a built-in function or
constructor
described is given more arguments than the function is specified to
allow, the extra arguments are evaluated by the call and then ignored by
the function. However, an implementation may define implementation
specific behaviour relating to such arguments as long as the behaviour
is not the throwing of a
TypeError
exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of
built-in functions are encouraged to do so by adding new functions
rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in
constructor
has the Function prototype object, which is the initial value of the expression
Function.prototype
19.2.3
), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the
Object prototype object, which is the initial value of the expression
Object.prototype
19.1.3
), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
Built-in function objects that are not identified as constructors
do not implement the [[Construct]] internal method unless otherwise
specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the
CreateBuiltinFunction
abstract operation (
9.3.3
).
Every built-in
function object
, including constructors, has a
"length"
property whose value is an integer. Unless otherwise specified, this
value is equal to the largest number of named arguments shown in the
subclause headings for the function description. Optional parameters
(which are indicated with brackets:
) or rest parameters (which are shown using the form «...name») are not included in the default argument count.
Note 2
For example, the
function object
that is the initial value of the
map
property of the Array prototype object is described under the subclause
heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the
two named arguments callbackFn and thisArg, the latter being optional;
therefore the value of the
"length"
property of that
function object
is 1.
Unless otherwise specified, the
"length"
property of a built-in
function object
has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Every built-in
function object
, including constructors, that is not identified as an anonymous function has a
name
property whose value is a String. Unless otherwise specified, this
value is the name that is given to the function in this specification.
For functions that are specified as properties of objects, the name
value is the
property name
string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have
"get "
or
"set "
prepended to the
property name
string. The value of the
name
property is explicitly specified for each built-in functions whose property key is a Symbol value.
Unless otherwise specified, the
name
property of a built-in
function object
, if it exists, has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Every other
data property
described in clauses 18 through 26 and in Annex
B.2
has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
} unless otherwise specified.
Every
accessor property
described in clauses 18 through 26 and in Annex
B.2
has the attributes { [[Enumerable]]:
false
, [[Configurable]]:
true
} unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value,
undefined
. If only a set accessor is described the get accessor is the default value,
undefined
18
The Global Object
The
global object
is created before control enters any
execution context
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is implementation-dependent.
may have host defined properties in addition to the properties
defined in this specification. This may include a property whose value
is the global object itself.
18.1
Value Properties of the Global Object
18.1.1
Infinity
The value of
Infinity
is
+∞
(see
6.1.6
). This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
18.1.2
NaN
The value of
NaN
is
NaN
(see
6.1.6
). This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
18.1.3
undefined
The value of
undefined
is
undefined
(see
6.1.1
). This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
18.2
Function Properties of the Global Object
18.2.1
eval (
The
eval
function is the
%eval%
intrinsic object. When the
eval
function is called with one argument
, the following steps are taken:
Assert
: The
execution context stack
has at least two elements.
Let
callerContext
be the second to top element of the
execution context stack
Let
callerRealm
be
callerContext
's
Realm
Let
calleeRealm
be
the current Realm Record
Perform ?
HostEnsureCanCompileStrings
callerRealm
calleeRealm
).
Return ?
PerformEval
calleeRealm
false
false
).
18.2.1.1
Runtime Semantics: PerformEval (
evalRealm
strictCaller
direct
The abstract operation PerformEval with arguments
evalRealm
strictCaller
, and
direct
performs the following steps:
Assert
: If
direct
is
false
, then
strictCaller
is also
false
If
Type
) is not String, return
Let
thisEnvRec
be !
GetThisEnvironment
().
If
thisEnvRec
is a
function Environment Record
, then
Let
be
thisEnvRec
.[[FunctionObject]].
Let
inFunction
be
true
Let
inMethod
be
thisEnvRec
.HasSuperBinding().
If
.[[ConstructorKind]] is
"derived"
, let
inDerivedConstructor
be
true
; otherwise, let
inDerivedConstructor
be
false
Else,
Let
inFunction
be
false
Let
inMethod
be
false
Let
inDerivedConstructor
be
false
Let
script
be the ECMAScript code that is the result of parsing
, interpreted as UTF-16 encoded Unicode text as described in
6.1.4
, for the
goal symbol
Script
. If
inFunction
is
false
, additional
early error
rules from
18.2.1.1.1
are applied. If
inMethod
is
false
, additional
early error
rules from
18.2.1.1.2
are applied. If
inDerivedConstructor
is
false
, additional
early error
rules from
18.2.1.1.3
are applied. If the parse fails, throw a
SyntaxError
exception. If any early errors are detected, throw a
SyntaxError
or a
ReferenceError
exception, depending on the type of the error (but see also clause
16
). Parsing and
early error
detection may be interweaved in an implementation-dependent manner.
If
script
Contains
ScriptBody
is
false
, return
undefined
Let
body
be the
ScriptBody
of
script
If
strictCaller
is
true
, let
strictEval
be
true
Else, let
strictEval
be IsStrict of
script
Let
ctx
be the
running execution context
NOTE: If
direct
is
true
ctx
will be the
execution context
that performed the
direct eval
. If
direct
is
false
ctx
will be the
execution context
for the invocation of the
eval
function.
If
direct
is
true
, then
Let
lexEnv
be
NewDeclarativeEnvironment
ctx
's LexicalEnvironment).
Let
varEnv
be
ctx
's VariableEnvironment.
Else,
Let
lexEnv
be
NewDeclarativeEnvironment
evalRealm
.[[GlobalEnv]]).
Let
varEnv
be
evalRealm
.[[GlobalEnv]].
If
strictEval
is
true
, set
varEnv
to
lexEnv
If
ctx
is not already suspended, suspend
ctx
Let
evalCxt
be a new ECMAScript code
execution context
Set the
evalCxt
's Function to
null
Set the
evalCxt
's
Realm
to
evalRealm
Set the
evalCxt
's ScriptOrModule to
ctx
's ScriptOrModule.
Set the
evalCxt
's VariableEnvironment to
varEnv
Set the
evalCxt
's LexicalEnvironment to
lexEnv
Push
evalCxt
on to the
execution context stack
evalCxt
is now the
running execution context
Let
result
be
EvalDeclarationInstantiation
body
varEnv
lexEnv
strictEval
).
If
result
.[[Type]] is
normal
, then
Set
result
to the result of evaluating
body
If
result
.[[Type]] is
normal
and
result
.[[Value]] is
empty
, then
Set
result
to
NormalCompletion
undefined
).
Suspend
evalCxt
and remove it from the
execution context stack
Resume the context that is now on the top of the
execution context stack
as the
running execution context
Return
Completion
result
).
Note
The eval code cannot instantiate variable or function
bindings in the variable environment of the calling context that invoked
the eval if the calling context is evaluating formal parameter
initializers or if either the code of the calling context or the eval
code is
strict mode code
Instead such bindings are instantiated in a new VariableEnvironment
that is only accessible to the eval code. Bindings introduced by
let
const
, or
class
declarations are always instantiated in a new LexicalEnvironment.
18.2.1.1.1
Additional Early Error Rules for Eval Outside Functions
These
static semantics
are applied by
PerformEval
when a
direct eval
call occurs outside of any function.
ScriptBody
StatementList
It is a Syntax Error if
StatementList
Contains
NewTarget
18.2.1.1.2
Additional Early Error Rules for Eval Outside Methods
These
static semantics
are applied by
PerformEval
when a
direct eval
call occurs outside of a
MethodDefinition
ScriptBody
StatementList
It is a Syntax Error if
StatementList
Contains
SuperProperty
18.2.1.1.3
Additional Early Error Rules for Eval Outside Constructor Methods
These
static semantics
are applied by
PerformEval
when a
direct eval
call occurs outside of the
constructor method
of a
ClassDeclaration
or
ClassExpression
ScriptBody
StatementList
It is a Syntax Error if
StatementList
Contains
SuperCall
18.2.1.2
HostEnsureCanCompileStrings (
callerRealm
calleeRealm
HostEnsureCanCompileStrings is an implementation-defined
abstract operation that allows host environments to block certain
ECMAScript functions which allow developers to compile strings into
ECMAScript code.
An implementation of HostEnsureCanCompileStrings may complete
normally or abruptly. Any abrupt completions will be propagated to its
callers. The default implementation of HostEnsureCanCompileStrings is to
unconditionally return an empty normal completion.
18.2.1.3
Runtime Semantics: EvalDeclarationInstantiation (
body
varEnv
lexEnv
strict
When the abstract operation EvalDeclarationInstantiation is called with arguments
body
varEnv
lexEnv
, and
strict
, the following steps are taken:
Let
varNames
be the VarDeclaredNames of
body
Let
varDeclarations
be the VarScopedDeclarations of
body
Let
lexEnvRec
be
lexEnv
's
EnvironmentRecord
Let
varEnvRec
be
varEnv
's
EnvironmentRecord
If
strict
is
false
, then
If
varEnvRec
is a global
Environment Record
, then
For each
name
in
varNames
, do
If
varEnvRec
.HasLexicalDeclaration(
name
) is
true
, throw a
SyntaxError
exception.
NOTE:
eval
will not create a global var declaration that would be shadowed by a global lexical declaration.
Let
thisLex
be
lexEnv
Assert
: The following loop will terminate.
Repeat, while
thisLex
is not the same as
varEnv
Let
thisEnvRec
be
thisLex
's
EnvironmentRecord
If
thisEnvRec
is not an object
Environment Record
, then
NOTE:
The environment of with statements cannot contain any lexical
declaration so it doesn't need to be checked for var/let hoisting
conflicts.
For each
name
in
varNames
, do
If
thisEnvRec
.HasBinding(
name
) is
true
, then
Throw a
SyntaxError
exception.
NOTE: Annex
B.3.5
defines alternate semantics for the above step.
NOTE: A
direct eval
will not hoist var declaration over a like-named lexical declaration.
Set
thisLex
to
thisLex
's outer environment reference.
Let
functionsToInitialize
be a new empty
List
Let
declaredFunctionNames
be a new empty
List
For each
in
varDeclarations
, in reverse list order, do
If
is neither a
VariableDeclaration
nor a
ForBinding
nor a
BindingIdentifier
, then
Assert
is either a
FunctionDeclaration
, a
GeneratorDeclaration
, an
AsyncFunctionDeclaration
, or an
AsyncGeneratorDeclaration
NOTE: If there are multiple function declarations for the same name, the last declaration is used.
Let
fn
be the sole element of the BoundNames of
If
fn
is not an element of
declaredFunctionNames
, then
If
varEnvRec
is a global
Environment Record
, then
Let
fnDefinable
be ?
varEnvRec
.CanDeclareGlobalFunction(
fn
).
If
fnDefinable
is
false
, throw a
TypeError
exception.
Append
fn
to
declaredFunctionNames
Insert
as the first element of
functionsToInitialize
NOTE: Annex
B.3.3.3
adds additional steps at this point.
Let
declaredVarNames
be a new empty
List
For each
in
varDeclarations
, do
If
is a
VariableDeclaration
, a
ForBinding
, or a
BindingIdentifier
, then
For each String
vn
in the BoundNames of
, do
If
vn
is not an element of
declaredFunctionNames
, then
If
varEnvRec
is a global
Environment Record
, then
Let
vnDefinable
be ?
varEnvRec
.CanDeclareGlobalVar(
vn
).
If
vnDefinable
is
false
, throw a
TypeError
exception.
If
vn
is not an element of
declaredVarNames
, then
Append
vn
to
declaredVarNames
NOTE: No abnormal terminations occur after this algorithm step unless
varEnvRec
is a global
Environment Record
and the
global object
is a Proxy
exotic object
Let
lexDeclarations
be the LexicallyScopedDeclarations of
body
For each element
in
lexDeclarations
, do
NOTE: Lexically declared names are only instantiated here but not initialized.
For each element
dn
of the BoundNames of
, do
If IsConstantDeclaration of
is
true
, then
Perform ?
lexEnvRec
.CreateImmutableBinding(
dn
true
).
Else,
Perform ?
lexEnvRec
.CreateMutableBinding(
dn
false
).
For each
Parse Node
in
functionsToInitialize
, do
Let
fn
be the sole element of the BoundNames of
Let
fo
be the result of performing InstantiateFunctionObject for
with argument
lexEnv
If
varEnvRec
is a global
Environment Record
, then
Perform ?
varEnvRec
.CreateGlobalFunctionBinding(
fn
fo
true
).
Else,
Let
bindingExists
be
varEnvRec
.HasBinding(
fn
).
If
bindingExists
is
false
, then
Let
status
be !
varEnvRec
.CreateMutableBinding(
fn
true
).
Assert
status
is not an
abrupt completion
because of validation preceding step 12.
Perform !
varEnvRec
.InitializeBinding(
fn
fo
).
Else,
Perform !
varEnvRec
.SetMutableBinding(
fn
fo
false
).
For each String
vn
in
declaredVarNames
, in list order, do
If
varEnvRec
is a global
Environment Record
, then
Perform ?
varEnvRec
.CreateGlobalVarBinding(
vn
true
).
Else,
Let
bindingExists
be
varEnvRec
.HasBinding(
vn
).
If
bindingExists
is
false
, then
Let
status
be !
varEnvRec
.CreateMutableBinding(
vn
true
).
Assert
status
is not an
abrupt completion
because of validation preceding step 12.
Perform !
varEnvRec
.InitializeBinding(
vn
undefined
).
Return
NormalCompletion
empty
).
Note
An alternative version of this algorithm is described in
B.3.5
18.2.2
isFinite (
number
The
isFinite
function is the
%isFinite%
intrinsic object. When the
isFinite
function is called with one argument
number
, the following steps are taken:
Let
num
be ?
ToNumber
number
).
If
num
is
NaN
+∞
, or
-∞
, return
false
Otherwise, return
true
18.2.3
isNaN (
number
The
isNaN
function is the
%isNaN%
intrinsic object. When the
isNaN
function is called with one argument
number
, the following steps are taken:
Let
num
be ?
ToNumber
number
).
If
num
is
NaN
, return
true
Otherwise, return
false
Note
A reliable way for ECMAScript code to test if a value
is a
NaN
is an expression of the form
X !== X
. The result will be
true
if and only if
is a
NaN
18.2.4
parseFloat (
string
The
parseFloat
function produces a Number value dictated by interpretation of the contents of the
string
argument as a decimal literal.
The
parseFloat
function is the
%parseFloat%
intrinsic object. When the
parseFloat
function is called with one argument
string
, the following steps are taken:
Let
inputString
be ?
ToString
string
).
Let
trimmedString
be a substring of
inputString
consisting of the leftmost code unit that is not a
StrWhiteSpaceChar
and all code units to the right of that code unit. (In other words, remove leading white space.) If
inputString
does not contain any such code units, let
trimmedString
be the empty string.
If neither
trimmedString
nor any prefix of
trimmedString
satisfies the syntax of a
StrDecimalLiteral
(see
7.1.3.1
), return
NaN
Let
numberString
be the longest prefix of
trimmedString
, which might be
trimmedString
itself, that satisfies the syntax of a
StrDecimalLiteral
Let
mathFloat
be MV of
numberString
If
mathFloat
= 0, then
If the first code unit of
trimmedString
is the code unit 0x002D (HYPHEN-MINUS), return
-0
Return
+0
Return the Number value for
mathFloat
Note
parseFloat
may interpret only a leading portion of
string
as a Number value; it ignores any code units that cannot be interpreted
as part of the notation of a decimal literal, and no indication is
given that any such code units were ignored.
18.2.5
parseInt (
string
radix
The
parseInt
function produces an integer value dictated by interpretation of the contents of the
string
argument according to the specified
radix
. Leading white space in
string
is ignored. If
radix
is
undefined
or 0, it is assumed to be 10 except when the number begins with the code unit pairs
0x
or
0X
, in which case a radix of 16 is assumed. If
radix
is 16, the number may also optionally begin with the code unit pairs
0x
or
0X
The
parseInt
function is the
%parseInt%
intrinsic object. When the
parseInt
function is called, the following steps are taken:
Let
inputString
be ?
ToString
string
).
Let
be a newly created substring of
inputString
consisting of the first code unit that is not a
StrWhiteSpaceChar
and all code units following that code unit. (In other words, remove leading white space.) If
inputString
does not contain any such code unit, let
be the empty string.
Let
sign
be 1.
If
is not empty and the first code unit of
is the code unit 0x002D (HYPHEN-MINUS), set
sign
to -1.
If
is not empty and the first code unit of
is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from
Let
be ?
ToInt32
radix
).
Let
stripPrefix
be
true
If
≠ 0, then
If
< 2 or
> 36, return
NaN
If
≠ 16, set
stripPrefix
to
false
Else
= 0,
Set
to 10.
If
stripPrefix
is
true
, then
If the length of
is at least 2 and the first two code units of
are either
"0x"
or
"0X"
, then
Remove the first two code units from
Set
to 16.
If
contains a code unit that is not a radix-
digit, let
be the substring of
consisting of all code units before the first such code unit; otherwise, let
be
If
is empty, return
NaN
Let
mathInt
be the mathematical integer value that is represented by
in radix-
notation, using the letters
and
for digits with values 10 through 35. (However, if
is 10 and
contains more than 20 significant digits, every significant digit after
the 20th may be replaced by a 0 digit, at the option of the
implementation; and if
is not 2, 4, 8, 10, 16, or 32, then
mathInt
may be an implementation-dependent approximation to the mathematical integer value that is represented by
in radix-
notation.)
If
mathInt
= 0, then
If
sign
= -1, return
-0
Return
+0
Let
number
be the Number value for
mathInt
Return
sign
number
Note
parseInt
may interpret only a leading portion of
string
as an integer value; it ignores any code units that cannot be
interpreted as part of the notation of an integer, and no indication is
given that any such code units were ignored.
18.2.6
URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that
identify resources (e.g. web pages or files) and transport protocols by
which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript
language itself does not provide any support for using URIs except for
functions that encode and decode URIs as described in
18.2.6.2
18.2.6.3
18.2.6.4
and
18.2.6.5
Note
Many implementations of ECMAScript provide additional
functions and methods that manipulate web pages; these functions are
beyond the scope of this standard.
18.2.6.1
URI Syntax and Semantics
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme
First
Second
Third
Fourth
where the italicized names represent components and “
”, “
”, “
” and “
” are reserved for use as separators. The
encodeURI
and
decodeURI
functions are intended to work with complete URIs; they assume that any
reserved code units in the URI are intended to have special meaning and
so are not encoded. The
encodeURIComponent
and
decodeURIComponent
functions are intended to work with the individual component parts of a
URI; they assume that any reserved code units represent text and so
must be encoded so that they are not interpreted as reserved code units
when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
Syntax
uri
:::
uriCharacters
opt
uriCharacters
:::
uriCharacter
uriCharacters
opt
uriCharacter
:::
uriReserved
uriUnescaped
uriEscaped
uriReserved
:::
one of
uriUnescaped
:::
uriAlpha
DecimalDigit
uriMark
uriEscaped
:::
HexDigit
HexDigit
uriAlpha
:::
one of
uriMark
:::
one of
Note
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above
or is not intended to have the special meaning sometimes given to the
reserved code units, that code unit must be encoded. The code unit is
transformed into its UTF-8 encoding, with
surrogate pairs
first converted from UTF-16 to the corresponding code point value.
(Note that for code units in the range [0, 127] this results in a single
octet with the same value.) The resulting sequence of octets is then
transformed into a String with each octet represented by an escape
sequence of the form
"%xx"
18.2.6.1.1
Runtime Semantics: Encode (
string
unescapedSet
The encoding and escaping process is described by the abstract operation Encode taking two String arguments
string
and
unescapedSet
Let
strLen
be the number of code units in
string
Let
be the empty String.
Let
be 0.
Repeat,
If
equals
strLen
, return
Let
be the code unit at index
within
string
If
is in
unescapedSet
, then
Let
be the String value containing only the code unit
Set
to the
string-concatenation
of the previous value of
and
Else
is not in
unescapedSet
If
is a
trailing surrogate
, throw a
URIError
exception.
If
is not a
leading surrogate
, then
Let
be the code point with the same numeric value as code unit
Else,
Increase
by 1.
If
equals
strLen
, throw a
URIError
exception.
Let
kChar
be the code unit at index
within
string
If
kChar
is not a
trailing surrogate
, throw a
URIError
exception.
Let
be
UTF16Decode
kChar
).
Let
Octets
be the
List
of octets resulting by applying the UTF-8 transformation to
For each element
octet
of
Octets
in
List
order, do
Let
be the
string-concatenation
of:
"%"
the String representation of
octet
, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
Set
to the
string-concatenation
of the previous value of
and
Increase
by 1.
18.2.6.1.2
Runtime Semantics: Decode (
string
reservedSet
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments
string
and
reservedSet
Let
strLen
be the number of code units in
string
Let
be the empty String.
Let
be 0.
Repeat,
If
equals
strLen
, return
Let
be the code unit at index
within
string
If
is not the code unit 0x0025 (PERCENT SIGN), then
Let
be the String value containing only the code unit
Else
is the code unit 0x0025 (PERCENT SIGN),
Let
start
be
If
+ 2 is greater than or equal to
strLen
, throw a
URIError
exception.
If the code units at index (
+ 1) and (
+ 2) within
string
do not represent hexadecimal digits, throw a
URIError
exception.
Let
be the 8-bit value represented by the two hexadecimal digits at index (
+ 1) and (
+ 2).
Increment
by 2.
If the most significant bit in
is 0, then
Let
be the code unit whose value is
If
is not in
reservedSet
, then
Let
be the String value containing only the code unit
Else
is in
reservedSet
Let
be the substring of
string
from index
start
to index
inclusive.
Else the most significant bit in
is 1,
Let
be the smallest nonnegative integer such that (
<<
) & 0x80 is equal to 0.
If
equals 1 or
is greater than 4, throw a
URIError
exception.
Let
Octets
be a
List
of 8-bit integers of size
Set
Octets
[0] to
If
+ (3 × (
- 1)) is greater than or equal to
strLen
, throw a
URIError
exception.
Let
be 1.
Repeat, while
Increment
by 1.
If the code unit at index
within
string
is not the code unit 0x0025 (PERCENT SIGN), throw a
URIError
exception.
If the code units at index (
+ 1) and (
+ 2) within
string
do not represent hexadecimal digits, throw a
URIError
exception.
Let
be the 8-bit value represented by the two hexadecimal digits at index (
+ 1) and (
+ 2).
If the two most significant bits in
are not 10, throw a
URIError
exception.
Increment
by 2.
Set
Octets
] to
Increment
by 1.
If
Octets
does not contain a valid UTF-8 encoding of a Unicode code point, throw a
URIError
exception.
Let
be the value obtained by applying the UTF-8 transformation to
Octets
, that is, from a
List
of octets into a 21-bit value.
Let
be the String value whose code units are, in order, the elements in
UTF16Encoding
).
Set
to the
string-concatenation
of the previous value of
and
Increase
by 1.
Note
This syntax of Uniform Resource Identifiers is based upon
RFC 2396 and does not reflect the more recent RFC 3986 which replaces
RFC 2396. A formal description and implementation of UTF-8 is given in
RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6
octets. The only octet of a sequence of one has the higher-order bit
set to 0, the remaining 7 bits being used to encode the character value.
In a sequence of n octets, n > 1, the initial octet has the n
higher-order bits set to 1, followed by a bit set to 0. The remaining
bits of that octet contain bits from the value of the character to be
encoded. The following octets all have the higher-order bit set to 1 and
the following bit set to 0, leaving 6 bits in each to contain bits from
the character to be encoded. The possible UTF-8 encodings of ECMAScript
characters are specified in
Table 46
Table 46 (Informative): UTF-8 Encodings
Code Unit Value
Representation
st
Octet
nd
Octet
rd
Octet
th
Octet
0x0000 - 0x007F
00000000 0
zzzzzzz
zzzzzzz
0x0080 - 0x07FF
00000
yyy yyzzzzzz
110
yyyyy
10
zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF
110110
vv vvwwwwxx
followed by
110111
yy yyzzzzzz
11110
uuu
10
uuwwww
10
xxyyyy
10
zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 - 0xDFFF
causes
URIError
0xDC00 - 0xDFFF
causes
URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110
xxxx
10
yyyyyy
10
zzzzzz
Where
uuuuu
vvvv
+ 1
to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).
The above transformation combines each
surrogate pair
(for which code unit values in the inclusive range 0xD800 to 0xDFFF are
reserved) into a UTF-32 representation and encodes the resulting 21-bit
value into UTF-8. Decoding reconstructs the
surrogate pair
RFC 3629 prohibits the decoding of invalid UTF-8 octet
sequences. For example, the invalid sequence C0 80 must not decode into
the code unit 0x0000. Implementations of the Decode algorithm are
required to throw a
URIError
when encountering such invalid sequences.
18.2.6.2
decodeURI (
encodedURI
The
decodeURI
function computes a new version of
a URI in which each escape sequence and UTF-8 encoding of the sort that
might be introduced by the
encodeURI
function is replaced
with the UTF-16 encoding of the code points that it represents. Escape
sequences that could not have been introduced by
encodeURI
are not replaced.
The
decodeURI
function is the
%decodeURI%
intrinsic object. When the
decodeURI
function is called with one argument
encodedURI
, the following steps are taken:
Let
uriString
be ?
ToString
encodedURI
).
Let
reservedURISet
be a String containing one instance of each code unit valid in
uriReserved
plus
"#"
Return ?
Decode
uriString
reservedURISet
).
Note
The code point
"#"
is not decoded from escape sequences even though it is not a reserved URI code point.
18.2.6.3
decodeURIComponent (
encodedURIComponent
The
decodeURIComponent
function computes a new
version of a URI in which each escape sequence and UTF-8 encoding of the
sort that might be introduced by the
encodeURIComponent
function is replaced with the UTF-16 encoding of the code points that it represents.
The
decodeURIComponent
function is the
%decodeURIComponent%
intrinsic object. When the
decodeURIComponent
function is called with one argument
encodedURIComponent
, the following steps are taken:
Let
componentString
be ?
ToString
encodedURIComponent
).
Let
reservedURIComponentSet
be the empty String.
Return ?
Decode
componentString
reservedURIComponentSet
).
18.2.6.4
encodeURI (
uri
The
encodeURI
function computes a new version of a UTF-16 encoded (
6.1.4
URI in which each instance of certain code points is replaced by one,
two, three, or four escape sequences representing the UTF-8 encoding of
the code points.
The
encodeURI
function is the
%encodeURI%
intrinsic object. When the
encodeURI
function is called with one argument
uri
, the following steps are taken:
Let
uriString
be ?
ToString
uri
).
Let
unescapedURISet
be a String containing one instance of each code unit valid in
uriReserved
and
uriUnescaped
plus
"#"
Return ?
Encode
uriString
unescapedURISet
).
Note
The code unit
"#"
is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.
18.2.6.5
encodeURIComponent (
uriComponent
The
encodeURIComponent
function computes a new version of a UTF-16 encoded (
6.1.4
URI in which each instance of certain code points is replaced by one,
two, three, or four escape sequences representing the UTF-8 encoding of
the code point.
The
encodeURIComponent
function is the
%encodeURIComponent%
intrinsic object. When the
encodeURIComponent
function is called with one argument
uriComponent
, the following steps are taken:
Let
componentString
be ?
ToString
uriComponent
).
Let
unescapedURIComponentSet
be a String containing one instance of each code unit valid in
uriUnescaped
Return ?
Encode
componentString
unescapedURIComponentSet
).
18.3
Constructor Properties of the Global Object
18.3.1
Array ( . . . )
See
22.1.1
18.3.2
ArrayBuffer ( . . . )
See
24.1.2
18.3.3
Boolean ( . . . )
See
19.3.1
18.3.4
DataView ( . . . )
See
24.3.2
18.3.5
Date ( . . . )
See
20.3.2
18.3.6
Error ( . . . )
See
19.5.1
18.3.7
EvalError ( . . . )
See
19.5.5.1
18.3.8
Float32Array ( . . . )
See
22.2.4
18.3.9
Float64Array ( . . . )
See
22.2.4
18.3.10
Function ( . . . )
See
19.2.1
18.3.11
Int8Array ( . . . )
See
22.2.4
18.3.12
Int16Array ( . . . )
See
22.2.4
18.3.13
Int32Array ( . . . )
See
22.2.4
18.3.14
Map ( . . . )
See
23.1.1
18.3.15
Number ( . . . )
See
20.1.1
18.3.16
Object ( . . . )
See
19.1.1
18.3.17
Promise ( . . . )
See
25.6.3
18.3.18
Proxy ( . . . )
See
26.2.1
18.3.19
RangeError ( . . . )
See
19.5.5.2
18.3.20
ReferenceError ( . . . )
See
19.5.5.3
18.3.21
RegExp ( . . . )
See
21.2.3
18.3.22
Set ( . . . )
See
23.2.1
18.3.23
SharedArrayBuffer ( . . . )
See
24.2.2
18.3.24
String ( . . . )
See
21.1.1
18.3.25
Symbol ( . . . )
See
19.4.1
18.3.26
SyntaxError ( . . . )
See
19.5.5.4
18.3.27
TypeError ( . . . )
See
19.5.5.5
18.3.28
Uint8Array ( . . . )
See
22.2.4
18.3.29
Uint8ClampedArray ( . . . )
See
22.2.4
18.3.30
Uint16Array ( . . . )
See
22.2.4
18.3.31
Uint32Array ( . . . )
See
22.2.4
18.3.32
URIError ( . . . )
See
19.5.5.6
18.3.33
WeakMap ( . . . )
See
23.3.1
18.3.34
WeakSet ( . . . )
See
23.4
18.4
Other Properties of the Global Object
18.4.1
Atomics
See
24.4
18.4.2
JSON
See
24.5
18.4.3
Math
See
20.2
18.4.4
Reflect
See
26.1
19
Fundamental Objects
19.1
Object Objects
19.1.1
The Object Constructor
The Object
constructor
is the intrinsic object
%Object%
is the initial value of the
Object
property of the
global object
creates a new ordinary object when called as a
constructor
performs a type conversion when called as a function rather than as a
constructor
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition.
19.1.1.1
Object ( [
value
] )
When the
Object
function is called with optional argument
value
, the following steps are taken:
If NewTarget is neither
undefined
nor the active function, then
Return ?
OrdinaryCreateFromConstructor
(NewTarget,
"%ObjectPrototype%"
).
If
value
is
null
undefined
or not supplied, return
ObjectCreate
%ObjectPrototype%
).
Return !
ToObject
value
).
The
"length"
property of the
Object
constructor
function is 1.
19.1.2
Properties of the Object Constructor
The Object
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has a
"length"
property.
has the following additional properties:
19.1.2.1
Object.assign (
target
, ...
sources
The
assign
function is used to copy the values of all of the enumerable own properties from one or more source objects to a
target
object. When the
assign
function is called, the following steps are taken:
Let
to
be ?
ToObject
target
).
If only one argument was passed, return
to
Let
sources
be the
List
of argument values starting with the second argument.
For each element
nextSource
of
sources
, in ascending index order, do
If
nextSource
is neither
undefined
nor
null
, then
Let
from
be !
ToObject
nextSource
).
Let
keys
be ?
from
.[[OwnPropertyKeys]]().
For each element
nextKey
of
keys
in
List
order, do
Let
desc
be ?
from
.[[GetOwnProperty]](
nextKey
).
If
desc
is not
undefined
and
desc
.[[Enumerable]] is
true
, then
Let
propValue
be ?
Get
from
nextKey
).
Perform ?
Set
to
nextKey
propValue
true
).
Return
to
The
"length"
property of the
assign
function is 2.
19.1.2.2
Object.create (
Properties
The
create
function creates a new object with a specified prototype. When the
create
function is called, the following steps are taken:
If
Type
) is neither Object nor Null, throw a
TypeError
exception.
Let
obj
be
ObjectCreate
).
If
Properties
is not
undefined
, then
Return ?
ObjectDefineProperties
obj
Properties
).
Return
obj
19.1.2.3
Object.defineProperties (
Properties
The
defineProperties
function is used to add own properties and/or update the attributes of existing own properties of an object. When the
defineProperties
function is called, the following steps are taken:
Return ?
ObjectDefineProperties
Properties
).
19.1.2.3.1
Runtime Semantics: ObjectDefineProperties (
Properties
The abstract operation ObjectDefineProperties with arguments
and
Properties
performs the following steps:
If
Type
) is not Object, throw a
TypeError
exception.
Let
props
be ?
ToObject
Properties
).
Let
keys
be ?
props
.[[OwnPropertyKeys]]().
Let
descriptors
be a new empty
List
For each element
nextKey
of
keys
in
List
order, do
Let
propDesc
be ?
props
.[[GetOwnProperty]](
nextKey
).
If
propDesc
is not
undefined
and
propDesc
.[[Enumerable]] is
true
, then
Let
descObj
be ?
Get
props
nextKey
).
Let
desc
be ?
ToPropertyDescriptor
descObj
).
Append the pair (a two element
List
) consisting of
nextKey
and
desc
to the end of
descriptors
For each
pair
from
descriptors
in list order, do
Let
be the first element of
pair
Let
desc
be the second element of
pair
Perform ?
DefinePropertyOrThrow
desc
).
Return
19.1.2.4
Object.defineProperty (
Attributes
The
defineProperty
function is used to add an own property and/or update the attributes of an existing own property of an object. When the
defineProperty
function is called, the following steps are taken:
If
Type
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
).
Let
desc
be ?
ToPropertyDescriptor
Attributes
).
Perform ?
DefinePropertyOrThrow
key
desc
).
Return
19.1.2.5
Object.entries (
When the
entries
function is called with argument
, the following steps are taken:
Let
obj
be ?
ToObject
).
Let
nameList
be ?
EnumerableOwnPropertyNames
obj
"key+value"
).
Return
CreateArrayFromList
nameList
).
19.1.2.6
Object.freeze (
When the
freeze
function is called, the following steps are taken:
If
Type
) is not Object, return
Let
status
be ?
SetIntegrityLevel
"frozen"
).
If
status
is
false
, throw a
TypeError
exception.
Return
19.1.2.7
Object.fromEntries (
iterable
When the
fromEntries
method is called with argument
iterable
, the following steps are taken:
Perform ?
RequireObjectCoercible
iterable
).
Let
obj
be
ObjectCreate
%ObjectPrototype%
).
Assert
obj
is an extensible ordinary object with no own properties.
Let
stepsDefine
be the algorithm steps defined in
CreateDataPropertyOnObject Functions
Let
adder
be
CreateBuiltinFunction
stepsDefine
, « »).
Return ?
AddEntriesFromIterable
obj
iterable
adder
).
Note
The function created for
adder
is never directly accessible to ECMAScript code.
19.1.2.7.1
CreateDataPropertyOnObject Functions
A CreateDataPropertyOnObject function is an anonymous
built-in function. When a CreateDataPropertyOnObject function is called
with arguments
key
and
value
, the following steps are taken:
Let
be the
this
value.
Assert
Type
) is Object.
Assert
is an extensible ordinary object.
Let
propertyKey
be ?
ToPropertyKey
key
).
Perform !
CreateDataPropertyOrThrow
propertyKey
value
).
Return
undefined
19.1.2.8
Object.getOwnPropertyDescriptor (
When the
getOwnPropertyDescriptor
function is called, the following steps are taken:
Let
obj
be ?
ToObject
).
Let
key
be ?
ToPropertyKey
).
Let
desc
be ?
obj
.[[GetOwnProperty]](
key
).
Return
FromPropertyDescriptor
desc
).
19.1.2.9
Object.getOwnPropertyDescriptors (
When the
getOwnPropertyDescriptors
function is called, the following steps are taken:
Let
obj
be ?
ToObject
).
Let
ownKeys
be ?
obj
.[[OwnPropertyKeys]]().
Let
descriptors
be !
ObjectCreate
%ObjectPrototype%
).
For each element
key
of
ownKeys
in
List
order, do
Let
desc
be ?
obj
.[[GetOwnProperty]](
key
).
Let
descriptor
be !
FromPropertyDescriptor
desc
).
If
descriptor
is not
undefined
, perform !
CreateDataProperty
descriptors
key
descriptor
).
Return
descriptors
19.1.2.10
Object.getOwnPropertyNames (
When the
getOwnPropertyNames
function is called, the following steps are taken:
Return ?
GetOwnPropertyKeys
, String).
19.1.2.11
Object.getOwnPropertySymbols (
When the
getOwnPropertySymbols
function is called with argument
, the following steps are taken:
Return ?
GetOwnPropertyKeys
, Symbol).
19.1.2.11.1
Runtime Semantics: GetOwnPropertyKeys (
type
The abstract operation GetOwnPropertyKeys is called with arguments
and
type
where
is an Object and
type
is one of the ECMAScript specification types String or Symbol. The following steps are taken:
Let
obj
be ?
ToObject
).
Let
keys
be ?
obj
.[[OwnPropertyKeys]]().
Let
nameList
be a new empty
List
For each element
nextKey
of
keys
in
List
order, do
If
Type
nextKey
) is
type
, then
Append
nextKey
as the last element of
nameList
Return
CreateArrayFromList
nameList
).
19.1.2.12
Object.getPrototypeOf (
When the
getPrototypeOf
function is called with argument
, the following steps are taken:
Let
obj
be ?
ToObject
).
Return ?
obj
.[[GetPrototypeOf]]().
19.1.2.13
Object.is (
value1
value2
When the
is
function is called with arguments
value1
and
value2
, the following steps are taken:
Return
SameValue
value1
value2
).
19.1.2.14
Object.isExtensible (
When the
isExtensible
function is called with argument
, the following steps are taken:
If
Type
) is not Object, return
false
Return ?
IsExtensible
).
19.1.2.15
Object.isFrozen (
When the
isFrozen
function is called with argument
, the following steps are taken:
If
Type
) is not Object, return
true
Return ?
TestIntegrityLevel
"frozen"
).
19.1.2.16
Object.isSealed (
When the
isSealed
function is called with argument
, the following steps are taken:
If
Type
) is not Object, return
true
Return ?
TestIntegrityLevel
"sealed"
).
19.1.2.17
Object.keys (
When the
keys
function is called with argument
, the following steps are taken:
Let
obj
be ?
ToObject
).
Let
nameList
be ?
EnumerableOwnPropertyNames
obj
"key"
).
Return
CreateArrayFromList
nameList
).
19.1.2.18
Object.preventExtensions (
When the
preventExtensions
function is called, the following steps are taken:
If
Type
) is not Object, return
Let
status
be ?
.[[PreventExtensions]]().
If
status
is
false
, throw a
TypeError
exception.
Return
19.1.2.19
Object.prototype
The initial value of
Object.prototype
is the intrinsic object
%ObjectPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.1.2.20
Object.seal (
When the
seal
function is called, the following steps are taken:
If
Type
) is not Object, return
Let
status
be ?
SetIntegrityLevel
"sealed"
).
If
status
is
false
, throw a
TypeError
exception.
Return
19.1.2.21
Object.setPrototypeOf (
proto
When the
setPrototypeOf
function is called with arguments
and
proto
, the following steps are taken:
Set
to ?
RequireObjectCoercible
).
If
Type
proto
) is neither Object nor Null, throw a
TypeError
exception.
If
Type
) is not Object, return
Let
status
be ?
.[[SetPrototypeOf]](
proto
).
If
status
is
false
, throw a
TypeError
exception.
Return
19.1.2.22
Object.values (
When the
values
function is called with argument
, the following steps are taken:
Let
obj
be ?
ToObject
).
Let
nameList
be ?
EnumerableOwnPropertyNames
obj
"value"
).
Return
CreateArrayFromList
nameList
).
19.1.3
Properties of the Object Prototype Object
The Object prototype object:
is the intrinsic object
%ObjectPrototype%
is an
immutable prototype exotic object
has a [[Prototype]] internal slot whose value is
null
19.1.3.1
Object.prototype.constructor
The initial value of
Object.prototype.constructor
is the intrinsic object
%Object%
19.1.3.2
Object.prototype.hasOwnProperty (
When the
hasOwnProperty
method is called with argument
, the following steps are taken:
Let
be ?
ToPropertyKey
).
Let
be ?
ToObject
this
value).
Return ?
HasOwnProperty
).
Note
The ordering of steps 1 and 2 is chosen to ensure that any
exception that would have been thrown by step 1 in previous editions of
this specification will continue to be thrown even if the
this
value is
undefined
or
null
19.1.3.3
Object.prototype.isPrototypeOf (
When the
isPrototypeOf
method is called with argument
, the following steps are taken:
If
Type
) is not Object, return
false
Let
be ?
ToObject
this
value).
Repeat,
Set
to ?
.[[GetPrototypeOf]]().
If
is
null
, return
false
If
SameValue
) is
true
, return
true
Note
The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where
is not an object and the
this
value is
undefined
or
null
19.1.3.4
Object.prototype.propertyIsEnumerable (
When the
propertyIsEnumerable
method is called with argument
, the following steps are taken:
Let
be ?
ToPropertyKey
).
Let
be ?
ToObject
this
value).
Let
desc
be ?
.[[GetOwnProperty]](
).
If
desc
is
undefined
, return
false
Return
desc
.[[Enumerable]].
Note 1
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any
exception that would have been thrown by step 1 in previous editions of
this specification will continue to be thrown even if the
this
value is
undefined
or
null
19.1.3.5
Object.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
When the
toLocaleString
method is called, the following steps are taken:
Let
be the
this
value.
Return ?
Invoke
"toString"
).
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402
toLocaleString
functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This function provides a generic
toLocaleString
implementation for objects that have no locale-specific
toString
behaviour.
Array
Number
Date
, and
Typed Arrays
provide their own locale-sensitive
toLocaleString
methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
19.1.3.6
Object.prototype.toString ( )
When the
toString
method is called, the following steps are taken:
If the
this
value is
undefined
, return
"[object Undefined]"
If the
this
value is
null
, return
"[object Null]"
Let
be !
ToObject
this
value).
Let
isArray
be ?
IsArray
).
If
isArray
is
true
, let
builtinTag
be
"Array"
Else if
is a String
exotic object
, let
builtinTag
be
"String"
Else if
has a [[ParameterMap]] internal slot, let
builtinTag
be
"Arguments"
Else if
has a [[Call]] internal method, let
builtinTag
be
"Function"
Else if
has an [[ErrorData]] internal slot, let
builtinTag
be
"Error"
Else if
has a [[BooleanData]] internal slot, let
builtinTag
be
"Boolean"
Else if
has a [[NumberData]] internal slot, let
builtinTag
be
"Number"
Else if
has a [[DateValue]] internal slot, let
builtinTag
be
"Date"
Else if
has a [[RegExpMatcher]] internal slot, let
builtinTag
be
"RegExp"
Else, let
builtinTag
be
"Object"
Let
tag
be ?
Get
, @@toStringTag).
If
Type
tag
) is not String, set
tag
to
builtinTag
Return the
string-concatenation
of
"[object "
tag
, and
"]"
This function is the
%ObjProto_toString%
intrinsic object.
Note
Historically, this function was occasionally used to access
the String value of the [[Class]] internal slot that was used in
previous editions of this specification as a nominal type tag for
various built-in objects. The above definition of
toString
preserves compatibility for legacy code that uses
toString
as a test for those specific kinds of built-in objects. It does not
provide a reliable type testing mechanism for other kinds of built-in or
program defined objects. In addition, programs can use @@toStringTag in
ways that will invalidate the reliability of such legacy type tests.
19.1.3.7
Object.prototype.valueOf ( )
When the
valueOf
method is called, the following steps are taken:
Return ?
ToObject
this
value).
This function is the
%ObjProto_valueOf%
intrinsic object.
19.1.4
Properties of Object Instances
Object instances have no special properties beyond those inherited from the Object prototype object.
19.2
Function Objects
19.2.1
The Function Constructor
The Function
constructor
is the intrinsic object
%Function%
is the initial value of the
Function
property of the
global object
creates and initializes a new
function object
when called as a function rather than as a
constructor
. Thus the function call
Function(…)
is equivalent to the object creation expression
new Function(…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Function
behaviour must include a
super
call to the
Function
constructor
to create and initialize a subclass instance with the internal slots
necessary for built-in function behaviour. All ECMAScript syntactic
forms for defining function objects create instances of
Function
. There is no syntactic means to create instances of
Function
subclasses except for the built-in
GeneratorFunction
AsyncFunction
, and
AsyncGeneratorFunction
subclasses.
19.2.1.1
Function (
p1
p2
, … ,
pn
body
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the
Function
function is called with some arguments
p1
p2
, … ,
pn
body
(where
might be 0, that is, there are no “
” arguments, and where
body
might also not be provided), the following steps are taken:
Let
be the
active function object
Let
args
be the
argumentsList
that was passed to this function by [[Call]] or [[Construct]].
Return ?
CreateDynamicFunction
, NewTarget,
"normal"
args
).
Note
It is permissible but not necessary to have one argument
for each formal parameter to be specified. For example, all three of the
following expressions produce the same result:
new
Function
"a"
"b"
"c"
"return a+b+c"
new
Function
"a, b, c"
"return a+b+c"
new
Function
"a,b"
"c"
"return a+b+c"
19.2.1.1.1
Runtime Semantics: CreateDynamicFunction (
constructor
newTarget
kind
args
The abstract operation CreateDynamicFunction is called with arguments
constructor
newTarget
kind
, and
args
constructor
is the
constructor
function that is performing this action,
newTarget
is the
constructor
that
new
was initially applied to,
kind
is either
"normal"
"generator"
"async"
, or
"async generator"
, and
args
is a
List
containing the actual argument values that were passed to
constructor
. The following steps are taken:
Assert
: The
execution context stack
has at least two elements.
Let
callerContext
be the second to top element of the
execution context stack
Let
callerRealm
be
callerContext
's
Realm
Let
calleeRealm
be
the current Realm Record
Perform ?
HostEnsureCanCompileStrings
callerRealm
calleeRealm
).
If
newTarget
is
undefined
, set
newTarget
to
constructor
If
kind
is
"normal"
, then
Let
goal
be the grammar symbol
FunctionBody
[~Yield, ~Await]
Let
parameterGoal
be the grammar symbol
FormalParameters
[~Yield, ~Await]
Let
fallbackProto
be
"%FunctionPrototype%"
Else if
kind
is
"generator"
, then
Let
goal
be the grammar symbol
GeneratorBody
Let
parameterGoal
be the grammar symbol
FormalParameters
[+Yield, ~Await]
Let
fallbackProto
be
"%Generator%"
Else if
kind
is
"async"
, then
Let
goal
be the grammar symbol
AsyncFunctionBody
Let
parameterGoal
be the grammar symbol
FormalParameters
[~Yield, +Await]
Let
fallbackProto
be
"%AsyncFunctionPrototype%"
Else,
Assert
kind
is
"async generator"
Let
goal
be the grammar symbol
AsyncGeneratorBody
Let
parameterGoal
be the grammar symbol
FormalParameters
[+Yield, +Await]
Let
fallbackProto
be
"%AsyncGenerator%"
Let
argCount
be the number of elements in
args
Let
be the empty String.
If
argCount
= 0, let
bodyText
be the empty String.
Else if
argCount
= 1, let
bodyText
be
args
[0].
Else
argCount
> 1,
Let
firstArg
be
args
[0].
Set
to ?
ToString
firstArg
).
Let
be 1.
Repeat, while
argCount
- 1
Let
nextArg
be
args
].
Let
nextArgString
be ?
ToString
nextArg
).
Set
to the
string-concatenation
of the previous value of
","
(a comma), and
nextArgString
Increase
by 1.
Let
bodyText
be
args
].
Set
bodyText
to ?
ToString
bodyText
).
Let
parameters
be the result of parsing
, interpreted as UTF-16 encoded Unicode text as described in
6.1.4
, using
parameterGoal
as the
goal symbol
. Throw a
SyntaxError
exception if the parse fails.
Let
body
be the result of parsing
bodyText
, interpreted as UTF-16 encoded Unicode text as described in
6.1.4
, using
goal
as the
goal symbol
. Throw a
SyntaxError
exception if the parse fails.
Let
strict
be ContainsUseStrict of
body
If any
static semantics
errors are detected for
parameters
or
body
, throw a
SyntaxError
or a
ReferenceError
exception, depending on the type of the error. If
strict
is
true
, the Early Error rules for
UniqueFormalParameters
FormalParameters
are applied. Parsing and
early error
detection may be interweaved in an implementation-dependent manner.
If
strict
is
true
and IsSimpleParameterList of
parameters
is
false
, throw a
SyntaxError
exception.
If any element of the BoundNames of
parameters
also occurs in the LexicallyDeclaredNames of
body
, throw a
SyntaxError
exception.
If
body
Contains
SuperCall
is
true
, throw a
SyntaxError
exception.
If
parameters
Contains
SuperCall
is
true
, throw a
SyntaxError
exception.
If
body
Contains
SuperProperty
is
true
, throw a
SyntaxError
exception.
If
parameters
Contains
SuperProperty
is
true
, throw a
SyntaxError
exception.
If
kind
is
"generator"
or
"async generator"
, then
If
parameters
Contains
YieldExpression
is
true
, throw a
SyntaxError
exception.
If
kind
is
"async"
or
"async generator"
, then
If
parameters
Contains
AwaitExpression
is
true
, throw a
SyntaxError
exception.
If
strict
is
true
, then
If BoundNames of
parameters
contains any duplicate elements, throw a
SyntaxError
exception.
Let
proto
be ?
GetPrototypeFromConstructor
newTarget
fallbackProto
).
Let
be
FunctionAllocate
proto
strict
kind
).
Let
realmF
be
.[[Realm]].
Let
scope
be
realmF
.[[GlobalEnv]].
Perform
FunctionInitialize
Normal
parameters
body
scope
).
If
kind
is
"generator"
, then
Let
prototype
be
ObjectCreate
%GeneratorPrototype%
).
Perform
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Else if
kind
is
"async generator"
, then
Let
prototype
be
ObjectCreate
%AsyncGeneratorPrototype%
).
Perform
DefinePropertyOrThrow
"prototype"
, PropertyDescriptor { [[Value]]:
prototype
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Else if
kind
is
"normal"
, perform
MakeConstructor
).
NOTE: Async functions are not constructable and do not have a [[Construct]] internal method or a
"prototype"
property.
Perform
SetFunctionName
"anonymous"
).
Let
prefix
be the prefix associated with
kind
in
Table 47
Let
sourceText
be the
string-concatenation
of
prefix
" anonymous("
, 0x000A (LINE FEED),
") {"
, 0x000A (LINE FEED),
bodyText
, 0x000A (LINE FEED), and
"}"
Set
.[[SourceText]] to
sourceText
Return
Note
prototype
property is created for every
non-async function created using CreateDynamicFunction to provide for
the possibility that the function will be used as a
constructor
Table 47: Dynamic Function SourceText Prefixes
Kind
Prefix
"normal"
"function"
"generator"
"function*"
"async"
"async function"
"async generator"
"async function*"
19.2.2
Properties of the Function Constructor
The Function
constructor
is itself a built-in
function object
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
19.2.2.1
Function.length
This is a
data property
with a value of 1. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
19.2.2.2
Function.prototype
The value of
Function.prototype
is
%FunctionPrototype%
, the intrinsic Function prototype object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.2.3
Properties of the Function Prototype Object
The Function prototype object:
is the intrinsic object
%FunctionPrototype%
is itself a built-in
function object
accepts any arguments and returns
undefined
when invoked.
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
does not have a
prototype
property.
has a
"length"
property whose value is 0.
has a
name
property whose value is the empty String.
Note
The Function prototype object is specified to be a
function object
to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
19.2.3.1
Function.prototype.apply (
thisArg
argArray
When the
apply
method is called with arguments
thisArg
and
argArray
, the following steps are taken:
Let
func
be the
this
value.
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
If
argArray
is
undefined
or
null
, then
Perform
PrepareForTailCall
().
Return ?
Call
func
thisArg
).
Let
argList
be ?
CreateListFromArrayLike
argArray
).
Perform
PrepareForTailCall
().
Return ?
Call
func
thisArg
argList
).
Note 1
The
thisArg
value is passed without modification as the
this
value. This is a change from Edition 3, where an
undefined
or
null
thisArg
is replaced with the
global object
and
ToObject
is applied to all other values and that result is passed as the
this
value. Even though the
thisArg
is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If
func
is an arrow function or a
bound function
then the
thisArg
will be ignored by the function [[Call]] in step 5.
19.2.3.2
Function.prototype.bind (
thisArg
, ...
args
When the
bind
method is called with argument
thisArg
and zero or more
args
, it performs the following steps:
Let
Target
be the
this
value.
If
IsCallable
Target
) is
false
, throw a
TypeError
exception.
Let
args
be a new (possibly empty)
List
consisting of all of the argument values provided after
thisArg
in order.
Let
be ?
BoundFunctionCreate
Target
thisArg
args
).
Let
targetHasLength
be ?
HasOwnProperty
Target
"length"
).
If
targetHasLength
is
true
, then
Let
targetLen
be ?
Get
Target
"length"
).
If
Type
targetLen
) is not Number, let
be 0.
Else,
Set
targetLen
to !
ToInteger
targetLen
).
Let
be the larger of 0 and the result of
targetLen
minus the number of elements of
args
Else, let
be 0.
Perform !
SetFunctionLength
).
Let
targetName
be ?
Get
Target
"name"
).
If
Type
targetName
) is not String, set
targetName
to the empty string.
Perform
SetFunctionName
targetName
"bound"
).
Return
Note 1
Function objects created using
Function.prototype.bind
are exotic objects. They also do not have a
prototype
property.
Note 2
If
Target
is an arrow function or a
bound function
then the
thisArg
passed to this method will not be used by subsequent calls to
19.2.3.3
Function.prototype.call (
thisArg
, ...
args
When the
call
method is called with argument
thisArg
and zero or more
args
, the following steps are taken:
Let
func
be the
this
value.
If
IsCallable
func
) is
false
, throw a
TypeError
exception.
Let
argList
be a new empty
List
If
this method was called with more than one argument, then in left to
right order, starting with the second argument, append each argument as
the last element of
argList
Perform
PrepareForTailCall
().
Return ?
Call
func
thisArg
argList
).
Note 1
The
thisArg
value is passed without modification as the
this
value. This is a change from Edition 3, where an
undefined
or
null
thisArg
is replaced with the
global object
and
ToObject
is applied to all other values and that result is passed as the
this
value. Even though the
thisArg
is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If
func
is an arrow function or a
bound function
then the
thisArg
will be ignored by the function [[Call]] in step 5.
19.2.3.4
Function.prototype.constructor
The initial value of
Function.prototype.constructor
is the intrinsic object
%Function%
19.2.3.5
Function.prototype.toString ( )
When the
toString
method is called, the following steps are taken:
Let
func
be the
this
value.
If
func
is a
Bound Function exotic object
or a
built-in function object
, then return an implementation-dependent String source code representation of
func
. The representation must have the syntax of a
NativeFunction
. Additionally, if
func
is a
Well-known Intrinsic Object
and is not identified as an anonymous function, the portion of the returned String that would be matched by
PropertyName
must be the initial value of the
name
property of
func
If
Type
func
) is Object and
func
has a [[SourceText]] internal slot and
Type
func
.[[SourceText]]) is String and ! HostHasSourceTextAvailable(
func
) is
true
, then return
func
.[[SourceText]].
If
Type
func
) is Object and
IsCallable
func
) is
true
, then return an implementation-dependent String source code representation of
func
. The representation must have the syntax of a
NativeFunction
Throw a
TypeError
exception.
NativeFunction
function
PropertyName
[~Yield, ~Await]
opt
FormalParameters
[~Yield, ~Await]
native
code
19.2.3.6
Function.prototype [ @@hasInstance ] (
When the
@@hasInstance
method of an object
is called with value
, the following steps are taken:
Let
be the
this
value.
Return ?
OrdinaryHasInstance
).
The value of the
name
property of this function is
"[Symbol.hasInstance]"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
This is the default implementation of
@@hasInstance
that most functions inherit.
@@hasInstance
is called by the
instanceof
operator to determine whether a value is an instance of a specific
constructor
. An expression such as
instanceof
evaluates as
F[@@hasInstance](v)
constructor
function can control which objects are recognized as its instances by
instanceof
by exposing a different
@@hasInstance
method on the function.
This property is non-writable and non-configurable to prevent
tampering that could be used to globally expose the target function of a
bound function
19.2.4
Function Instances
Every Function instance is an ECMAScript
function object
and has the internal slots listed in
Table 27
. Function objects created using the
Function.prototype.bind
method (
19.2.3.2
) have the internal slots listed in
Table 28
Function instances have the following properties:
19.2.4.1
length
The value of the
"length"
property is an integer
that indicates the typical number of arguments expected by the
function. However, the language permits the function to be invoked with
some other number of arguments. The behaviour of a function when invoked
on a number of arguments other than the number specified by its
"length"
property depends on the function. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
19.2.4.2
name
The value of the
name
property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable or
property name
that is used to refer to the function at its point of definition in
ECMAScript code. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Anonymous functions objects that do not have a contextual name associated with them by this specification do not have a
name
own property but inherit the
name
property of
%FunctionPrototype%
19.2.4.3
prototype
Function instances that can be used as a
constructor
have a
prototype
property. Whenever such a Function instance is created another ordinary
object is also created and is the initial value of the function's
prototype
property. Unless otherwise specified, the value of the
prototype
property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a
constructor
This property has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
Function objects created using
Function.prototype.bind
, or by evaluating a
MethodDefinition
(that is not a
GeneratorMethod
or
AsyncGeneratorMethod
) or an
ArrowFunction
do not have a
prototype
property.
19.2.5
HostHasSourceTextAvailable (
func
HostHasSourceTextAvailable is an implementation-defined
abstract operation that allows host environments to prevent the source
text from being provided for a given function.
An implementation of HostHasSourceTextAvailable must complete
normally in all cases. This operation must be deterministic with respect
to its parameters. Each time it is called with a specific
func
as its argument, it must return the same completion record. The default
implementation of HostHasSourceTextAvailable is to unconditionally
return a normal completion with a value of
true
19.3
Boolean Objects
19.3.1
The Boolean Constructor
The Boolean
constructor
is the intrinsic object
%Boolean%
is the initial value of the
Boolean
property of the
global object
creates and initializes a new Boolean object when called as a
constructor
performs a type conversion when called as a function rather than as a
constructor
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Boolean
behaviour must include a
super
call to the
Boolean
constructor
to create and initialize the subclass instance with a [[BooleanData]] internal slot.
19.3.1.1
Boolean (
value
When
Boolean
is called with argument
value
, the following steps are taken:
Let
be
ToBoolean
value
).
If NewTarget is
undefined
, return
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%BooleanPrototype%"
, « [[BooleanData]] »).
Set
.[[BooleanData]] to
Return
19.3.2
Properties of the Boolean Constructor
The Boolean
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
19.3.2.1
Boolean.prototype
The initial value of
Boolean.prototype
is the intrinsic object
%BooleanPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.3.3
Properties of the Boolean Prototype Object
The Boolean prototype object:
is the intrinsic object
%BooleanPrototype%
is an ordinary object.
is itself a Boolean object; it has a [[BooleanData]] internal slot with the value
false
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
The abstract operation
thisBooleanValue
value
) performs the following steps:
If
Type
value
) is Boolean, return
value
If
Type
value
) is Object and
value
has a [[BooleanData]] internal slot, then
Let
be
value
.[[BooleanData]].
Assert
Type
) is Boolean.
Return
Throw a
TypeError
exception.
19.3.3.1
Boolean.prototype.constructor
The initial value of
Boolean.prototype.constructor
is the intrinsic object
%Boolean%
19.3.3.2
Boolean.prototype.toString ( )
The following steps are taken:
Let
be ?
thisBooleanValue
this
value).
If
is
true
, return
"true"
; else return
"false"
19.3.3.3
Boolean.prototype.valueOf ( )
The following steps are taken:
Return ?
thisBooleanValue
this
value).
19.3.4
Properties of Boolean Instances
Boolean instances are ordinary objects that inherit properties
from the Boolean prototype object. Boolean instances have a
[[BooleanData]] internal slot. The [[BooleanData]] internal slot is the
Boolean value represented by this Boolean object.
19.4
Symbol Objects
19.4.1
The Symbol Constructor
The Symbol
constructor
is the intrinsic object
%Symbol%
is the initial value of the
Symbol
property of the
global object
returns a new Symbol value when called as a function.
is not intended to be used with the
new
operator.
is not intended to be subclassed.
may be used as the value of an
extends
clause of a class definition but a
super
call to it will cause an exception.
19.4.1.1
Symbol ( [
description
] )
When
Symbol
is called with optional argument
description
, the following steps are taken:
If NewTarget is not
undefined
, throw a
TypeError
exception.
If
description
is
undefined
, let
descString
be
undefined
Else, let
descString
be ?
ToString
description
).
Return a new unique Symbol value whose [[Description]] value is
descString
19.4.2
Properties of the Symbol Constructor
The Symbol
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
19.4.2.1
Symbol.asyncIterator
The initial value of
Symbol.asyncIterator
is the well known symbol @@asyncIterator (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.2
Symbol.for (
key
When
Symbol.for
is called with argument
key
it performs the following steps:
Let
stringKey
be ?
ToString
key
).
For each element
of the GlobalSymbolRegistry
List
, do
If
SameValue
.[[Key]],
stringKey
) is
true
, return
.[[Symbol]].
Assert
: GlobalSymbolRegistry does not currently contain an entry for
stringKey
Let
newSymbol
be a new unique Symbol value whose [[Description]] value is
stringKey
Append the
Record
{ [[Key]]:
stringKey
, [[Symbol]]:
newSymbol
} to the GlobalSymbolRegistry
List
Return
newSymbol
The GlobalSymbolRegistry is a
List
that is globally available. It is shared by all realms. Prior to the
evaluation of any ECMAScript code it is initialized as a new empty
List
. Elements of the GlobalSymbolRegistry are Records with the structure defined in
Table 48
Table 48: GlobalSymbolRegistry
Record
Fields
Field Name
Value
Usage
[[Key]]
A String
A string key used to globally identify a Symbol.
[[Symbol]]
A Symbol
A symbol that can be retrieved from any
realm
19.4.2.3
Symbol.hasInstance
The initial value of
Symbol.hasInstance
is the well-known symbol @@hasInstance (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.4
Symbol.isConcatSpreadable
The initial value of
Symbol.isConcatSpreadable
is the well-known symbol @@isConcatSpreadable (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.5
Symbol.iterator
The initial value of
Symbol.iterator
is the well-known symbol @@iterator (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.6
Symbol.keyFor (
sym
When
Symbol.keyFor
is called with argument
sym
it performs the following steps:
If
Type
sym
) is not Symbol, throw a
TypeError
exception.
For each element
of the GlobalSymbolRegistry
List
(see
19.4.2.2
), do
If
SameValue
.[[Symbol]],
sym
) is
true
, return
.[[Key]].
Assert
: GlobalSymbolRegistry does not currently contain an entry for
sym
Return
undefined
19.4.2.7
Symbol.match
The initial value of
Symbol.match
is the well-known symbol @@match (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.8
Symbol.prototype
The initial value of
Symbol.prototype
is the intrinsic object
%SymbolPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.9
Symbol.replace
The initial value of
Symbol.replace
is the well-known symbol @@replace (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.10
Symbol.search
The initial value of
Symbol.search
is the well-known symbol @@search (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.11
Symbol.species
The initial value of
Symbol.species
is the well-known symbol @@species (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.12
Symbol.split
The initial value of
Symbol.split
is the well-known symbol @@split (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.13
Symbol.toPrimitive
The initial value of
Symbol.toPrimitive
is the well-known symbol @@toPrimitive (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.14
Symbol.toStringTag
The initial value of
Symbol.toStringTag
is the well-known symbol @@toStringTag (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.2.15
Symbol.unscopables
The initial value of
Symbol.unscopables
is the well-known symbol @@unscopables (
Table 1
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.4.3
Properties of the Symbol Prototype Object
The Symbol prototype object:
is the intrinsic object
%SymbolPrototype%
is an ordinary object.
is not a Symbol instance and does not have a [[SymbolData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
The abstract operation
thisSymbolValue
value
) performs the following steps:
If
Type
value
) is Symbol, return
value
If
Type
value
) is Object and
value
has a [[SymbolData]] internal slot, then
Let
be
value
.[[SymbolData]].
Assert
Type
) is Symbol.
Return
Throw a
TypeError
exception.
19.4.3.1
Symbol.prototype.constructor
The initial value of
Symbol.prototype.constructor
is the intrinsic object
%Symbol%
19.4.3.2
get Symbol.prototype.description
Symbol.prototype.description
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
Let
sym
be ?
thisSymbolValue
).
Return
sym
.[[Description]].
19.4.3.3
Symbol.prototype.toString ( )
The following steps are taken:
Let
sym
be ?
thisSymbolValue
this
value).
Return
SymbolDescriptiveString
sym
).
19.4.3.3.1
Runtime Semantics: SymbolDescriptiveString (
sym
When the abstract operation SymbolDescriptiveString is called with argument
sym
, the following steps are taken:
Assert
Type
sym
) is Symbol.
Let
desc
be
sym
's [[Description]] value.
If
desc
is
undefined
, set
desc
to the empty string.
Assert
Type
desc
) is String.
Return the
string-concatenation
of
"Symbol("
desc
, and
")"
19.4.3.4
Symbol.prototype.valueOf ( )
The following steps are taken:
Return ?
thisSymbolValue
this
value).
19.4.3.5
Symbol.prototype [ @@toPrimitive ] (
hint
This function is called by ECMAScript language operators to
convert a Symbol object to a primitive value. The allowed values for
hint
are
"default"
"number"
, and
"string"
When the
@@toPrimitive
method is called with argument
hint
, the following steps are taken:
Return ?
thisSymbolValue
this
value).
The value of the
name
property of this function is
"[Symbol.toPrimitive]"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
19.4.3.6
Symbol.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Symbol"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
19.4.4
Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties
from the Symbol prototype object. Symbol instances have a [[SymbolData]]
internal slot. The [[SymbolData]] internal slot is the Symbol value
represented by this Symbol object.
19.5
Error Objects
Instances of Error objects are thrown as exceptions when runtime
errors occur. The Error objects may also serve as base objects for
user-defined exception classes.
19.5.1
The Error Constructor
The Error
constructor
is the intrinsic object
%Error%
is the initial value of the
Error
property of the
global object
creates and initializes a new Error object when called as a function rather than as a
constructor
. Thus the function call
Error(…)
is equivalent to the object creation expression
new Error(…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Error
behaviour must include a
super
call to the
Error
constructor
to create and initialize subclass instances with an [[ErrorData]] internal slot.
19.5.1.1
Error (
message
When the
Error
function is called with argument
message
, the following steps are taken:
If NewTarget is
undefined
, let
newTarget
be the
active function object
, else let
newTarget
be NewTarget.
Let
be ?
OrdinaryCreateFromConstructor
newTarget
"%ErrorPrototype%"
, « [[ErrorData]] »).
If
message
is not
undefined
, then
Let
msg
be ?
ToString
message
).
Let
msgDesc
be the PropertyDescriptor { [[Value]]:
msg
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Perform !
DefinePropertyOrThrow
"message"
msgDesc
).
Return
19.5.2
Properties of the Error Constructor
The Error
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
19.5.2.1
Error.prototype
The initial value of
Error.prototype
is the intrinsic object
%ErrorPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.5.3
Properties of the Error Prototype Object
The Error prototype object:
is the intrinsic object
%ErrorPrototype%
is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
19.5.3.1
Error.prototype.constructor
The initial value of
Error.prototype.constructor
is the intrinsic object
%Error%
19.5.3.2
Error.prototype.message
The initial value of
Error.prototype.message
is the empty String.
19.5.3.3
Error.prototype.name
The initial value of
Error.prototype.name
is
"Error"
19.5.3.4
Error.prototype.toString ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
name
be ?
Get
"name"
).
If
name
is
undefined
, set
name
to
"Error"
; otherwise set
name
to ?
ToString
name
).
Let
msg
be ?
Get
"message"
).
If
msg
is
undefined
, set
msg
to the empty String; otherwise set
msg
to ?
ToString
msg
).
If
name
is the empty String, return
msg
If
msg
is the empty String, return
name
Return the
string-concatenation
of
name
, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and
msg
19.5.4
Properties of Error Instances
Error instances are ordinary objects that inherit properties
from the Error prototype object and have an [[ErrorData]] internal slot
whose value is
undefined
. The only specified uses of [[ErrorData]] is to identify Error and
NativeError
instances as Error objects within
Object.prototype.toString
19.5.5
Native Error Types Used in This Standard
A new instance of one of the
NativeError
objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in
19.5.6
19.5.5.1
EvalError
This exception is not currently used within this
specification. This object remains for compatibility with previous
editions of this specification.
19.5.5.2
RangeError
Indicates a value that is not in the set or range of allowable values.
19.5.5.3
ReferenceError
Indicate that an invalid reference value has been detected.
19.5.5.4
SyntaxError
Indicates that a parsing error has occurred.
19.5.5.5
TypeError
TypeError is used to indicate an unsuccessful operation when none of the other
NativeError
objects are an appropriate indication of the failure cause.
19.5.5.6
URIError
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.
19.5.6
NativeError
Object Structure
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError
objects defined in
19.5.5
. Each of these objects has the structure described below, differing only in the name used as the
constructor
name instead of
NativeError
, in the
name
property of the prototype object, and in the implementation-defined
message
property of the prototype object.
For each error object, references to
NativeError
in the definition should be replaced with the appropriate error object name from
19.5.5
19.5.6.1
The
NativeError
Constructors
Each
NativeError
constructor
creates and initializes a new
NativeError
object when called as a function rather than as a
constructor
. A call of the object as a function is equivalent to calling it as a
constructor
with the same arguments. Thus the function call
NativeError
(…)
is equivalent to the object creation expression
new
NativeError
(…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
NativeError
behaviour must include a
super
call to the
NativeError
constructor
to create and initialize subclass instances with an [[ErrorData]] internal slot.
19.5.6.1.1
NativeError (
message
When a
NativeError
function is called with argument
message
, the following steps are taken:
If NewTarget is
undefined
, let
newTarget
be the
active function object
, else let
newTarget
be NewTarget.
Let
be ?
OrdinaryCreateFromConstructor
newTarget
"%
NativeError
Prototype%"
, « [[ErrorData]] »).
If
message
is not
undefined
, then
Let
msg
be ?
ToString
message
).
Let
msgDesc
be the PropertyDescriptor { [[Value]]:
msg
, [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Perform !
DefinePropertyOrThrow
"message"
msgDesc
).
Return
The actual value of the string passed in step 2 is either
"%EvalErrorPrototype%"
"%RangeErrorPrototype%"
"%ReferenceErrorPrototype%"
"%SyntaxErrorPrototype%"
"%TypeErrorPrototype%"
, or
"%URIErrorPrototype%"
corresponding to which
NativeError
constructor
is being defined.
19.5.6.2
Properties of the
NativeError
Constructors
Each
NativeError
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%Error%
has a
name
property whose value is the String value `"
NativeError
"`.
has the following properties:
19.5.6.2.1
NativeError.prototype
The initial value of
NativeError
.prototype
is a
NativeError
prototype object (
19.5.6.3
). Each
NativeError
constructor
has a distinct prototype object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
19.5.6.3
Properties of the
NativeError
Prototype Objects
Each
NativeError
prototype object:
is an ordinary object.
is not an Error instance and does not have an [[ErrorData]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ErrorPrototype%
19.5.6.3.1
NativeError
.prototype.constructor
The initial value of the
constructor
property of the prototype for a given
NativeError
constructor
is the corresponding intrinsic object %
NativeError
% (
19.5.6.1
).
19.5.6.3.2
NativeError
.prototype.message
The initial value of the
message
property of the prototype for a given
NativeError
constructor
is the empty String.
19.5.6.3.3
NativeError
.prototype.name
The initial value of the
name
property of the prototype for a given
NativeError
constructor
is the String value consisting of the name of the
constructor
(the name used instead of
NativeError
).
19.5.6.4
Properties of
NativeError
Instances
NativeError
instances are ordinary objects that inherit properties from their
NativeError
prototype object and have an [[ErrorData]] internal slot whose value is
undefined
. The only specified use of [[ErrorData]] is by
Object.prototype.toString
19.1.3.6
) to identify Error or
NativeError
instances.
20
Numbers and Dates
20.1
Number Objects
20.1.1
The Number Constructor
The Number
constructor
is the intrinsic object
%Number%
is the initial value of the
Number
property of the
global object
creates and initializes a new Number object when called as a
constructor
performs a type conversion when called as a function rather than as a
constructor
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Number
behaviour must include a
super
call to the
Number
constructor
to create and initialize the subclass instance with a [[NumberData]] internal slot.
20.1.1.1
Number (
value
When
Number
is called with argument
value
, the following steps are taken:
If no arguments were passed to this function invocation, let
be
+0
Else, let
be ?
ToNumber
value
).
If NewTarget is
undefined
, return
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%NumberPrototype%"
, « [[NumberData]] »).
Set
.[[NumberData]] to
Return
20.1.2
Properties of the Number Constructor
The Number
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
20.1.2.1
Number.EPSILON
The value of Number.EPSILON is the difference between 1 and
the smallest value greater than 1 that is representable as a Number
value, which is approximately 2.2204460492503130808472633361816 x 10
- 16
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.2
Number.isFinite (
number
When
Number.isFinite
is called with one argument
number
, the following steps are taken:
If
Type
number
) is not Number, return
false
If
number
is
NaN
+∞
, or
-∞
, return
false
Otherwise, return
true
20.1.2.3
Number.isInteger (
number
When
Number.isInteger
is called with one argument
number
, the following steps are taken:
If
Type
number
) is not Number, return
false
If
number
is
NaN
+∞
, or
-∞
, return
false
Let
integer
be !
ToInteger
number
).
If
integer
is not equal to
number
, return
false
Otherwise, return
true
20.1.2.4
Number.isNaN (
number
When
Number.isNaN
is called with one argument
number
, the following steps are taken:
If
Type
number
) is not Number, return
false
If
number
is
NaN
, return
true
Otherwise, return
false
Note
This function differs from the global isNaN function (
18.2.3
) in that it does not convert its argument to a Number before determining whether it is
NaN
20.1.2.5
Number.isSafeInteger (
number
When
Number.isSafeInteger
is called with one argument
number
, the following steps are taken:
If
Type
number
) is not Number, return
false
If
number
is
NaN
+∞
, or
-∞
, return
false
Let
integer
be !
ToInteger
number
).
If
integer
is not equal to
number
, return
false
If
abs
integer
) ≤ 2
53
- 1, return
true
Otherwise, return
false
20.1.2.6
Number.MAX_SAFE_INTEGER
Note
The value of
Number.MAX_SAFE_INTEGER
is the largest integer n such that n and n + 1 are both exactly representable as a Number value.
The value of Number.MAX_SAFE_INTEGER is 9007199254740991 (2
53
- 1).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.7
Number.MAX_VALUE
The value of
Number.MAX_VALUE
is the largest positive finite value of the Number type, which is approximately
1.7976931348623157 × 10
308
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.8
Number.MIN_SAFE_INTEGER
Note
The value of
Number.MIN_SAFE_INTEGER
is the smallest integer n such that n and n - 1 are both exactly representable as a Number value.
The value of Number.MIN_SAFE_INTEGER is -9007199254740991 (-(2
53
- 1)).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.9
Number.MIN_VALUE
The value of
Number.MIN_VALUE
is the smallest positive value of the Number type, which is approximately
5 × 10
-324
In the IEEE 754-2008 double precision binary representation,
the smallest possible value is a denormalized number. If an
implementation does not support denormalized values, the value of
Number.MIN_VALUE
must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.10
Number.NaN
The value of
Number.NaN
is
NaN
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.11
Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is
-∞
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.12
Number.parseFloat (
string
The value of the
Number.parseFloat
data property
is the same built-in
function object
that is the value of the
parseFloat
property of the
global object
defined in
18.2.4
20.1.2.13
Number.parseInt (
string
radix
The value of the
Number.parseInt
data property
is the same built-in
function object
that is the value of the
parseInt
property of the
global object
defined in
18.2.5
20.1.2.14
Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is
+∞
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.2.15
Number.prototype
The initial value of
Number.prototype
is the intrinsic object
%NumberPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.1.3
Properties of the Number Prototype Object
The Number prototype object:
is the intrinsic object
%NumberPrototype%
is an ordinary object.
is itself a Number object; it has a [[NumberData]] internal slot with the value
+0
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the
this
value passed to them must be either a Number value or an object that
has a [[NumberData]] internal slot that has been initialized to a Number
value.
The abstract operation
thisNumberValue
value
) performs the following steps:
If
Type
value
) is Number, return
value
If
Type
value
) is Object and
value
has a [[NumberData]] internal slot, then
Let
be
value
.[[NumberData]].
Assert
Type
) is Number.
Return
Throw a
TypeError
exception.
The phrase “this Number value” within the specification of a
method refers to the result returned by calling the abstract operation
thisNumberValue
with the
this
value of the method invocation passed as the argument.
20.1.3.1
Number.prototype.constructor
The initial value of
Number.prototype.constructor
is the intrinsic object
%Number%
20.1.3.2
Number.prototype.toExponential (
fractionDigits
Return a String containing this Number value represented in
decimal exponential notation with one digit before the significand's
decimal point and
fractionDigits
digits after the significand's decimal point. If
fractionDigits
is
undefined
, include as many significand digits as necessary to uniquely specify the Number (just like in
ToString
except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:
Let
be ?
thisNumberValue
this
value).
Let
be ?
ToInteger
fractionDigits
).
Assert
: If
fractionDigits
is
undefined
, then
is 0.
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
< 0, then
Set
to
"-"
Set
to -
If
+∞
, then
Return the
string-concatenation
of
and
"Infinity"
If
< 0 or
> 100, throw a
RangeError
exception.
If
= 0, then
Let
be the String value consisting of
+ 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let
be 0.
Else
≠ 0,
If
fractionDigits
is not
undefined
, then
Let
and
be integers such that 10
< 10
+ 1
and for which the exact mathematical value of
× 10
is as close to zero as possible. If there are two such sets of
and
, pick the
and
for which
× 10
is larger.
Else
fractionDigits
is
undefined
Let
, and
be integers such that
≥ 0, 10
< 10
+ 1
, the Number value for
× 10
is
, and
is as small as possible. Note that the decimal representation of
has
+ 1 digits,
is not divisible by 10, and the least significant digit of
is not necessarily uniquely determined by these criteria.
Let
be the String value consisting of the digits of the decimal representation of
(in order, with no leading zeroes).
If
≠ 0, then
Let
be the first code unit of
, and let
be the remaining
code units of
Set
to the
string-concatenation
of
"."
, and
If
= 0, then
Let
be
"+"
Let
be
"0"
Else,
If
> 0, let
be
"+"
Else
≤ 0,
Let
be
"-"
Set
to -
Let
be the String value consisting of the digits of the decimal representation of
(in order, with no leading zeroes).
Set
to the
string-concatenation
of
"e"
, and
Return the
string-concatenation
of
and
Note
For implementations that provide more accurate conversions
than required by the rules above, it is recommended that the following
alternative version of step 10.b.i be used as a guideline:
Let
, and
be integers such that
≥ 0, 10
< 10
+ 1
, the Number value for
× 10
is
, and
is as small as possible. If there are multiple possibilities for
, choose the value of
for which
× 10
is closest in value to
. If there are two such possible values of
, choose the one that is even.
20.1.3.3
Number.prototype.toFixed (
fractionDigits
Note 1
toFixed
returns a String containing this Number value represented in decimal fixed-point notation with
fractionDigits
digits after the decimal point. If
fractionDigits
is
undefined
, 0 is assumed.
The following steps are performed:
Let
be ?
thisNumberValue
this
value).
Let
be ?
ToInteger
fractionDigits
).
Assert
: If
fractionDigits
is
undefined
, then
is 0.
If
< 0 or
> 100, throw a
RangeError
exception.
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
< 0, then
Set
to
"-"
Set
to -
If
≥ 10
21
, then
Let
be !
ToString
).
Else
< 10
21
Let
be an integer for which the exact mathematical value of
÷ 10
is as close to zero as possible. If there are two such
, pick the larger
If
= 0, let
be the String
"0"
. Otherwise, let
be the String value consisting of the digits of the decimal representation of
(in order, with no leading zeroes).
If
≠ 0, then
Let
be the length of
If
, then
Let
be the String value consisting of
+ 1 -
occurrences of the code unit 0x0030 (DIGIT ZERO).
Set
to the
string-concatenation
of
and
Set
to
+ 1.
Let
be the first
code units of
, and let
be the remaining
code units of
Set
to the
string-concatenation
of
"."
, and
Return the
string-concatenation
of
and
Note 2
The output of
toFixed
may be more precise than
toString
for some values because toString only prints enough significant digits
to distinguish the number from adjacent number values. For example,
(1000000000000000128).toString()
returns
"1000000000000000100"
, while
(1000000000000000128).toFixed(0)
returns
"1000000000000000128"
20.1.3.4
Number.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Number.prototype.toLocaleString
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleString
method is used.
Produces a String value that represents this Number value
formatted according to the conventions of the host environment's current
locale. This function is implementation-dependent, and it is
permissible, but not encouraged, for it to return the same thing as
toString
The meanings of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
20.1.3.5
Number.prototype.toPrecision (
precision
Return a String containing this Number value represented
either in decimal exponential notation with one digit before the
significand's decimal point and
precision
- 1
digits after the significand's decimal point or in decimal fixed notation with
precision
significant digits. If
precision
is
undefined
, call
ToString
instead. Specifically, perform the following steps:
Let
be ?
thisNumberValue
this
value).
If
precision
is
undefined
, return !
ToString
).
Let
be ?
ToInteger
precision
).
If
is
NaN
, return the String
"NaN"
Let
be the empty String.
If
< 0, then
Set
to the code unit 0x002D (HYPHEN-MINUS).
Set
to -
If
+∞
, then
Return the
string-concatenation
of
and
"Infinity"
If
< 1 or
> 100, throw a
RangeError
exception.
If
= 0, then
Let
be the String value consisting of
occurrences of the code unit 0x0030 (DIGIT ZERO).
Let
be 0.
Else
≠ 0,
Let
and
be integers such that 10
- 1
< 10
and for which the exact mathematical value of
× 10
+ 1
is as close to zero as possible. If there are two such sets of
and
, pick the
and
for which
× 10
+ 1
is larger.
Let
be the String value consisting of the digits of the decimal representation of
(in order, with no leading zeroes).
If
< -6 or
, then
Assert
≠ 0.
If
≠ 1, then
Let
be the first code unit of
, and let
be the remaining
- 1 code units of
Set
to the
string-concatenation
of
"."
, and
If
> 0, then
Let
be the code unit 0x002B (PLUS SIGN).
Else
< 0,
Let
be the code unit 0x002D (HYPHEN-MINUS).
Set
to -
Let
be the String value consisting of the digits of the decimal representation of
(in order, with no leading zeroes).
Return the
string-concatenation
of
, the code unit 0x0065 (LATIN SMALL LETTER E),
, and
If
- 1, return the
string-concatenation
of
and
If
≥ 0, then
Set
to the
string-concatenation
of the first
+ 1 code units of
, the code unit 0x002E (FULL STOP), and the remaining
- (
+ 1) code units of
Else
< 0,
Set
to the
string-concatenation
of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(
+ 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String
Return the
string-concatenation
of
and
20.1.3.6
Number.prototype.toString ( [
radix
] )
Note
The optional
radix
should be an integer value in the inclusive range 2 to 36. If
radix
is not present or is
undefined
the Number 10 is used as the value of
radix
The following steps are performed:
Let
be ?
thisNumberValue
this
value).
If
radix
is not present, let
radixNumber
be 10.
Else if
radix
is
undefined
, let
radixNumber
be 10.
Else, let
radixNumber
be ?
ToInteger
radix
).
If
radixNumber
< 2 or
radixNumber
> 36, throw a
RangeError
exception.
If
radixNumber
= 10, return !
ToString
).
Return the String representation of this Number value using the radix specified by
radixNumber
. Letters
are used for digits with values 10 through 35. The precise algorithm is
implementation-dependent, however the algorithm should be a
generalization of that specified in
7.1.12.1
The
toString
function is not generic; it throws a
TypeError
exception if its
this
value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
The
"length"
property of the
toString
method is 1.
20.1.3.7
Number.prototype.valueOf ( )
Return ?
thisNumberValue
this
value).
20.1.4
Properties of Number Instances
Number instances are ordinary objects that inherit properties
from the Number prototype object. Number instances also have a
[[NumberData]] internal slot. The [[NumberData]] internal slot is the
Number value represented by this Number object.
20.2
The Math Object
The Math object:
is the intrinsic object
%Math%
is the initial value of the
Math
property of the
global object
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is not a
function object
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “the Number value for
” has a technical meaning defined in
6.1.6
20.2.1
Value Properties of the Math Object
20.2.1.1
Math.E
The Number value for
, the base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.2.1.2
Math.LN10
The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.2.1.3
Math.LN2
The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.2.1.4
Math.LOG10E
The Number value for the base-10 logarithm of
, the base of the natural logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
The value of
Math.LOG10E
is approximately the reciprocal of the value of
Math.LN10
20.2.1.5
Math.LOG2E
The Number value for the base-2 logarithm of
, the base of the natural logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
The value of
Math.LOG2E
is approximately the reciprocal of the value of
Math.LN2
20.2.1.6
Math.PI
The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.2.1.7
Math.SQRT1_2
The Number value for the square root of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
The value of
Math.SQRT1_2
is approximately the reciprocal of the value of
Math.SQRT2
20.2.1.8
Math.SQRT2
The Number value for the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.2.1.9
Math [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Math"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
20.2.2
Function Properties of the Math Object
Each of the following
Math
object functions applies the
ToNumber
abstract operation to each of its arguments (in left-to-right order if there is more than one). If
ToNumber
returns an
abrupt completion
, that
Completion Record
is immediately returned. Otherwise, the function performs a computation
on the resulting Number value(s). The value returned by each function
is a Number.
In the function descriptions below, the symbols
NaN
-0
+0
-∞
and
+∞
refer to the Number values described in
6.1.6
Note
The behaviour of the functions
acos
acosh
asin
asinh
atan
atanh
atan2
cbrt
cos
cosh
exp
expm1
hypot
log
log1p
log2
log10
pow
random
sin
sinh
sqrt
tan
, and
tanh
is not precisely specified here except to require specific results for
certain argument values that represent boundary cases of interest. For
other argument values, these functions are intended to compute
approximations to the results of familiar mathematical functions, but
some latitude is allowed in the choice of approximation algorithms. The
general intent is that an implementer should be able to use the same
mathematical library for ECMAScript on a given hardware platform that is
available to C programmers on that platform.
Although the choice of algorithms is left to the
implementation, it is recommended (but not specified by this standard)
that implementations use the approximation algorithms for IEEE 754-2008
arithmetic contained in
fdlibm
, the freely distributable mathematical library from Sun Microsystems (
).
20.2.2.1
Math.abs (
Returns the absolute value of
; the result has the same magnitude as
but has positive sign.
If
is
NaN
, the result is
NaN
If
is
-0
, the result is
+0
If
is
-∞
, the result is
+∞
20.2.2.2
Math.acos (
Returns an implementation-dependent approximation to the arc cosine of
. The result is expressed in radians and ranges from
+0
to +π.
If
is
NaN
, the result is
NaN
If
is greater than 1, the result is
NaN
If
is less than -1, the result is
NaN
If
is exactly 1, the result is
+0
20.2.2.3
Math.acosh (
Returns an implementation-dependent approximation to the inverse hyperbolic cosine of
If
is
NaN
, the result is
NaN
If x is less than 1, the result is
NaN
If x is 1, the result is
+0
If
is
+∞
, the result is
+∞
20.2.2.4
Math.asin (
Returns an implementation-dependent approximation to the arc sine of
. The result is expressed in radians and ranges from -π / 2 to +π / 2.
If
is
NaN
, the result is
NaN
If
is greater than 1, the result is
NaN
If
is less than -1, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
20.2.2.5
Math.asinh (
Returns an implementation-dependent approximation to the inverse hyperbolic sine of
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If x is
-∞
, the result is
-∞
20.2.2.6
Math.atan (
Returns an implementation-dependent approximation to the arc tangent of
. The result is expressed in radians and ranges from -π / 2 to +π / 2.
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is an implementation-dependent approximation to +π / 2.
If
is
-∞
, the result is an implementation-dependent approximation to -π / 2.
20.2.2.7
Math.atanh (
Returns an implementation-dependent approximation to the inverse hyperbolic tangent of
If
is
NaN
, the result is
NaN
If
is less than -1, the result is
NaN
If
is greater than 1, the result is
NaN
If
is -1, the result is
-∞
If
is +1, the result is
+∞
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
20.2.2.8
Math.atan2 (
Returns an implementation-dependent approximation to the arc tangent of the quotient
of the arguments
and
, where the signs of
and
are used to determine the quadrant of the result. Note that it is
intentional and traditional for the two-argument arc tangent function
that the argument named
be first and the argument named
be second. The result is expressed in radians and ranges from -π to +π.
If either
or
is
NaN
, the result is
NaN
If
> 0 and
is
+0
, the result is an implementation-dependent approximation to +π / 2.
If
> 0 and
is
-0
, the result is an implementation-dependent approximation to +π / 2.
If
is
+0
and
> 0, the result is
+0
If
is
+0
and
is
+0
, the result is
+0
If
is
+0
and
is
-0
, the result is an implementation-dependent approximation to +π.
If
is
+0
and
< 0, the result is an implementation-dependent approximation to +π.
If
is
-0
and
> 0, the result is
-0
If
is
-0
and
is
+0
, the result is
-0
If
is
-0
and
is
-0
, the result is an implementation-dependent approximation to -π.
If
is
-0
and
< 0, the result is an implementation-dependent approximation to -π.
If
< 0 and
is
+0
, the result is an implementation-dependent approximation to -π / 2.
If
< 0 and
is
-0
, the result is an implementation-dependent approximation to -π / 2.
If
> 0 and
is finite and
is
+∞
, the result is
+0
If
> 0 and
is finite and
is
-∞
, the result is an implementation-dependent approximation to +π.
If
< 0 and
is finite and
is
+∞
, the result is
-0
If
< 0 and
is finite and
is
-∞
, the result is an implementation-dependent approximation to -π.
If
is
+∞
and
is finite, the result is an implementation-dependent approximation to +π / 2.
If
is
-∞
and
is finite, the result is an implementation-dependent approximation to -π / 2.
If
is
+∞
and
is
+∞
, the result is an implementation-dependent approximation to +π / 4.
If
is
+∞
and
is
-∞
, the result is an implementation-dependent approximation to +3π / 4.
If
is
-∞
and
is
+∞
, the result is an implementation-dependent approximation to -π / 4.
If
is
-∞
and
is
-∞
, the result is an implementation-dependent approximation to -3π / 4.
20.2.2.9
Math.cbrt (
Returns an implementation-dependent approximation to the cube root of
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
20.2.2.10
Math.ceil (
Returns the smallest (closest to
-∞
) Number value that is not less than
and is equal to a mathematical integer. If
is already an integer, the result is
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
If
is less than 0 but greater than -1, the result is
-0
The value of
Math.ceil(x)
is the same as the value of
-Math.floor(-x)
20.2.2.11
Math.clz32 (
When
Math.clz32
is called with one argument
, the following steps are taken:
Let
be ?
ToUint32
).
Let
be the number of leading zero bits in the 32-bit binary representation of
Return
Note
If
is 0,
will be 32. If the most significant bit of the 32-bit binary encoding of
is 1,
will be 0.
20.2.2.12
Math.cos (
Returns an implementation-dependent approximation to the cosine of
. The argument is expressed in radians.
If
is
NaN
, the result is
NaN
If
is
+0
, the result is 1.
If
is
-0
, the result is 1.
If
is
+∞
, the result is
NaN
If
is
-∞
, the result is
NaN
20.2.2.13
Math.cosh (
Returns an implementation-dependent approximation to the hyperbolic cosine of
If
is
NaN
, the result is
NaN
If
is
+0
, the result is 1.
If
is
-0
, the result is 1.
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
+∞
Note
The value of cosh(x) is the same as
(exp(x) + exp(-x)) / 2
20.2.2.14
Math.exp (
Returns an implementation-dependent approximation to the exponential function of
raised to the power of
, where
is the base of the natural logarithms).
If
is
NaN
, the result is
NaN
If
is
+0
, the result is 1.
If
is
-0
, the result is 1.
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
+0
20.2.2.15
Math.expm1 (
Returns an implementation-dependent approximation to subtracting 1 from the exponential function of
raised to the power of
, where
is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close 0.
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is -1.
20.2.2.16
Math.floor (
Returns the greatest (closest to
+∞
) Number value that is not greater than
and is equal to a mathematical integer. If
is already an integer, the result is
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
If
is greater than 0 but less than 1, the result is
+0
Note
The value of
Math.floor(x)
is the same as the value of
-Math.ceil(-x)
20.2.2.17
Math.fround (
When
Math.fround
is called with argument
, the following steps are taken:
If
is
NaN
, return
NaN
If
is one of
+0
-0
+∞
-∞
, return
Let
x32
be the result of converting
to a value in IEEE 754-2008 binary32 format using roundTiesToEven.
Let
x64
be the result of converting
x32
to a value in IEEE 754-2008 binary64 format.
Return the ECMAScript Number value corresponding to
x64
20.2.2.18
Math.hypot (
value1
value2
, ...
values
Math.hypot
returns an implementation-dependent approximation of the square root of the sum of squares of its arguments.
If no arguments are passed, the result is
+0
If any argument is
+∞
, the result is
+∞
If any argument is
-∞
, the result is
+∞
If no argument is
+∞
or
-∞
, and any argument is
NaN
, the result is
NaN
If all arguments are either
+0
or
-0
, the result is
+0
Note
Implementations should take care to avoid the loss of
precision from overflows and underflows that are prone to occur in naive
implementations when this function is called with two or more
arguments.
20.2.2.19
Math.imul (
When
Math.imul
is called with arguments
and
, the following steps are taken:
Let
be ?
ToUint32
).
Let
be ?
ToUint32
).
Let
product
be (
modulo
32
If
product
≥ 2
31
, return
product
- 2
32
; otherwise return
product
20.2.2.20
Math.log (
Returns an implementation-dependent approximation to the natural logarithm of
If
is
NaN
, the result is
NaN
If
is less than 0, the result is
NaN
If
is
+0
or
-0
, the result is
-∞
If
is 1, the result is
+0
If
is
+∞
, the result is
+∞
20.2.2.21
Math.log1p (
Returns an implementation-dependent approximation to the natural logarithm of 1 +
. The result is computed in a way that is accurate even when the value of x is close to zero.
If
is
NaN
, the result is
NaN
If
is less than -1, the result is
NaN
If x is -1, the result is
-∞
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
20.2.2.22
Math.log10 (
Returns an implementation-dependent approximation to the base 10 logarithm of
If
is
NaN
, the result is
NaN
If
is less than 0, the result is
NaN
If
is
+0
, the result is
-∞
If
is
-0
, the result is
-∞
If
is 1, the result is
+0
If
is
+∞
, the result is
+∞
20.2.2.23
Math.log2 (
Returns an implementation-dependent approximation to the base 2 logarithm of
If
is
NaN
, the result is
NaN
If
is less than 0, the result is
NaN
If
is
+0
, the result is
-∞
If
is
-0
, the result is
-∞
If
is 1, the result is
+0
If
is
+∞
, the result is
+∞
20.2.2.24
Math.max (
value1
value2
, ...
values
Given zero or more arguments, calls
ToNumber
on each of the arguments and returns the largest of the resulting values.
If no arguments are given, the result is
-∞
If any value is
NaN
, the result is
NaN
The comparison of values to determine the largest value is done using the
Abstract Relational Comparison
algorithm except that
+0
is considered to be larger than
-0
20.2.2.25
Math.min (
value1
value2
, ...
values
Given zero or more arguments, calls
ToNumber
on each of the arguments and returns the smallest of the resulting values.
If no arguments are given, the result is
+∞
If any value is
NaN
, the result is
NaN
The comparison of values to determine the smallest value is done using the
Abstract Relational Comparison
algorithm except that
+0
is considered to be larger than
-0
20.2.2.26
Math.pow (
base
exponent
Return the result of
Applying the ** operator
with
base
and
exponent
as specified in
12.6.4
20.2.2.27
Math.random ( )
Returns a Number value with positive sign, greater than or
equal to 0 but less than 1, chosen randomly or pseudo randomly with
approximately uniform distribution over that range, using an
implementation-dependent algorithm or strategy. This function takes no
arguments.
Each
Math.random
function created for distinct realms must produce a distinct sequence of values from successive calls.
20.2.2.28
Math.round (
Returns the Number value that is closest to
and is equal to a mathematical integer. If two integer Number values are equally close to
, then the result is the Number value that is closer to
+∞
. If
is already an integer, the result is
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
If
is greater than 0 but less than 0.5, the result is
+0
If
is less than 0 but greater than or equal to -0.5, the result is
-0
Note 1
Math.round(3.5)
returns 4, but
Math.round(-3.5)
returns -3.
Note 2
The value of
Math.round(x)
is not always the same as the value of
Math.floor(x + 0.5)
. When
is
-0
or is less than 0 but greater than or equal to -0.5,
Math.round(x)
returns
-0
, but
Math.floor(x + 0.5)
returns
+0
Math.round(x)
may also differ from the value of
Math.floor(x + 0.5)
because of internal rounding when computing
x + 0.5
20.2.2.29
Math.sign (
Returns the sign of
, indicating whether
is positive, negative, or zero.
If
is
NaN
, the result is
NaN
If
is
-0
, the result is
-0
If
is
+0
, the result is
+0
If
is negative and not
-0
, the result is -1.
If
is positive and not
+0
, the result is +1.
20.2.2.30
Math.sin (
Returns an implementation-dependent approximation to the sine of
. The argument is expressed in radians.
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
or
-∞
, the result is
NaN
20.2.2.31
Math.sinh (
Returns an implementation-dependent approximation to the hyperbolic sine of
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
Note
The value of sinh(x) is the same as
(exp(x) - exp(-x)) / 2
20.2.2.32
Math.sqrt (
Returns an implementation-dependent approximation to the square root of
If
is
NaN
, the result is
NaN
If
is less than 0, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is
+∞
20.2.2.33
Math.tan (
Returns an implementation-dependent approximation to the tangent of
. The argument is expressed in radians.
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
or
-∞
, the result is
NaN
20.2.2.34
Math.tanh (
Returns an implementation-dependent approximation to the hyperbolic tangent of
If
is
NaN
, the result is
NaN
If
is
+0
, the result is
+0
If
is
-0
, the result is
-0
If
is
+∞
, the result is +1.
If
is
-∞
, the result is -1.
Note
The value of tanh(x) is the same as
(exp(x) - exp(-x))/(exp(x) + exp(-x))
20.2.2.35
Math.trunc (
Returns the integral part of the number
, removing any fractional digits. If
is already an integer, the result is
If
is
NaN
, the result is
NaN
If
is
-0
, the result is
-0
If
is
+0
, the result is
+0
If
is
+∞
, the result is
+∞
If
is
-∞
, the result is
-∞
If
is greater than 0 but less than 1, the result is
+0
If
is less than 0 but greater than -1, the result is
-0
20.3
Date Objects
20.3.1
Overview of Date Objects and Definitions of Abstract Operations
The following functions are
abstract operations
that operate on time values (defined in
20.3.1.1
). Note that, in every case, if any argument to one of these functions is
NaN
, the result will be
NaN
20.3.1.1
Time Values and Time Range
A Date object contains a Number representing an instant in time with millisecond precision. Such a Number is called a
time value
. A time value may also be
NaN
, indicating that the Date object does not represent a specific instant in time.
Time is measured in ECMAScript as milliseconds since midnight
at the beginning of 01 January, 1970 UTC. Time in ECMAScript does not
observe leap seconds; they are ignored. Time calculations assume each
and every day contains exactly
60 × 60 × 24 × 1000 = 86,400,000
milliseconds, to align with the POSIX specification of each and every day containing exactly 86,400 seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (
20.1.2.8
and
20.1.2.6
).
A time value supports a slightly smaller range of exactly -100,000,000
days to 100,000,000 days measured relative to midnight at the beginning
of 01 January, 1970 UTC. This yields an exact supported time value range
of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds
relative to midnight at the beginning of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the time value
+0
Note
The 400 year cycle of the Gregorian calendar contains 97
leap years. This yields an average of 365.2425 days per year, or an
average of 31,556,952,000 milliseconds per year under the Gregorian
calendar. ECMAScript applies a proleptic Gregorian calendar for all time
computations.
As specified by this section, the maximum year range a
Number can represent exactly with millisecond precision is approximately
-285,426 to 285,426 years relative to midnight at the beginning of 01
January, 1970 UTC.
As specified by this section, the maximum year range a time
value can represent is approximately -273,790 to 273,790 years relative
to midnight at the beginning of 01 January, 1970 UTC.
20.3.1.2
Day Number and Time within Day
A given
time value
belongs to day number
Day(
) =
floor
msPerDay
where the number of milliseconds per day is
msPerDay = 86400000
The remainder is called the time within the day:
TimeWithinDay(
) =
modulo
msPerDay
20.3.1.3
Year Number
ECMAScript uses a proleptic Gregorian calendar to map a day
number to a year number and to determine the month and date within that
year. In this calendar, leap years are precisely those which are
(divisible by 4) and ((not divisible by 100) or (divisible by 400)). The
number of days in year number
is therefore defined by
DaysInYear(
= 365 if (
modulo
4) ≠ 0
= 366 if (
modulo
4) = 0 and (
modulo
100) ≠ 0
= 365 if (
modulo
100) = 0 and (
modulo
400) ≠ 0
= 366 if (
modulo
400) = 0
All non-leap years have 365 days with the usual number of
days per month and leap years have an extra day in February. The day
number of the first day of year
is given by:
DayFromYear(
) = 365 × (
- 1970) +
floor
((
- 1969) / 4) -
floor
((
- 1901) / 100) +
floor
((
- 1601) / 400)
The
time value
of the start of a year is:
TimeFromYear(
) =
msPerDay
DayFromYear
time value
determines a year by:
YearFromTime(
) = the largest integer
(closest to positive infinity) such that
TimeFromYear
) ≤
The leap-year function is 1 for a time within a leap year and otherwise is zero:
InLeapYear(
= 0 if
DaysInYear
YearFromTime
)) = 365
= 1 if
DaysInYear
YearFromTime
)) = 366
20.3.1.4
Month Number
Months are identified by an integer in the range 0 to 11, inclusive. The mapping
MonthFromTime
) from a
time value
to a month number is defined by:
MonthFromTime(
= 0 if 0 ≤
DayWithinYear
) < 31
= 1 if 31 ≤
DayWithinYear
) < 59 +
InLeapYear
= 2 if 59 +
InLeapYear
) ≤
DayWithinYear
) < 90 +
InLeapYear
= 3 if 90 +
InLeapYear
) ≤
DayWithinYear
) < 120 +
InLeapYear
= 4 if 120 +
InLeapYear
) ≤
DayWithinYear
) < 151 +
InLeapYear
= 5 if 151 +
InLeapYear
) ≤
DayWithinYear
) < 181 +
InLeapYear
= 6 if 181 +
InLeapYear
) ≤
DayWithinYear
) < 212 +
InLeapYear
= 7 if 212 +
InLeapYear
) ≤
DayWithinYear
) < 243 +
InLeapYear
= 8 if 243 +
InLeapYear
) ≤
DayWithinYear
) < 273 +
InLeapYear
= 9 if 273 +
InLeapYear
) ≤
DayWithinYear
) < 304 +
InLeapYear
= 10 if 304 +
InLeapYear
) ≤
DayWithinYear
) < 334 +
InLeapYear
= 11 if 334 +
InLeapYear
) ≤
DayWithinYear
) < 365 +
InLeapYear
where
DayWithinYear(
) =
Day
) -
DayFromYear
YearFromTime
))
A month value of 0 specifies January; 1 specifies February; 2
specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6
specifies July; 7 specifies August; 8 specifies September; 9 specifies
October; 10 specifies November; and 11 specifies December. Note that
MonthFromTime
(0) = 0
, corresponding to Thursday, 01 January, 1970.
20.3.1.5
Date Number
A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(
) from a
time value
to a date number is defined by:
DateFromTime(
DayWithinYear
) + 1 if
MonthFromTime
) = 0
DayWithinYear
) - 30 if
MonthFromTime
) = 1
DayWithinYear
) - 58 -
InLeapYear
) if
MonthFromTime
) = 2
DayWithinYear
) - 89 -
InLeapYear
) if
MonthFromTime
) = 3
DayWithinYear
) - 119 -
InLeapYear
) if
MonthFromTime
) = 4
DayWithinYear
) - 150 -
InLeapYear
) if
MonthFromTime
) = 5
DayWithinYear
) - 180 -
InLeapYear
) if
MonthFromTime
) = 6
DayWithinYear
) - 211 -
InLeapYear
) if
MonthFromTime
) = 7
DayWithinYear
) - 242 -
InLeapYear
) if
MonthFromTime
) = 8
DayWithinYear
) - 272 -
InLeapYear
) if
MonthFromTime
) = 9
DayWithinYear
) - 303 -
InLeapYear
) if
MonthFromTime
) = 10
DayWithinYear
) - 333 -
InLeapYear
) if
MonthFromTime
) = 11
20.3.1.6
Week Day
The weekday for a particular
time value
is defined as
WeekDay(
) = (
Day
) + 4)
modulo
A weekday value of 0 specifies Sunday; 1 specifies Monday; 2
specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5
specifies Friday; and 6 specifies Saturday. Note that
WeekDay(0) = 4
, corresponding to Thursday, 01 January, 1970.
20.3.1.7
LocalTZA (
isUTC
LocalTZA(
isUTC
) is an
implementation-defined algorithm that must return a number representing
milliseconds suitable for adding to a Time Value. The local political
rules for standard time and daylight saving time in effect at
should be used to determine the result in the way specified in the following three paragraphs.
When
isUTC
is true,
LocalTZA(
, true )
should return the offset of the local time zone from UTC measured in milliseconds at time represented by
time value
(UTC). When the result is added to
(UTC), it should yield the local time.
When
isUTC
is false,
LocalTZA(
, false )
should return the offset of the local time zone from UTC measured in milliseconds at local time represented by
time value
local
. When the result is subtracted from the local time
local
, it should yield the corresponding UTC.
When
local
represents local time repeating multiple times at a negative time zone
transition (e.g. when the daylight saving time ends or the time zone
adjustment is decreased due to a time zone rule change) or skipped local
time at a positive time zone transitions (e.g. when the daylight saving
time starts or the time zone adjustment is increased due to a time zone
rule change),
local
must be interpreted with the time zone adjustment before the transition.
If an implementation does not support a conversion described above or if political rules for time
are not available within the implementation, the result must be 0.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database
1:30 AM on November 5, 2017 in America/New_York is repeated
twice (fall backward), but it must be interpreted as 1:30 AM UTC-04
instead of 1:30 AM UTC-05. LocalTZA(
TimeClip
MakeDate
MakeDay
(2017, 10, 5),
MakeTime
(1, 30, 0, 0))), false) is
-4 ×
msPerHour
2:30 AM on March 12, 2017 in America/New_York does not
exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30
AM UTC-04). LocalTZA(
TimeClip
MakeDate
MakeDay
(2017, 2, 12),
MakeTime
(2, 30, 0, 0))), false) is
-5 ×
msPerHour
20.3.1.8
LocalTime (
The abstract operation LocalTime with argument
converts
from UTC to local time by performing the following steps:
Return
LocalTZA
true
).
Note
Two different time values (
(UTC)) are converted to the same local time
local
at a negative time zone transition when there are repeated times (e.g.
the daylight saving time ends or the time zone adjustment is
decreased.).
20.3.1.9
UTC (
The abstract operation UTC with argument
converts
from local time to UTC. It performs the following steps:
Return
LocalTZA
false
).
Note
UTC(
LocalTime
))
is not necessarily always equal to
LocalTime
(UTC(
local
))
is not necessarily always equal to
local
, either.
20.3.1.10
Hours, Minutes, Second, and Milliseconds
The following
abstract operations
are useful in decomposing time values:
HourFromTime(
) =
floor
msPerHour
modulo
HoursPerDay
MinFromTime(
) =
floor
msPerMinute
modulo
MinutesPerHour
SecFromTime(
) =
floor
msPerSecond
modulo
SecondsPerMinute
msFromTime(
) =
modulo
msPerSecond
where
HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000
msPerMinute = 60000 =
msPerSecond
SecondsPerMinute
msPerHour = 3600000 =
msPerMinute
MinutesPerHour
20.3.1.11
MakeTime (
hour
min
sec
ms
The abstract operation MakeTime calculates a number of
milliseconds from its four arguments, which must be ECMAScript Number
values. This operator functions as follows:
If
hour
is not finite or
min
is not finite or
sec
is not finite or
ms
is not finite, return
NaN
Let
be !
ToInteger
hour
).
Let
be !
ToInteger
min
).
Let
be !
ToInteger
sec
).
Let
milli
be !
ToInteger
ms
).
Let
be
msPerHour
msPerMinute
msPerSecond
milli
, performing the arithmetic according to IEEE 754-2008 rules (that is, as if using the ECMAScript operators
and
).
Return
20.3.1.12
MakeDay (
year
month
date
The abstract operation MakeDay calculates a number of days
from its three arguments, which must be ECMAScript Number values. This
operator functions as follows:
If
year
is not finite or
month
is not finite or
date
is not finite, return
NaN
Let
be !
ToInteger
year
).
Let
be !
ToInteger
month
).
Let
dt
be !
ToInteger
date
).
Let
ym
be
floor
/ 12).
Let
mn
be
modulo
12.
Find a value
such that
YearFromTime
) is
ym
and
MonthFromTime
) is
mn
and
DateFromTime
) is 1; but if this is not possible (because some argument is out of range), return
NaN
Return
Day
) +
dt
- 1.
20.3.1.13
MakeDate (
day
time
The abstract operation MakeDate calculates a number of
milliseconds from its two arguments, which must be ECMAScript Number
values. This operator functions as follows:
If
day
is not finite or
time
is not finite, return
NaN
Return
day
msPerDay
time
20.3.1.14
TimeClip (
time
The abstract operation TimeClip calculates a number of
milliseconds from its argument, which must be an ECMAScript Number
value. This operator functions as follows:
If
time
is not finite, return
NaN
If
abs
time
) > 8.64 × 10
15
, return
NaN
Let
clippedTime
be !
ToInteger
time
).
If
clippedTime
is
-0
, set
clippedTime
to
+0
Return
clippedTime
Note
The point of step 4 is that an implementation is permitted a
choice of internal representations of time values, for example as a
64-bit signed integer or as a 64-bit floating-point value. Depending on
the implementation, this internal representation may or may not
distinguish
-0
and
+0
20.3.1.15
Date Time String Format
ECMAScript defines a string interchange format for date-times
based upon a simplification of the ISO 8601 calendar date extended
format. The format is as follows:
YYYY-MM-DDTHH:mm:ss.sssZ
Where the fields are as follows:
YYYY
is the decimal digits of the year 0000 to 9999 in the proleptic Gregorian calendar.
"-"
(hyphen) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12 (December).
DD
is the day of the month from 01 to 31.
"T"
appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
":"
(colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
"."
(dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
is the time zone offset specified as
"Z"
(for UTC) or either
"+"
or
"-"
followed by a time expression
HH:mm
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the
above date-only forms immediately followed by one of the following time
forms with an optional time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base 10. If the
MM
or
DD
fields are absent
"01"
is used as the value. If the
HH
mm
, or
ss
fields are absent
"00"
is used as the value and the value of an absent
sss
field is
"000"
When the time zone offset is absent, date-only forms are interpreted as
a UTC time and date-time forms are interpreted as a local time.
A string containing out-of-bounds or nonconforming fields is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations
00:00
and
24:00
are available to distinguish the two midnights that can be associated
with one date. This means that the following two notations refer to
exactly the same point in time:
1995-02-04T24:00
and
1995-02-05T00:00
This interpretation of the latter form as "end of a calendar day" is
consistent with ISO 8601, even though that specification reserves it for
describing time intervals and does not permit it within representations
of single points in time.
Note 2
There exists no international standard that specifies
abbreviations for civil time zones like CET, EST, etc. and sometimes the
same abbreviation is even used for two very different time zones. For
this reason, both ISO 8601 and this format specify numeric
representations of time zone offsets.
20.3.1.15.1
Expanded Years
Covering
the full
time value
range of approximately 273,790 years forward or backward from 01 January, 1970 (
20.3.1.1
requires representing years before 0 or after 9999. ISO 8601 permits
expansion of the year representation, but only by mutual agreement of
the partners in information interchange. In the simplified ECMAScript
format, such an expanded year representation shall have 6 digits and is
always prefixed with a + or - sign. The year 0 is considered positive
and hence prefixed with a + sign. Strings matching the
Date Time String Format
with expanded years representing instants in time outside the range of a
time value
are treated as unrecognizable by
Date.parse
and cause that function to return
NaN
without falling back to implementation-specific behavior or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z
271822 B.C.
-000001-01-01T00:00:00Z
2 B.C.
+000000-01-01T00:00:00Z
1 B.C.
+000001-01-01T00:00:00Z
1 A.D.
+001970-01-01T00:00:00Z
1970 A.D.
+002009-12-15T00:00:00Z
2009 A.D.
+275760-09-13T00:00:00Z
275760 A.D.
20.3.2
The Date Constructor
The Date
constructor
is the intrinsic object
%Date%
is the initial value of the
Date
property of the
global object
creates and initializes a new Date object when called as a
constructor
returns a String representing the current time (UTC) when called as a function rather than as a
constructor
is a single function whose behaviour is overloaded based upon the number and types of its arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Date
behaviour must include a
super
call to the
Date
constructor
to create and initialize the subclass instance with a [[DateValue]] internal slot.
has a
"length"
property whose value is 7.
20.3.2.1
Date (
year
month
[ ,
date
[ ,
hours
[ ,
minutes
[ ,
seconds
[ ,
ms
] ] ] ] ] )
This description applies only if the Date
constructor
is called with at least two arguments.
When the
Date
function is called, the following steps are taken:
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
≥ 2.
If NewTarget is
undefined
, then
Let
now
be the Number that is the
time value
(UTC) identifying the current time.
Return
ToDateString
now
).
Else,
Let
be ?
ToNumber
year
).
Let
be ?
ToNumber
month
).
If
date
is present, let
dt
be ?
ToNumber
date
); else let
dt
be 1.
If
hours
is present, let
be ?
ToNumber
hours
); else let
be 0.
If
minutes
is present, let
min
be ?
ToNumber
minutes
); else let
min
be 0.
If
seconds
is present, let
be ?
ToNumber
seconds
); else let
be 0.
If
ms
is present, let
milli
be ?
ToNumber
ms
); else let
milli
be 0.
If
is
NaN
, let
yr
be
NaN
Else,
Let
yi
be !
ToInteger
).
If 0 ≤
yi
≤ 99, let
yr
be 1900 +
yi
; otherwise, let
yr
be
Let
finalDate
be
MakeDate
MakeDay
yr
dt
),
MakeTime
min
milli
)).
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%DatePrototype%"
, « [[DateValue]] »).
Set
.[[DateValue]] to
TimeClip
UTC
finalDate
)).
Return
20.3.2.2
Date (
value
This description applies only if the Date
constructor
is called with exactly one argument.
When the
Date
function is called, the following steps are taken:
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
= 1.
If NewTarget is
undefined
, then
Let
now
be the Number that is the
time value
(UTC) identifying the current time.
Return
ToDateString
now
).
Else,
If
Type
value
) is Object and
value
has a [[DateValue]] internal slot, then
Let
tv
be
thisTimeValue
value
).
Else,
Let
be ?
ToPrimitive
value
).
If
Type
) is String, then
Assert
: The next step never returns an
abrupt completion
because
Type
) is String.
Let
tv
be the result of parsing
as a date, in exactly the same manner as for the
parse
method (
20.3.3.2
).
Else,
Let
tv
be ?
ToNumber
).
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%DatePrototype%"
, « [[DateValue]] »).
Set
.[[DateValue]] to
TimeClip
tv
).
Return
20.3.2.3
Date ( )
This description applies only if the Date
constructor
is called with no arguments.
When the
Date
function is called, the following steps are taken:
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
= 0.
If NewTarget is
undefined
, then
Let
now
be the Number that is the
time value
(UTC) identifying the current time.
Return
ToDateString
now
).
Else,
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%DatePrototype%"
, « [[DateValue]] »).
Set
.[[DateValue]] to the
time value
(UTC) identifying the current time.
Return
20.3.3
Properties of the Date Constructor
The Date
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
20.3.3.1
Date.now ( )
The
now
function returns a Number value that is the
time value
designating the UTC date and time of the occurrence of the call to
now
20.3.3.2
Date.parse (
string
The
parse
function applies the
ToString
operator to its argument. If
ToString
results in an
abrupt completion
the
Completion Record
is immediately returned. Otherwise,
parse
interprets the resulting String as a date and time; it returns a Number, the UTC
time value
corresponding to the date and time. The String may be interpreted as a
local time, a UTC time, or a time in some other time zone, depending on
the contents of the String. The function first attempts to parse the
String according to the format described in Date Time String Format (
20.3.1.15
),
including expanded years. If the String does not conform to that format
the function may fall back to any implementation-specific heuristics or
implementation-specific date formats. Strings that are unrecognizable
or contain out-of-bounds format field values shall cause
Date.parse
to return
NaN
If
is any Date object whose milliseconds
amount is zero within a particular implementation of ECMAScript, then
all of the following expressions should produce the same numeric value
in that implementation, if all the properties referenced have their
initial values:
x.valueOf()
Date
.parse(x.toString())
Date
.parse(x.toUTCString())
Date
.parse(x.toISOString())
However, the expression
Date
.parse(x.toLocaleString())
is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by
Date.parse
is implementation-dependent when given any String value that does not conform to the Date Time String Format (
20.3.1.15
) and that could not be produced in that implementation by the
toString
or
toUTCString
method.
20.3.3.3
Date.prototype
The initial value of
Date.prototype
is the intrinsic object
%DatePrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
20.3.3.4
Date.UTC (
year
[ ,
month
[ ,
date
[ ,
hours
[ ,
minutes
[ ,
seconds
[ ,
ms
] ] ] ] ] ] )
When the
UTC
function is called, the following steps are taken:
Let
be ?
ToNumber
year
).
If
month
is present, let
be ?
ToNumber
month
); else let
be 0.
If
date
is present, let
dt
be ?
ToNumber
date
); else let
dt
be 1.
If
hours
is present, let
be ?
ToNumber
hours
); else let
be 0.
If
minutes
is present, let
min
be ?
ToNumber
minutes
); else let
min
be 0.
If
seconds
is present, let
be ?
ToNumber
seconds
); else let
be 0.
If
ms
is present, let
milli
be ?
ToNumber
ms
); else let
milli
be 0.
If
is
NaN
, let
yr
be
NaN
Else,
Let
yi
be !
ToInteger
).
If 0 ≤
yi
≤ 99, let
yr
be 1900 +
yi
; otherwise, let
yr
be
Return
TimeClip
MakeDate
MakeDay
yr
dt
),
MakeTime
min
milli
))).
The
"length"
property of the
UTC
function is 7.
Note
The
UTC
function differs from the
Date
constructor
in two ways: it returns a
time value
as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
20.3.4
Properties of the Date Prototype Object
The Date prototype object:
is the intrinsic object
%DatePrototype%
is itself an ordinary object.
is not a Date instance and does not have a [[DateValue]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the
this
value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a
time value
The abstract operation
thisTimeValue
value
) performs the following steps:
If
Type
value
) is Object and
value
has a [[DateValue]] internal slot, then
Return
value
.[[DateValue]].
Throw a
TypeError
exception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “
this Date object
” refers to the object that is the
this
value for the invocation of the function. If the Type of the
this
value is not Object, a
TypeError
exception is thrown. The phrase “
this time value
” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the
this
value of the method invocation passed as the argument.
20.3.4.1
Date.prototype.constructor
The initial value of
Date.prototype.constructor
is the intrinsic object
%Date%
20.3.4.2
Date.prototype.getDate ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
DateFromTime
LocalTime
)).
20.3.4.3
Date.prototype.getDay ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
WeekDay
LocalTime
)).
20.3.4.4
Date.prototype.getFullYear ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
YearFromTime
LocalTime
)).
20.3.4.5
Date.prototype.getHours ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
HourFromTime
LocalTime
)).
20.3.4.6
Date.prototype.getMilliseconds ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
msFromTime
LocalTime
)).
20.3.4.7
Date.prototype.getMinutes ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
MinFromTime
LocalTime
)).
20.3.4.8
Date.prototype.getMonth ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
MonthFromTime
LocalTime
)).
20.3.4.9
Date.prototype.getSeconds ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
SecFromTime
LocalTime
)).
20.3.4.10
Date.prototype.getTime ( )
The following steps are performed:
Return ?
thisTimeValue
this
value).
20.3.4.11
Date.prototype.getTimezoneOffset ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return (
LocalTime
)) /
msPerMinute
20.3.4.12
Date.prototype.getUTCDate ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
DateFromTime
).
20.3.4.13
Date.prototype.getUTCDay ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
WeekDay
).
20.3.4.14
Date.prototype.getUTCFullYear ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
YearFromTime
).
20.3.4.15
Date.prototype.getUTCHours ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
HourFromTime
).
20.3.4.16
Date.prototype.getUTCMilliseconds ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
msFromTime
).
20.3.4.17
Date.prototype.getUTCMinutes ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
MinFromTime
).
20.3.4.18
Date.prototype.getUTCMonth ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
MonthFromTime
).
20.3.4.19
Date.prototype.getUTCSeconds ( )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
SecFromTime
).
20.3.4.20
Date.prototype.setDate (
date
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
MonthFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
20.3.4.21
Date.prototype.setFullYear (
year
[ ,
month
[ ,
date
] ] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, set
to
+0
; otherwise, set
to
LocalTime
).
Let
be ?
ToNumber
year
).
If
month
is not present, let
be
MonthFromTime
); otherwise, let
be ?
ToNumber
month
).
If
date
is not present, let
dt
be
DateFromTime
); otherwise, let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setFullYear
method is 3.
Note
If
month
is not present, this method behaves as if
month
was present with the value
getMonth()
. If
date
is not present, it behaves as if
date
was present with the value
getDate()
20.3.4.22
Date.prototype.setHours (
hour
[ ,
min
[ ,
sec
[ ,
ms
] ] ] )
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Let
be ?
ToNumber
hour
).
If
min
is not present, let
be
MinFromTime
); otherwise, let
be ?
ToNumber
min
).
If
sec
is not present, let
be
SecFromTime
); otherwise, let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
); otherwise, let
milli
be ?
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setHours
method is 4.
Note
If
min
is not present, this method behaves as if
min
was present with the value
getMinutes()
. If
sec
is not present, it behaves as if
sec
was present with the value
getSeconds()
. If
ms
is not present, it behaves as if
ms
was present with the value
getMilliseconds()
20.3.4.23
Date.prototype.setMilliseconds (
ms
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Set
ms
to ?
ToNumber
ms
).
Let
time
be
MakeTime
HourFromTime
),
MinFromTime
),
SecFromTime
),
ms
).
Let
be
TimeClip
UTC
MakeDate
Day
),
time
))).
Set the [[DateValue]] internal slot of
this Date object
to
Return
20.3.4.24
Date.prototype.setMinutes (
min
[ ,
sec
[ ,
ms
] ] )
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Let
be ?
ToNumber
min
).
If
sec
is not present, let
be
SecFromTime
); otherwise, let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
); otherwise, let
milli
be ?
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setMinutes
method is 3.
Note
If
sec
is not present, this method behaves as if
sec
was present with the value
getSeconds()
. If
ms
is not present, this behaves as if
ms
was present with the value
getMilliseconds()
20.3.4.25
Date.prototype.setMonth (
month
[ ,
date
] )
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Let
be ?
ToNumber
month
).
If
date
is not present, let
dt
be
DateFromTime
); otherwise, let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
UTC
newDate
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setMonth
method is 2.
Note
If
date
is not present, this method behaves as if
date
was present with the value
getDate()
20.3.4.26
Date.prototype.setSeconds (
sec
[ ,
ms
] )
The following steps are performed:
Let
be
LocalTime
(?
thisTimeValue
this
value)).
Let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
); otherwise, let
milli
be ?
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
MinFromTime
),
milli
)).
Let
be
TimeClip
UTC
date
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setSeconds
method is 2.
Note
If
ms
is not present, this method behaves as if
ms
was present with the value
getMilliseconds()
20.3.4.27
Date.prototype.setTime (
time
The following steps are performed:
Perform ?
thisTimeValue
this
value).
Let
be ?
ToNumber
time
).
Let
be
TimeClip
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
20.3.4.28
Date.prototype.setUTCDate (
date
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
MonthFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
20.3.4.29
Date.prototype.setUTCFullYear (
year
[ ,
month
[ ,
date
] ] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, set
to
+0
Let
be ?
ToNumber
year
).
If
month
is not present, let
be
MonthFromTime
); otherwise, let
be ?
ToNumber
month
).
If
date
is not present, let
dt
be
DateFromTime
); otherwise, let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setUTCFullYear
method is 3.
Note
If
month
is not present, this method behaves as if
month
was present with the value
getUTCMonth()
. If
date
is not present, it behaves as if
date
was present with the value
getUTCDate()
20.3.4.30
Date.prototype.setUTCHours (
hour
[ ,
min
[ ,
sec
[ ,
ms
] ] ] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
be ?
ToNumber
hour
).
If
min
is not present, let
be
MinFromTime
); otherwise, let
be ?
ToNumber
min
).
If
sec
is not present, let
be
SecFromTime
); otherwise, let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
); otherwise, let
milli
be ?
ToNumber
ms
).
Let
newDate
be
MakeDate
Day
),
MakeTime
milli
)).
Let
be
TimeClip
newDate
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setUTCHours
method is 4.
Note
If
min
is not present, this method behaves as if
min
was present with the value
getUTCMinutes()
. If
sec
is not present, it behaves as if
sec
was present with the value
getUTCSeconds()
. If
ms
is not present, it behaves as if
ms
was present with the value
getUTCMilliseconds()
20.3.4.31
Date.prototype.setUTCMilliseconds (
ms
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
milli
be ?
ToNumber
ms
).
Let
time
be
MakeTime
HourFromTime
),
MinFromTime
),
SecFromTime
),
milli
).
Let
be
TimeClip
MakeDate
Day
),
time
)).
Set the [[DateValue]] internal slot of
this Date object
to
Return
20.3.4.32
Date.prototype.setUTCMinutes (
min
[ ,
sec
[ ,
ms
] ] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
be ?
ToNumber
min
).
If
sec
is not present, let
be
SecFromTime
).
Else,
Let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
).
Else,
Let
milli
be ?
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
milli
)).
Let
be
TimeClip
date
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setUTCMinutes
method is 3.
Note
If
sec
is not present, this method behaves as if
sec
was present with the value
getUTCSeconds()
. If
ms
is not present, it function behaves as if
ms
was present with the value return by
getUTCMilliseconds()
20.3.4.33
Date.prototype.setUTCMonth (
month
[ ,
date
] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
be ?
ToNumber
month
).
If
date
is not present, let
dt
be
DateFromTime
).
Else,
Let
dt
be ?
ToNumber
date
).
Let
newDate
be
MakeDate
MakeDay
YearFromTime
),
dt
),
TimeWithinDay
)).
Let
be
TimeClip
newDate
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setUTCMonth
method is 2.
Note
If
date
is not present, this method behaves as if
date
was present with the value
getUTCDate()
20.3.4.34
Date.prototype.setUTCSeconds (
sec
[ ,
ms
] )
The following steps are performed:
Let
be ?
thisTimeValue
this
value).
Let
be ?
ToNumber
sec
).
If
ms
is not present, let
milli
be
msFromTime
).
Else,
Let
milli
be ?
ToNumber
ms
).
Let
date
be
MakeDate
Day
),
MakeTime
HourFromTime
),
MinFromTime
),
milli
)).
Let
be
TimeClip
date
).
Set the [[DateValue]] internal slot of
this Date object
to
Return
The
"length"
property of the
setUTCSeconds
method is 2.
Note
If
ms
is not present, this method behaves as if
ms
was present with the value
getUTCMilliseconds()
20.3.4.35
Date.prototype.toDateString ( )
The following steps are performed:
Let
be
this Date object
Let
tv
be ?
thisTimeValue
).
If
tv
is
NaN
, return
"Invalid Date"
Let
be
LocalTime
tv
).
Return
DateString
).
20.3.4.36
Date.prototype.toISOString ( )
This function returns a String value representing the instance in time corresponding to
this time value
. The format of the String is the Date Time string format defined in
20.3.1.15
. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If
this time value
is not a finite Number or if the year is not a value that can be
represented in that format (if necessary using expanded year format), a
RangeError
exception is thrown.
20.3.4.37
Date.prototype.toJSON (
key
This function provides a String representation of a Date object for use by
JSON.stringify
24.5.2
).
When the
toJSON
method is called with argument
key
, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
tv
be ?
ToPrimitive
, hint Number).
If
Type
tv
) is Number and
tv
is not finite, return
null
Return ?
Invoke
"toISOString"
).
Note 1
The argument is ignored.
Note 2
The
toJSON
function is intentionally generic; it does not require that its
this
value be a Date object. Therefore, it can be transferred to other kinds
of objects for use as a method. However, it does require that any such
object have a
toISOString
method.
20.3.4.38
Date.prototype.toLocaleDateString ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleDateString
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleDateString
method is used.
This function returns a String value. The contents of the
String are implementation-dependent, but are intended to represent the
“date” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
20.3.4.39
Date.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleString
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleString
method is used.
This function returns a String value. The contents of the
String are implementation-dependent, but are intended to represent the
Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
20.3.4.40
Date.prototype.toLocaleTimeString ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Date.prototype.toLocaleTimeString
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleTimeString
method is used.
This function returns a String value. The contents of the
String are implementation-dependent, but are intended to represent the
“time” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
20.3.4.41
Date.prototype.toString ( )
The following steps are performed:
Let
tv
be ?
thisTimeValue
this
value).
Return
ToDateString
tv
).
Note 1
For any Date object
whose milliseconds amount is zero, the result of
Date.parse(d.toString())
is equal to
d.valueOf()
. See
20.3.3.2
Note 2
The
toString
function is not generic; it throws a
TypeError
exception if its
this
value is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
20.3.4.41.1
Runtime Semantics: TimeString (
tv
The following steps are performed:
Assert
Type
tv
) is Number.
Assert
tv
is not
NaN
Let
hour
be the String representation of
HourFromTime
tv
), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
minute
be the String representation of
MinFromTime
tv
), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
second
be the String representation of
SecFromTime
tv
), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Return the
string-concatenation
of
hour
":"
minute
":"
second
, the code unit 0x0020 (SPACE), and
"GMT"
20.3.4.41.2
Runtime Semantics: DateString (
tv
The following steps are performed:
Assert
Type
tv
) is Number.
Assert
tv
is not
NaN
Let
weekday
be the Name of the entry in
Table 49
with the Number
WeekDay
tv
).
Let
month
be the Name of the entry in
Table 50
with the Number
MonthFromTime
tv
).
Let
day
be the String representation of
DateFromTime
tv
), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
year
be the String representation of
YearFromTime
tv
), formatted as a decimal number of at least four digits, padded to the left with zeroes if necessary.
Return the
string-concatenation
of
weekday
, the code unit 0x0020 (SPACE),
month
, the code unit 0x0020 (SPACE),
day
, the code unit 0x0020 (SPACE), and
year
Table 49: Names of days of the week
Number
Name
"Sun"
"Mon"
"Tue"
"Wed"
"Thu"
"Fri"
"Sat"
Table 50: Names of months of the year
Number
Name
"Jan"
"Feb"
"Mar"
"Apr"
"May"
"Jun"
"Jul"
"Aug"
"Sep"
"Oct"
10
"Nov"
11
"Dec"
20.3.4.41.3
Runtime Semantics: TimeZoneString (
tv
The following steps are performed:
Assert
Type
tv
) is Number.
Assert
tv
is not
NaN
Let
offset
be
LocalTZA
tv
true
).
If
offset
≥ 0, let
offsetSign
be
"+"
; otherwise, let
offsetSign
be
"-"
Let
offsetMin
be the String representation of
MinFromTime
abs
offset
)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
offsetHour
be the String representation of
HourFromTime
abs
offset
)), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
tzName
be an implementation-defined string that is either the empty string or the
string-concatenation
of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT
PARENTHESIS), an implementation-dependent timezone name, and the code
unit 0x0029 (RIGHT PARENTHESIS).
Return the
string-concatenation
of
offsetSign
offsetHour
offsetMin
, and
tzName
20.3.4.41.4
Runtime Semantics: ToDateString (
tv
The following steps are performed:
Assert
Type
tv
) is Number.
If
tv
is
NaN
, return
"Invalid Date"
Let
be
LocalTime
tv
).
Return the
string-concatenation
of
DateString
), the code unit 0x0020 (SPACE),
TimeString
), and
TimeZoneString
tv
).
20.3.4.42
Date.prototype.toTimeString ( )
The following steps are performed:
Let
be
this Date object
Let
tv
be ?
thisTimeValue
).
If
tv
is
NaN
, return
"Invalid Date"
Let
be
LocalTime
tv
).
Return the
string-concatenation
of
TimeString
) and
TimeZoneString
tv
).
20.3.4.43
Date.prototype.toUTCString ( )
The following steps are performed:
Let
be
this Date object
Let
tv
be ?
thisTimeValue
).
If
tv
is
NaN
, return
"Invalid Date"
Let
weekday
be the Name of the entry in
Table 49
with the Number
WeekDay
tv
).
Let
month
be the Name of the entry in
Table 50
with the Number
MonthFromTime
tv
).
Let
day
be the String representation of
DateFromTime
tv
), formatted as a two-digit decimal number, padded to the left with a zero if necessary.
Let
year
be the String representation of
YearFromTime
tv
), formatted as a decimal number of at least four digits, padded to the left with zeroes if necessary.
Return the
string-concatenation
of
weekday
","
, the code unit 0x0020 (SPACE),
day
, the code unit 0x0020 (SPACE),
month
, the code unit 0x0020 (SPACE),
year
, the code unit 0x0020 (SPACE), and
TimeString
tv
).
20.3.4.44
Date.prototype.valueOf ( )
The following steps are performed:
Return ?
thisTimeValue
this
value).
20.3.4.45
Date.prototype [ @@toPrimitive ] (
hint
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for
hint
are
"default"
"number"
, and
"string"
. Date objects, are unique among built-in ECMAScript object in that they treat
"default"
as being equivalent to
"string"
, All other built-in ECMAScript objects treat
"default"
as being equivalent to
"number"
When the
@@toPrimitive
method is called with argument
hint
, the following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
hint
is the String value
"string"
or the String value
"default"
, then
Let
tryFirst
be
"string"
Else if
hint
is the String value
"number"
, then
Let
tryFirst
be
"number"
Else, throw a
TypeError
exception.
Return ?
OrdinaryToPrimitive
tryFirst
).
The value of the
name
property of this function is
"[Symbol.toPrimitive]"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
20.3.5
Properties of Date Instances
Date instances are ordinary objects that inherit properties
from the Date prototype object. Date instances also have a [[DateValue]]
internal slot. The [[DateValue]] internal slot is the
time value
represented by
this Date object
21
Text Processing
21.1
String Objects
21.1.1
The String Constructor
The String
constructor
is the intrinsic object
%String%
is the initial value of the
String
property of the
global object
creates and initializes a new String object when called as a
constructor
performs a type conversion when called as a function rather than as a
constructor
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
String
behaviour must include a
super
call to the
String
constructor
to create and initialize the subclass instance with a [[StringData]] internal slot.
21.1.1.1
String (
value
When
String
is called with argument
value
, the following steps are taken:
If no arguments were passed to this function invocation, let
be
""
Else,
If NewTarget is
undefined
and
Type
value
) is Symbol, return
SymbolDescriptiveString
value
).
Let
be ?
ToString
value
).
If NewTarget is
undefined
, return
Return !
StringCreate
, ?
GetPrototypeFromConstructor
(NewTarget,
"%StringPrototype%"
)).
21.1.2
Properties of the String Constructor
The String
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
21.1.2.1
String.fromCharCode ( ...
codeUnits
The
String.fromCharCode
function may be called with any number of arguments which form the rest parameter
codeUnits
. The following steps are taken:
Let
codeUnits
be a
List
containing the arguments passed to this function.
Let
length
be the number of elements in
codeUnits
Let
elements
be a new empty
List
Let
nextIndex
be 0.
Repeat, while
nextIndex
length
Let
next
be
codeUnits
nextIndex
].
Let
nextCU
be ?
ToUint16
next
).
Append
nextCU
to the end of
elements
Increase
nextIndex
by 1.
Return the String value whose code units are, in order, the elements in the
List
elements
. If
length
is 0, the empty string is returned.
The
"length"
property of the
fromCharCode
function is 1.
21.1.2.2
String.fromCodePoint ( ...
codePoints
The
String.fromCodePoint
function may be called with any number of arguments which form the rest parameter
codePoints
. The following steps are taken:
Let
codePoints
be a
List
containing the arguments passed to this function.
Let
length
be the number of elements in
codePoints
Let
elements
be a new empty
List
Let
nextIndex
be 0.
Repeat, while
nextIndex
length
Let
next
be
codePoints
nextIndex
].
Let
nextCP
be ?
ToNumber
next
).
If
SameValue
nextCP
, !
ToInteger
nextCP
)) is
false
, throw a
RangeError
exception.
If
nextCP
< 0 or
nextCP
> 0x10FFFF, throw a
RangeError
exception.
Append the elements of the
UTF16Encoding
of
nextCP
to the end of
elements
Increase
nextIndex
by 1.
Return the String value whose code units are, in order, the elements in the
List
elements
. If
length
is 0, the empty string is returned.
The
"length"
property of the
fromCodePoint
function is 1.
21.1.2.3
String.prototype
The initial value of
String.prototype
is the intrinsic object
%StringPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
21.1.2.4
String.raw (
template
, ...
substitutions
The
String.raw
function may be called with a variable number of arguments. The first argument is
template
and the remainder of the arguments form the
List
substitutions
. The following steps are taken:
Let
substitutions
be a
List
consisting of all of the arguments passed to this function, starting
with the second argument. If fewer than two arguments were passed, the
List
is empty.
Let
numberOfSubstitutions
be the number of elements in
substitutions
Let
cooked
be ?
ToObject
template
).
Let
raw
be ?
ToObject
(?
Get
cooked
"raw"
)).
Let
literalSegments
be ?
ToLength
(?
Get
raw
"length"
)).
If
literalSegments
≤ 0, return the empty string.
Let
stringElements
be a new empty
List
Let
nextIndex
be 0.
Repeat,
Let
nextKey
be !
ToString
nextIndex
).
Let
nextSeg
be ?
ToString
(?
Get
raw
nextKey
)).
Append in order the code unit elements of
nextSeg
to the end of
stringElements
If
nextIndex
+ 1 =
literalSegments
, then
Return the String value whose code units are, in order, the elements in the
List
stringElements
. If
stringElements
has no elements, the empty string is returned.
If
nextIndex
numberOfSubstitutions
, let
next
be
substitutions
nextIndex
].
Else, let
next
be the empty String.
Let
nextSub
be ?
ToString
next
).
Append in order the code unit elements of
nextSub
to the end of
stringElements
Increase
nextIndex
by 1.
Note
String.raw is intended for use as a tag function of a Tagged Template (
12.3.7
).
When called as such, the first argument will be a well formed template
object and the rest parameter will contain the substitution values.
21.1.3
Properties of the String Prototype Object
The String prototype object:
is the intrinsic object
%StringPrototype%
is a String
exotic object
and has the internal methods specified for such objects.
has a [[StringData]] internal slot whose value is the empty String.
has a
"length"
property whose initial value is 0 and whose attributes are { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the
this
value passed to them must be either a String value or an object that
has a [[StringData]] internal slot that has been initialized to a String
value.
The abstract operation
thisStringValue
value
) performs the following steps:
If
Type
value
) is String, return
value
If
Type
value
) is Object and
value
has a [[StringData]] internal slot, then
Let
be
value
.[[StringData]].
Assert
Type
) is String.
Return
Throw a
TypeError
exception.
21.1.3.1
String.prototype.charAt (
pos
Note 1
Returns a single element String containing the code unit at index
pos
within the String value resulting from converting this object to a
String. If there is no element at that index, the result is the empty
String. The result is a String value, not a String object.
If
pos
is a value of Number type that is an integer, then the result of
x.charAt(pos)
is equal to the result of
x.substring(pos, pos + 1)
When the
charAt
method is called with one argument
pos
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
position
be ?
ToInteger
pos
).
Let
size
be the length of
If
position
< 0 or
position
size
, return the empty String.
Return the String value of length 1, containing one code unit from
, namely the code unit at index
position
Note 2
The
charAt
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.2
String.prototype.charCodeAt (
pos
Note 1
Returns a Number (a nonnegative integer less than 2
16
) that is the numeric value of the code unit at index
pos
within the String resulting from converting this object to a String. If there is no element at that index, the result is
NaN
When the
charCodeAt
method is called with one argument
pos
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
position
be ?
ToInteger
pos
).
Let
size
be the length of
If
position
< 0 or
position
size
, return
NaN
Return a value of Number type, whose value is the numeric value of the code unit at index
position
within the String
Note 2
The
charCodeAt
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
21.1.3.3
String.prototype.codePointAt (
pos
Note 1
Returns a nonnegative integer Number less than 0x110000 that is the code point value of the UTF-16 encoded code point (
6.1.4
) starting at the string element at index
pos
within the String resulting from converting this object to a String. If there is no element at that index, the result is
undefined
. If a valid UTF-16
surrogate pair
does not begin at
pos
, the result is the code unit at
pos
When the
codePointAt
method is called with one argument
pos
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
position
be ?
ToInteger
pos
).
Let
size
be the length of
If
position
< 0 or
position
size
, return
undefined
Let
first
be the numeric value of the code unit at index
position
within the String
If
first
< 0xD800 or
first
> 0xDBFF or
position
+ 1 =
size
, return
first
Let
second
be the numeric value of the code unit at index
position
+ 1 within the String
If
second
< 0xDC00 or
second
> 0xDFFF, return
first
Return
UTF16Decode
first
second
).
Note 2
The
codePointAt
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
21.1.3.4
String.prototype.concat ( ...
args
Note 1
When the
concat
method is called it returns the String value consisting of the code units of the
this
object (converted to a String) followed by the code units of each of
the arguments converted to a String. The result is a String value, not a
String object.
When the
concat
method is called with zero or more arguments, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
args
be a
List
whose elements are the arguments passed to this function.
Let
be
Repeat, while
args
is not empty
Remove the first element from
args
and let
next
be the value of that element.
Let
nextString
be ?
ToString
next
).
Set
to the
string-concatenation
of the previous value of
and
nextString
Return
The
"length"
property of the
concat
method is 1.
Note 2
The
concat
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
21.1.3.5
String.prototype.constructor
The initial value of
String.prototype.constructor
is the intrinsic object
%String%
21.1.3.6
String.prototype.endsWith (
searchString
[ ,
endPosition
] )
The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
isRegExp
be ?
IsRegExp
searchString
).
If
isRegExp
is
true
, throw a
TypeError
exception.
Let
searchStr
be ?
ToString
searchString
).
Let
len
be the length of
If
endPosition
is
undefined
, let
pos
be
len
, else let
pos
be ?
ToInteger
endPosition
).
Let
end
be
min
max
pos
, 0),
len
).
Let
searchLength
be the length of
searchStr
Let
start
be
end
searchLength
If
start
is less than 0, return
false
If the sequence of code units of
starting at
start
of length
searchLength
is the same as the full code unit sequence of
searchStr
, return
true
Otherwise, return
false
Note 1
Returns
true
if the sequence of code units of
searchString
converted to a String is the same as the corresponding code units of this object (converted to a String) starting at
endPosition
- length(this). Otherwise returns
false
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define extensions that
allow such argument values.
Note 3
The
endsWith
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.7
String.prototype.includes (
searchString
[ ,
position
] )
The
includes
method takes two arguments,
searchString
and
position
, and performs the following steps:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
isRegExp
be ?
IsRegExp
searchString
).
If
isRegExp
is
true
, throw a
TypeError
exception.
Let
searchStr
be ?
ToString
searchString
).
Let
pos
be ?
ToInteger
position
).
Assert
: If
position
is
undefined
, then
pos
is 0.
Let
len
be the length of
Let
start
be
min
max
pos
, 0),
len
).
Let
searchLen
be the length of
searchStr
If there exists any integer
not smaller than
start
such that
searchLen
is not greater than
len
, and for all nonnegative integers
less than
searchLen
, the code unit at index
within
is the same as the code unit at index
within
searchStr
, return
true
; but if there is no such integer
, return
false
Note 1
If
searchString
appears as a substring of the
result of converting this object to a String, at one or more indices
that are greater than or equal to
position
, return
true
; otherwise, returns
false
. If
position
is
undefined
, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define extensions that
allow such argument values.
Note 3
The
includes
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.8
String.prototype.indexOf (
searchString
[ ,
position
] )
Note 1
If
searchString
appears as a substring of the
result of converting this object to a String, at one or more indices
that are greater than or equal to
position
, then the smallest such index is returned; otherwise, -1 is returned. If
position
is
undefined
, 0 is assumed, so as to search all of the String.
The
indexOf
method takes two arguments,
searchString
and
position
, and performs the following steps:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
searchStr
be ?
ToString
searchString
).
Let
pos
be ?
ToInteger
position
).
Assert
: If
position
is
undefined
, then
pos
is 0.
Let
len
be the length of
Let
start
be
min
max
pos
, 0),
len
).
Let
searchLen
be the length of
searchStr
Return the smallest possible integer
not smaller than
start
such that
searchLen
is not greater than
len
, and for all nonnegative integers
less than
searchLen
, the code unit at index
within
is the same as the code unit at index
within
searchStr
; but if there is no such integer
, return the value -1.
Note 2
The
indexOf
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.9
String.prototype.lastIndexOf (
searchString
[ ,
position
] )
Note 1
If
searchString
appears as a substring of the
result of converting this object to a String at one or more indices that
are smaller than or equal to
position
, then the greatest such index is returned; otherwise, -1 is returned. If
position
is
undefined
, the length of the String value is assumed, so as to search all of the String.
The
lastIndexOf
method takes two arguments,
searchString
and
position
, and performs the following steps:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
searchStr
be ?
ToString
searchString
).
Let
numPos
be ?
ToNumber
position
).
Assert
: If
position
is
undefined
, then
numPos
is
NaN
If
numPos
is
NaN
, let
pos
be
+∞
; otherwise, let
pos
be !
ToInteger
numPos
).
Let
len
be the length of
Let
start
be
min
max
pos
, 0),
len
).
Let
searchLen
be the length of
searchStr
Return the largest possible nonnegative integer
not larger than
start
such that
searchLen
is not greater than
len
, and for all nonnegative integers
less than
searchLen
, the code unit at index
within
is the same as the code unit at index
within
searchStr
; but if there is no such integer
, return the value -1.
Note 2
The
lastIndexOf
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.10
String.prototype.localeCompare (
that
[ ,
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
localeCompare
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
localeCompare
method is used.
When the
localeCompare
method is called with argument
that
, it returns a Number other than
NaN
that represents the result of a locale-sensitive String comparison of the
this
value (converted to a String) with
that
(converted to a String). The two Strings are
and
That
The two Strings are compared in an implementation-defined fashion. The
result is intended to order String values in the sort order specified by
a host default locale, and will be negative, zero, or positive,
depending on whether
comes before
That
in the sort order, the Strings are equal, or
comes after
That
in the sort order, respectively.
Before performing the comparisons, the following steps are performed to prepare the Strings:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
That
be ?
ToString
that
).
The meaning of the optional second and third parameters to
this method are defined in the ECMA-402 specification; implementations
that do not include ECMA-402 support must not assign any other
interpretation to those parameter positions.
The
localeCompare
method, if considered as a function of two arguments
this
and
that
, is a consistent comparison function (as defined in
22.1.3.27
) on the set of all Strings.
The actual return values are implementation-defined to permit
implementers to encode additional information in the value, but the
function is required to define a total ordering on all Strings. This
function must treat Strings that are canonically equivalent according to
the Unicode standard as identical and must return
when comparing Strings that are considered canonically equivalent.
Note 1
The
localeCompare
method itself is not directly suitable as an argument to
Array.prototype.sort
because the latter requires a function of two arguments.
Note 2
This function is intended to rely on whatever
language-sensitive comparison functionality is available to the
ECMAScript environment from the host environment, and to compare
according to the rules of the host environment's current locale.
However, regardless of the host provided comparison capabilities, this
function must treat Strings that are canonically equivalent according to
the Unicode standard as identical. It is recommended that this function
should not honour Unicode compatibility equivalences or decompositions.
For a definition and discussion of canonical equivalence see the
Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex
#15, Unicode Normalization Forms (
) and Unicode Technical Note #5, Canonical Equivalence in Applications (
). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (
).
Note 3
The
localeCompare
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.11
String.prototype.match (
regexp
When the
match
method is called with argument
regexp
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
If
regexp
is neither
undefined
nor
null
, then
Let
matcher
be ?
GetMethod
regexp
, @@match).
If
matcher
is not
undefined
, then
Return ?
Call
matcher
regexp
, «
»).
Let
be ?
ToString
).
Let
rx
be ?
RegExpCreate
regexp
undefined
).
Return ?
Invoke
rx
, @@match, «
»).
Note
The
match
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.12
String.prototype.normalize ( [
form
] )
When the
normalize
method is called with one argument
form
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
If
form
is not present or
form
is
undefined
, set
form
to
"NFC"
Let
be ?
ToString
form
).
If
is not one of
"NFC"
"NFD"
"NFKC"
, or
"NFKD"
, throw a
RangeError
exception.
Let
ns
be the String value that is the result of normalizing
into the normalization form named by
as specified in
Return
ns
Note
The
normalize
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
21.1.3.13
String.prototype.padEnd (
maxLength
[ ,
fillString
] )
When the
padEnd
method is called, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
intMaxLength
be ?
ToLength
maxLength
).
Let
stringLength
be the length of
If
intMaxLength
is not greater than
stringLength
, return
If
fillString
is
undefined
, let
filler
be the String value consisting solely of the code unit 0x0020 (SPACE).
Else, let
filler
be ?
ToString
fillString
).
If
filler
is the empty String, return
Let
fillLen
be
intMaxLength
stringLength
Let
truncatedStringFiller
be the String value consisting of repeated concatenations of
filler
truncated to length
fillLen
Return the
string-concatenation
of
and
truncatedStringFiller
Note 1
The first argument
maxLength
will be clamped such that it can be no smaller than the length of the
this
value.
Note 2
The optional second argument
fillString
defaults to
" "
(the String value consisting of the code unit 0x0020 SPACE).
21.1.3.14
String.prototype.padStart (
maxLength
[ ,
fillString
] )
When the
padStart
method is called, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
intMaxLength
be ?
ToLength
maxLength
).
Let
stringLength
be the length of
If
intMaxLength
is not greater than
stringLength
, return
If
fillString
is
undefined
, let
filler
be the String value consisting solely of the code unit 0x0020 (SPACE).
Else, let
filler
be ?
ToString
fillString
).
If
filler
is the empty String, return
Let
fillLen
be
intMaxLength
stringLength
Let
truncatedStringFiller
be the String value consisting of repeated concatenations of
filler
truncated to length
fillLen
Return the
string-concatenation
of
truncatedStringFiller
and
Note 1
The first argument
maxLength
will be clamped such that it can be no smaller than the length of the
this
value.
Note 2
The optional second argument
fillString
defaults to
" "
(the String value consisting of the code unit 0x0020 SPACE).
21.1.3.15
String.prototype.repeat (
count
The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
be ?
ToInteger
count
).
If
< 0, throw a
RangeError
exception.
If
is
+∞
, throw a
RangeError
exception.
If
is 0, return the empty String.
Return the String value that is made from
copies of
appended together.
Note 1
This method creates the String value consisting of the code units of the
this
object (converted to String) repeated
count
times.
Note 2
The
repeat
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.16
String.prototype.replace (
searchValue
replaceValue
When the
replace
method is called with arguments
searchValue
and
replaceValue
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
If
searchValue
is neither
undefined
nor
null
, then
Let
replacer
be ?
GetMethod
searchValue
, @@replace).
If
replacer
is not
undefined
, then
Return ?
Call
replacer
searchValue
, «
replaceValue
»).
Let
string
be ?
ToString
).
Let
searchString
be ?
ToString
searchValue
).
Let
functionalReplace
be
IsCallable
replaceValue
).
If
functionalReplace
is
false
, then
Set
replaceValue
to ?
ToString
replaceValue
).
string
for the first occurrence of
searchString
and let
pos
be the index within
string
of the first code unit of the matched substring and let
matched
be
searchString
. If no occurrences of
searchString
were found, return
string
If
functionalReplace
is
true
, then
Let
replValue
be ?
Call
replaceValue
undefined
, «
matched
pos
string
»).
Let
replStr
be ?
ToString
replValue
).
Else,
Let
captures
be a new empty
List
Let
replStr
be
GetSubstitution
matched
string
pos
captures
undefined
replaceValue
).
Let
tailPos
be
pos
+ the number of code units in
matched
Let
newString
be the
string-concatenation
of the first
pos
code units of
string
replStr
, and the trailing substring of
string
starting at index
tailPos
. If
pos
is 0, the first element of the concatenation will be the empty String.
Return
newString
Note
The
replace
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.16.1
Runtime Semantics: GetSubstitution (
matched
str
position
captures
namedCaptures
replacement
The abstract operation GetSubstitution performs the following steps:
Assert
Type
matched
) is String.
Let
matchLength
be the number of code units in
matched
Assert
Type
str
) is String.
Let
stringLength
be the number of code units in
str
Assert
position
is a nonnegative integer.
Assert
position
stringLength
Assert
captures
is a possibly empty
List
of Strings.
Assert
Type
replacement
) is String.
Let
tailPos
be
position
matchLength
Let
be the number of elements in
captures
If
namedCaptures
is not
undefined
, then
Set
namedCaptures
to ?
ToObject
namedCaptures
).
Let
result
be the String value derived from
replacement
by copying code unit elements from
replacement
to
result
while performing replacements as specified in
Table 51
. These
replacements are done left-to-right, and, once such a replacement is
performed, the new replacement text is not subject to further
replacements.
Return
result
Table 51: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
If
position
is 0, the replacement is the empty String. Otherwise the replacement is the substring of
str
that starts at index 0 and whose last code unit is at index
position
- 1.
0x0024, 0x0027
$'
If
tailPos
stringLength
, the replacement is the empty String. Otherwise the replacement is the substring of
str
that starts at index
tailPos
and continues to the end of
str
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n
where
is one of
1 2 3 4 5 6 7 8 9
and
$n
is not followed by a decimal digit
The
th
element of
captures
, where
is a single digit in the range 1 to 9. If
and the
th
element of
captures
is
undefined
, use the empty String instead. If
, no replacement is done.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn
where
is one of
0 1 2 3 4 5 6 7 8 9
The
nn
th
element of
captures
, where
nn
is a two-digit decimal number in the range 01 to 99. If
nn
and the
nn
th
element of
captures
is
undefined
, use the empty String instead. If
nn
is 00 or
nn
, no replacement is done.
0x0024, 0x003C
$<
If
namedCaptures
is
undefined
, the replacement text is the String
"$<"
Else,
Scan until the next
U+003E (GREATER-THAN SIGN).
If none is found, the replacement text is the String
"$<"
Else,
Let
groupName
be the enclosed substring.
Let
capture
be ?
Get
namedCaptures
groupName
).
If
capture
is
undefined
, replace the text through
with the empty string.
Otherwise, replace the text through
with ?
ToString
capture
).
0x0024
in any context that does not match any of the above.
21.1.3.17
String.prototype.search (
regexp
When the
method is called with argument
regexp
, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
If
regexp
is neither
undefined
nor
null
, then
Let
searcher
be ?
GetMethod
regexp
, @@search).
If
searcher
is not
undefined
, then
Return ?
Call
searcher
regexp
, «
»).
Let
string
be ?
ToString
).
Let
rx
be ?
RegExpCreate
regexp
undefined
).
Return ?
Invoke
rx
, @@search, «
string
»).
Note
The
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.18
String.prototype.slice (
start
end
The
slice
method takes two arguments,
start
and
end
, and returns a substring of the result of converting this object to a String, starting from index
start
and running to, but not including, index
end
(or through the end of the String if
end
is
undefined
). If
start
is negative, it is treated as
sourceLength
start
where
sourceLength
is the length of the String. If
end
is negative, it is treated as
sourceLength
end
where
sourceLength
is the length of the String. The result is a String value, not a String object. The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
len
be the length of
Let
intStart
be ?
ToInteger
start
).
If
end
is
undefined
, let
intEnd
be
len
; else let
intEnd
be ?
ToInteger
end
).
If
intStart
< 0, let
from
be
max
len
intStart
, 0); otherwise let
from
be
min
intStart
len
).
If
intEnd
< 0, let
to
be
max
len
intEnd
, 0); otherwise let
to
be
min
intEnd
len
).
Let
span
be
max
to
from
, 0).
Return the String value containing
span
consecutive code units from
beginning with the code unit at index
from
Note
The
slice
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
21.1.3.19
String.prototype.split (
separator
limit
Returns an Array object into which substrings of the result
of converting this object to a String have been stored. The substrings
are determined by searching from left to right for occurrences of
separator
; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of
separator
may be a String of any length or it may be an object, such as a RegExp, that has a @@split method.
When the
split
method is called, the following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
If
separator
is neither
undefined
nor
null
, then
Let
splitter
be ?
GetMethod
separator
, @@split).
If
splitter
is not
undefined
, then
Return ?
Call
splitter
separator
, «
limit
»).
Let
be ?
ToString
).
Let
be !
ArrayCreate
(0).
Let
lengthA
be 0.
If
limit
is
undefined
, let
lim
be 2
32
- 1; else let
lim
be ?
ToUint32
limit
).
Let
be the length of
Let
be 0.
Let
be ?
ToString
separator
).
If
lim
= 0, return
If
separator
is
undefined
, then
Perform !
CreateDataProperty
"0"
).
Return
If
= 0, then
Let
be
SplitMatch
, 0,
).
If
is not
false
, return
Perform !
CreateDataProperty
"0"
).
Return
Let
be
Repeat, while
Let
be
SplitMatch
).
If
is
false
, increase
by 1.
Else
is an
integer index
If
, increase
by 1.
Else
Let
be the String value equal to the substring of
consisting of the code units at indices
(inclusive) through
(exclusive).
Perform !
CreateDataProperty
, !
ToString
lengthA
),
).
Increment
lengthA
by 1.
If
lengthA
lim
, return
Set
to
Set
to
Let
be the String value equal to the substring of
consisting of the code units at indices
(inclusive) through
(exclusive).
Perform !
CreateDataProperty
, !
ToString
lengthA
),
).
Return
Note 1
The value of
separator
may be an empty String. In this case,
separator
does not match the empty substring at the beginning or end of the input
String, nor does it match the empty substring at the end of the
previous separator match. If
separator
is the empty String,
the String is split up into individual code unit elements; the length of
the result array equals the length of the String, and each substring
contains one code unit.
If the
this
object is (or converts to) the empty String, the result depends on whether
separator
can match the empty String. If it can, the result array contains no
elements. Otherwise, the result array contains one element, which is the
empty String.
If
separator
is
undefined
, then the result array contains just one String, which is the
this
value (converted to a String). If
limit
is not
undefined
, then the output array is truncated so that it contains no more than
limit
elements.
Note 2
The
split
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.19.1
Runtime Semantics: SplitMatch (
The abstract operation SplitMatch takes three parameters, a String
, an integer
, and a String
, and performs the following steps in order to return either
false
or the end index of a match:
Assert
Type
) is String.
Let
be the number of code units in
Let
be the number of code units in
If
, return
false
If there exists an integer
between 0 (inclusive) and
(exclusive) such that the code unit at index
within
is different from the code unit at index
within
, return
false
Return
21.1.3.20
String.prototype.startsWith (
searchString
[ ,
position
] )
The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
isRegExp
be ?
IsRegExp
searchString
).
If
isRegExp
is
true
, throw a
TypeError
exception.
Let
searchStr
be ?
ToString
searchString
).
Let
pos
be ?
ToInteger
position
).
Assert
: If
position
is
undefined
, then
pos
is 0.
Let
len
be the length of
Let
start
be
min
max
pos
, 0),
len
).
Let
searchLength
be the length of
searchStr
If
searchLength
start
is greater than
len
, return
false
If the sequence of code units of
starting at
start
of length
searchLength
is the same as the full code unit sequence of
searchStr
, return
true
Otherwise, return
false
Note 1
This method returns
true
if the sequence of code units of
searchString
converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index
position
. Otherwise returns
false
Note 2
Throwing an exception if the first argument is a RegExp is
specified in order to allow future editions to define extensions that
allow such argument values.
Note 3
The
startsWith
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.21
String.prototype.substring (
start
end
The
substring
method takes two arguments,
start
and
end
, and returns a substring of the result of converting this object to a String, starting from index
start
and running to, but not including, index
end
of the String (or through the end of the String if
end
is
undefined
). The result is a String value, not a String object.
If either argument is
NaN
or negative, it
is replaced with zero; if either argument is larger than the length of
the String, it is replaced with the length of the String.
If
start
is larger than
end
, they are swapped.
The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
len
be the length of
Let
intStart
be ?
ToInteger
start
).
If
end
is
undefined
, let
intEnd
be
len
; else let
intEnd
be ?
ToInteger
end
).
Let
finalStart
be
min
max
intStart
, 0),
len
).
Let
finalEnd
be
min
max
intEnd
, 0),
len
).
Let
from
be
min
finalStart
finalEnd
).
Let
to
be
max
finalStart
finalEnd
).
Return the String value whose length is
to
from
, containing code units from
, namely the code units with indices
from
through
to
- 1, in ascending order.
Note
The
substring
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.22
String.prototype.toLocaleLowerCase ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
toLocaleLowerCase
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleLowerCase
method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
This function works exactly the same as
toLowerCase
except that its result is intended to yield the correct result for the
host environment's current locale, rather than a locale-independent
result. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular
Unicode case mappings.
The meaning of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
Note
The
toLocaleLowerCase
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.23
String.prototype.toLocaleUpperCase ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
toLocaleUpperCase
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleUpperCase
method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
This function works exactly the same as
toUpperCase
except that its result is intended to yield the correct result for the
host environment's current locale, rather than a locale-independent
result. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular
Unicode case mappings.
The meaning of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
Note
The
toLocaleUpperCase
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.24
String.prototype.toLowerCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
. The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
cpList
be a
List
containing in order the code points as defined in
6.1.4
of
, starting at the first element of
Let
cuList
be a
List
where the elements are the result of toLowercase(
cpList
), according to the Unicode Default Case Conversion algorithm.
Let
be the String value whose code units are the
UTF16Encoding
of the code points of
cuList
Return
The result must be derived according to the
locale-insensitive case mappings in the Unicode Character Database (this
explicitly includes not only the UnicodeData.txt file, but also all
locale-insensitive mappings in the SpecialCasings.txt file that
accompanies it).
Note 1
The case mapping of some code points may produce multiple
code points. In this case the result String may not be the same length
as the source String. Because both
toUpperCase
and
toLowerCase
have context-sensitive behaviour, the functions are not symmetrical. In other words,
s.toUpperCase().toLowerCase()
is not necessarily equal to
s.toLowerCase()
Note 2
The
toLowerCase
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.25
String.prototype.toString ( )
When the
toString
method is called, the following steps are taken:
Return ?
thisStringValue
this
value).
Note
For a String object, the
toString
method happens to return the same thing as the
valueOf
method.
21.1.3.26
String.prototype.toUpperCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
This function behaves in exactly the same way as
String.prototype.toLowerCase
, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
The
toUpperCase
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.27
String.prototype.trim ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
The following steps are taken:
Let
be
this
value.
Return ?
TrimString
"start+end"
).
Note
The
trim
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.27.1
Runtime Semantics: TrimString (
string
where
The abstract operation TrimString is called with arguments
string
and
where
, and interprets the String value
string
as a sequence of UTF-16 encoded code points, as described in
6.1.4
. It performs the following steps:
Let
str
be ?
RequireObjectCoercible
string
).
Let
be ?
ToString
str
).
If
where
is
"start"
, let
be the String value that is a copy of
with leading white space removed.
Else if
where
is
"end"
, let
be the String value that is a copy of
with trailing white space removed.
Else,
Assert
where
is
"start+end"
Let
be the String value that is a copy of
with both leading and trailing white space removed.
Return
The definition of white space is the union of
WhiteSpace
and
LineTerminator
When determining whether a Unicode code point is in Unicode general
category “Space_Separator” (“Zs”), code unit sequences are interpreted
as UTF-16 encoded code point sequences as specified in
6.1.4
21.1.3.28
String.prototype.trimEnd ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
The following steps are taken:
Let
be
this
value.
Return ?
TrimString
"end"
).
Note
The
trimEnd
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.29
String.prototype.trimStart ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
The following steps are taken:
Let
be
this
value.
Return ?
TrimString
"start"
).
Note
The
trimStart
function is intentionally generic; it does not require that its
this
value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.30
String.prototype.valueOf ( )
When the
valueOf
method is called, the following steps are taken:
Return ?
thisStringValue
this
value).
21.1.3.31
String.prototype [ @@iterator ] ( )
When the
@@iterator
method is called it returns an Iterator object (
25.1.1.2
that iterates over the code points of a String value, returning each
code point as a String value. The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Return
CreateStringIterator
).
The value of the
name
property of this function is
"[Symbol.iterator]"
21.1.4
Properties of String Instances
String instances are String exotic objects and have the
internal methods specified for such objects. String instances inherit
properties from the String prototype object. String instances also have a
[[StringData]] internal slot.
String instances have a
"length"
property, and a set of enumerable properties with integer-indexed names.
21.1.4.1
length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
21.1.5
String Iterator Objects
A String Iterator is an object, that represents a specific
iteration over some specific String instance object. There is not a
named
constructor
for String Iterator objects. Instead, String iterator objects are
created by calling certain methods of String instance objects.
21.1.5.1
CreateStringIterator (
string
Several methods of String objects return Iterator objects. The abstract operation CreateStringIterator with argument
string
is used to create such iterator objects. It performs the following steps:
Assert
Type
string
) is String.
Let
iterator
be
ObjectCreate
%StringIteratorPrototype%
, « [[IteratedString]], [[StringIteratorNextIndex]] »).
Set
iterator
.[[IteratedString]] to
string
Set
iterator
.[[StringIteratorNextIndex]] to 0.
Return
iterator
21.1.5.2
The %StringIteratorPrototype% Object
The
%StringIteratorPrototype%
object:
has properties that are inherited by all String Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%IteratorPrototype%
has the following properties:
21.1.5.2.1
%StringIteratorPrototype%.next ( )
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have all of the internal slots of a String Iterator Instance (
21.1.5.3
), throw a
TypeError
exception.
Let
be
.[[IteratedString]].
If
is
undefined
, return
CreateIterResultObject
undefined
true
).
Let
position
be
.[[StringIteratorNextIndex]].
Let
len
be the length of
If
position
len
, then
Set
.[[IteratedString]] to
undefined
Return
CreateIterResultObject
undefined
true
).
Let
first
be the numeric value of the code unit at index
position
within
If
first
< 0xD800 or
first
> 0xDBFF or
position
+ 1 =
len
, let
resultString
be the String value consisting of the single code unit
first
Else,
Let
second
be the numeric value of the code unit at index
position
+ 1 within the String
If
second
< 0xDC00 or
second
> 0xDFFF, let
resultString
be the String value consisting of the single code unit
first
Else, let
resultString
be the
string-concatenation
of the code unit
first
and the code unit
second
Let
resultSize
be the number of code units in
resultString
Set
.[[StringIteratorNextIndex]] to
position
resultSize
Return
CreateIterResultObject
resultString
false
).
21.1.5.2.2
%StringIteratorPrototype% [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"String Iterator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
21.1.5.3
Properties of String Iterator Instances
String Iterator instances are ordinary objects that inherit properties from the
%StringIteratorPrototype%
intrinsic object. String Iterator instances are initially created with the internal slots listed in
Table 52
Table 52: Internal Slots of String Iterator Instances
Internal Slot
Description
[[IteratedString]]
The String value whose code units are being iterated.
[[StringIteratorNextIndex]]
The
integer index
of the next string index to be examined by this iteration.
21.2
RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled
after the regular expression facility in the Perl 5 programming
language.
21.2.1
Patterns
The
RegExp
constructor
applies the following grammar to the input pattern String. An error
occurs if the grammar cannot interpret the String as an expansion of
Pattern
Syntax
Pattern
[U, N]
::
Disjunction
[?U, ?N]
Disjunction
[U, N]
::
Alternative
[?U, ?N]
Alternative
[?U, ?N]
Disjunction
[?U, ?N]
Alternative
[U, N]
::
[empty]
Alternative
[?U, ?N]
Term
[?U, ?N]
Term
[U, N]
::
Assertion
[?U, ?N]
Atom
[?U, ?N]
Atom
[?U, ?N]
Quantifier
Assertion
[U, N]
::
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
<=
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
Quantifier
::
QuantifierPrefix
QuantifierPrefix
QuantifierPrefix
::
DecimalDigits
DecimalDigits
DecimalDigits
DecimalDigits
Atom
[U, N]
::
PatternCharacter
AtomEscape
[?U, ?N]
CharacterClass
[?U]
GroupSpecifier
[?U]
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
SyntaxCharacter
::
one of
PatternCharacter
::
SourceCharacter
but not
SyntaxCharacter
AtomEscape
[U, N]
::
DecimalEscape
CharacterClassEscape
[?U]
CharacterEscape
[?U]
[+N]
GroupName
[?U]
CharacterEscape
[U]
::
ControlEscape
ControlLetter
[lookahead ∉
DecimalDigit
HexEscapeSequence
RegExpUnicodeEscapeSequence
[?U]
IdentityEscape
[?U]
ControlEscape
::
one of
ControlLetter
::
one of
GroupSpecifier
[U]
::
[empty]
GroupName
[?U]
GroupName
[U]
::
RegExpIdentifierName
[?U]
RegExpIdentifierName
[U]
::
RegExpIdentifierStart
[?U]
RegExpIdentifierName
[?U]
RegExpIdentifierPart
[?U]
RegExpIdentifierStart
[U]
::
UnicodeIDStart
RegExpUnicodeEscapeSequence
[?U]
RegExpIdentifierPart
[U]
::
UnicodeIDContinue
RegExpUnicodeEscapeSequence
[?U]


RegExpUnicodeEscapeSequence
[U]
::
[+U]
LeadSurrogate
\u
TrailSurrogate
[+U]
LeadSurrogate
[+U]
TrailSurrogate
[+U]
NonSurrogate
[~U]
Hex4Digits
[+U]
u{
CodePoint
Each
\u
TrailSurrogate
for which the choice of associated
LeadSurrogate
is ambiguous shall be associated with the nearest possible
LeadSurrogate
that would otherwise have no corresponding
\u
TrailSurrogate
LeadSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is in the inclusive range 0xD800 to 0xDBFF
TrailSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is in the inclusive range 0xDC00 to 0xDFFF
NonSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is not in the inclusive range 0xD800 to 0xDFFF
IdentityEscape
[U]
::
[+U]
SyntaxCharacter
[+U]
[~U]
SourceCharacter
but not
UnicodeIDContinue
DecimalEscape
::
NonZeroDigit
DecimalDigits
opt
[lookahead ∉
DecimalDigit
CharacterClassEscape
[U]
::
[+U]
p{
UnicodePropertyValueExpression
[+U]
P{
UnicodePropertyValueExpression
UnicodePropertyValueExpression
::
UnicodePropertyName
UnicodePropertyValue
LoneUnicodePropertyNameOrValue
UnicodePropertyName
::
UnicodePropertyNameCharacters
UnicodePropertyNameCharacters
::
UnicodePropertyNameCharacter
UnicodePropertyNameCharacters
opt
UnicodePropertyValue
::
UnicodePropertyValueCharacters
LoneUnicodePropertyNameOrValue
::
UnicodePropertyValueCharacters
UnicodePropertyValueCharacters
::
UnicodePropertyValueCharacter
UnicodePropertyValueCharacters
opt
UnicodePropertyValueCharacter
::
UnicodePropertyNameCharacter
UnicodePropertyNameCharacter
::
ControlLetter
CharacterClass
[U]
::
[lookahead ∉ {
}]
ClassRanges
[?U]
ClassRanges
[?U]
ClassRanges
[U]
::
[empty]
NonemptyClassRanges
[?U]
NonemptyClassRanges
[U]
::
ClassAtom
[?U]
ClassAtom
[?U]
NonemptyClassRangesNoDash
[?U]
ClassAtom
[?U]
ClassAtom
[?U]
ClassRanges
[?U]
NonemptyClassRangesNoDash
[U]
::
ClassAtom
[?U]
ClassAtomNoDash
[?U]
NonemptyClassRangesNoDash
[?U]
ClassAtomNoDash
[?U]
ClassAtom
[?U]
ClassRanges
[?U]
ClassAtom
[U]
::
ClassAtomNoDash
[?U]
ClassAtomNoDash
[U]
::
SourceCharacter
but not one of
or
or
ClassEscape
[?U]
ClassEscape
[U]
::
[+U]
CharacterClassEscape
[?U]
CharacterEscape
[?U]
21.2.1.1
Static Semantics: Early Errors
Pattern
::
Disjunction
It is a Syntax Error if
NcapturingParens
≥ 2
32
- 1.
It is a Syntax Error if
Pattern
contains multiple
GroupSpecifier
s whose enclosed
RegExpIdentifierName
s have the same StringValue.
QuantifierPrefix
::
DecimalDigits
DecimalDigits
It is a Syntax Error if the MV of the first
DecimalDigits
is larger than the MV of the second
DecimalDigits
AtomEscape
::
GroupName
It is a Syntax Error if the enclosing
Pattern
does not contain a
GroupSpecifier
with an enclosed
RegExpIdentifierName
whose StringValue equals the StringValue of the
RegExpIdentifierName
of this production's
GroupName
AtomEscape
::
DecimalEscape
It is a Syntax Error if the CapturingGroupNumber of
DecimalEscape
is larger than
NcapturingParens
21.2.2.1
).
NonemptyClassRanges
::
ClassAtom
ClassAtom
ClassRanges
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is
true
or IsCharacterClass of the second
ClassAtom
is
true
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is
false
and IsCharacterClass of the second
ClassAtom
is
false
and the CharacterValue of the first
ClassAtom
is larger than the CharacterValue of the second
ClassAtom
NonemptyClassRangesNoDash
::
ClassAtomNoDash
ClassAtom
ClassRanges
It is a Syntax Error if IsCharacterClass of
ClassAtomNoDash
is
true
or IsCharacterClass of
ClassAtom
is
true
It is a Syntax Error if IsCharacterClass of
ClassAtomNoDash
is
false
and IsCharacterClass of
ClassAtom
is
false
and the CharacterValue of
ClassAtomNoDash
is larger than the CharacterValue of
ClassAtom
RegExpIdentifierStart
[U]
::
RegExpUnicodeEscapeSequence
[?U]
It is a Syntax Error if SV(
RegExpUnicodeEscapeSequence
) is none of
"$"
, or
"_"
, or the
UTF16Encoding
of a code point matched by the
UnicodeIDStart
lexical grammar production.
RegExpIdentifierPart
[U]
::
RegExpUnicodeEscapeSequence
[?U]
It is a Syntax Error if SV(
RegExpUnicodeEscapeSequence
) is none of
"$"
, or
"_"
, or the
UTF16Encoding
of either or , or the
UTF16Encoding
of a Unicode code point that would be matched by the
UnicodeIDContinue
lexical grammar production.
UnicodePropertyValueExpression
::
UnicodePropertyName
UnicodePropertyValue
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
UnicodePropertyName
is not identical to a
List
of Unicode code points that is a Unicode
property name
or property alias listed in the “
Property name
and aliases” column of
Table 54
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
UnicodePropertyValue
is not identical to a
List
of Unicode code points that is a value or value alias for the Unicode property or property alias given by SourceText of
UnicodePropertyName
listed in the “Property value and aliases” column of the corresponding tables
Table 56
or
Table 57
UnicodePropertyValueExpression
::
LoneUnicodePropertyNameOrValue
It is a Syntax Error if the
List
of Unicode code points that is SourceText of
LoneUnicodePropertyNameOrValue
is not identical to a
List
of Unicode code points that is a Unicode general category or general
category alias listed in the “Property value and aliases” column of
Table 56
, nor a binary property or binary property alias listed in the “
Property name
and aliases” column of
Table 55
21.2.1.2
Static Semantics: CapturingGroupNumber
DecimalEscape
::
NonZeroDigit
Return the MV of
NonZeroDigit
DecimalEscape
::
NonZeroDigit
DecimalDigits
Let
be the number of code points in
DecimalDigits
Return (the MV of
NonZeroDigit
× 10
) plus the MV of
DecimalDigits
The definitions of “the MV of
NonZeroDigit
” and “the MV of
DecimalDigits
” are in
11.8.3
21.2.1.3
Static Semantics: IsCharacterClass
ClassAtom
::
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
ClassEscape
::
ClassEscape
::
ClassEscape
::
CharacterEscape
Return
false
ClassEscape
::
CharacterClassEscape
Return
true
21.2.1.4
Static Semantics: CharacterValue
ClassAtom
::
Return the code point value of U+002D (HYPHEN-MINUS).
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
Let
ch
be the code point matched by
SourceCharacter
Return the code point value of
ch
ClassEscape
::
Return the code point value of U+0008 (BACKSPACE).
ClassEscape
::
Return the code point value of U+002D (HYPHEN-MINUS).
CharacterEscape
::
ControlEscape
Return the code point value according to
Table 53
Table 53: ControlEscape Code Point Values
ControlEscape
Code Point Value
Code Point
Unicode Name
Symbol
U+0009
CHARACTER TABULATION

10
U+000A
LINE FEED (LF)

11
U+000B
LINE TABULATION

12
U+000C
FORM FEED (FF)

13
U+000D
CARRIAGE RETURN (CR)

CharacterEscape
::
ControlLetter
Let
ch
be the code point matched by
ControlLetter
Let
be
ch
's code point value.
Return the remainder of dividing
by 32.
CharacterEscape
::
[lookahead ∉
DecimalDigit
Return the code point value of U+0000 (NULL).
Note
\0
represents the character and cannot be followed by a decimal digit.
CharacterEscape
::
HexEscapeSequence
Return the numeric value of the code unit that is the SV of
HexEscapeSequence
RegExpUnicodeEscapeSequence
::
LeadSurrogate
\u
TrailSurrogate
Let
lead
be the CharacterValue of
LeadSurrogate
Let
trail
be the CharacterValue of
TrailSurrogate
Let
cp
be
UTF16Decode
lead
trail
).
Return the code point value of
cp
RegExpUnicodeEscapeSequence
::
LeadSurrogate
Return the CharacterValue of
LeadSurrogate
RegExpUnicodeEscapeSequence
::
TrailSurrogate
Return the CharacterValue of
TrailSurrogate
RegExpUnicodeEscapeSequence
::
NonSurrogate
Return the CharacterValue of
NonSurrogate
RegExpUnicodeEscapeSequence
::
Hex4Digits
Return the MV of
Hex4Digits
RegExpUnicodeEscapeSequence
::
u{
CodePoint
Return the MV of
CodePoint
LeadSurrogate
::
Hex4Digits
TrailSurrogate
::
Hex4Digits
NonSurrogate
::
Hex4Digits
Return the MV of
HexDigits
CharacterEscape
::
IdentityEscape
Let
ch
be the code point matched by
IdentityEscape
Return the code point value of
ch
21.2.1.5
Static Semantics: SourceText
UnicodePropertyNameCharacters
::
UnicodePropertyNameCharacter
UnicodePropertyNameCharacters
opt
UnicodePropertyValueCharacters
::
UnicodePropertyValueCharacter
UnicodePropertyValueCharacters
opt
Return the
List
, in source text order, of Unicode code points in the source text matched by this production.
21.2.1.6
Static Semantics: StringValue
RegExpIdentifierName
[U]
::
RegExpIdentifierStart
[?U]
RegExpIdentifierName
[?U]
RegExpIdentifierPart
[?U]
Return the String value consisting of the sequence of code units corresponding to
RegExpIdentifierName
. In determining the sequence any occurrences of
RegExpUnicodeEscapeSequence
are first replaced with the code point represented by the
RegExpUnicodeEscapeSequence
and then the code points of the entire
RegExpIdentifierName
are converted to code units by
UTF16Encoding
each code point.
21.2.2
Pattern Semantics
A regular expression pattern is converted into an internal
procedure using the process described below. An implementation is
encouraged to use more efficient algorithms than the ones listed below,
as long as the results are the same. The internal procedure is used as
the value of a RegExp object's [[RegExpMatcher]] internal slot.
Pattern
is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a
"u"
A BMP pattern matches against a String interpreted as consisting of a
sequence of 16-bit values that are Unicode code points in the range of
the Basic Multilingual Plane. A Unicode pattern matches against a String
interpreted as consisting of Unicode code points encoded using UTF-16.
In the context of describing the behaviour of a BMP pattern “character”
means a single 16-bit Unicode BMP code point. In the context of
describing the behaviour of a Unicode pattern “character” means a UTF-16
encoded code point (
6.1.4
). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of
Pattern
is defined as if the source code for the
Pattern
was a
List
of
SourceCharacter
values where each
SourceCharacter
corresponds to a Unicode code point. If a BMP pattern contains a non-BMP
SourceCharacter
the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the
List
Note
For example, consider a pattern expressed in source text as
the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF).
Interpreted as a Unicode pattern, it would be a single element
(character)
List
consisting of the single code point 0x1D11E. However, interpreted as a
BMP pattern, it is first UTF-16 encoded to produce a two element
List
consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp
constructor
as ECMAScript String values in which non-BMP characters are UTF-16
encoded. For example, the single character MUSICAL SYMBOL G CLEF
pattern, expressed as a String value, is a String of length 2 whose
elements were the code units 0xD834 and 0xDD1E. So no further
translation of the string would be necessary to process it as a BMP
pattern consisting of two pattern characters. However, to process it as a
Unicode pattern
UTF16Decode
must be used in producing a
List
consisting of a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations
to or from UTF-16, but the semantics of this specification requires that
the result of pattern matching be as if such translations were
performed.
21.2.2.1
Notation
The descriptions below use the following variables:
Input
is a
List
consisting of all of the characters, in order, of the String being
matched by the regular expression pattern. Each character is either a
code unit or a code point, depending upon the kind of pattern involved.
The notation
Input
] means the
th
character of
Input
, where
can range between 0 (inclusive) and
InputLength
(exclusive).
InputLength
is the number of characters in
Input
NcapturingParens
is the total number of left-capturing parentheses (i.e. the total number of
Atom
::
GroupSpecifier
Disjunction
Parse Nodes) in the pattern. A left-capturing parenthesis is any
pattern character that is matched by the
terminal of the
Atom
::
GroupSpecifier
Disjunction
production.
DotAll
is
true
if the RegExp object's [[OriginalFlags]] internal slot contains
"s"
and otherwise is
false
IgnoreCase
is
true
if the RegExp object's [[OriginalFlags]] internal slot contains
"i"
and otherwise is
false
Multiline
is
true
if the RegExp object's [[OriginalFlags]] internal slot contains
"m"
and otherwise is
false
Unicode
is
true
if the RegExp object's [[OriginalFlags]] internal slot contains
"u"
and otherwise is
false
Furthermore, the descriptions below use the following internal data structures:
CharSet
is a mathematical set of characters, either code units or code points depending up the state of the
Unicode
flag. “All characters” means either all code unit values or all code point values also depending upon the state of
Unicode
State
is an ordered pair (
endIndex
captures
) where
endIndex
is an integer and
captures
is a
List
of
NcapturingParens
values. States are used to represent partial match states in the regular expression matching algorithms. The
endIndex
is one plus the index of the last input character matched so far by the pattern, while
captures
holds the results of capturing parentheses. The
th
element of
captures
is either a
List
that represents the value obtained by the
th
set of capturing parentheses or
undefined
if the
th
set of capturing parentheses hasn't been reached yet. Due to
backtracking, many States may be in use at any time during the matching
process.
MatchResult
is either a State or the special token
failure
that indicates that the match failed.
Continuation
procedure is an internal closure
(i.e. an internal procedure with some arguments already bound to values)
that takes one State argument and returns a MatchResult result. If an
internal closure references variables which are bound in the function
that creates the closure, the closure uses the values that these
variables had at the time the closure was created. The Continuation
attempts to match the remaining portion (specified by the closure's
already-bound arguments) of the pattern against
Input
starting at the intermediate state given by its State argument. If the
match succeeds, the Continuation returns the final State that it
reached; if the match fails, the Continuation returns
failure
Matcher
procedure is an internal closure that
takes two arguments — a State and a Continuation — and returns a
MatchResult result. A Matcher attempts to match a middle subpattern
(specified by the closure's already-bound arguments) of the pattern
against
Input
, starting at the intermediate state given by
its State argument. The Continuation argument should be a closure that
matches the rest of the pattern. After matching the subpattern of a
pattern to obtain a new State, the Matcher then calls Continuation on
that new State to test if the rest of the pattern can match as well. If
it can, the Matcher returns the State returned by Continuation; if not,
the Matcher may try different choices at its choice points, repeatedly
calling Continuation until it either succeeds or all possibilities have
been exhausted.
An
AssertionTester
procedure is an internal
closure that takes a State argument and returns a Boolean result. The
assertion tester tests a specific condition (specified by the closure's
already-bound arguments) against the current place in
Input
and returns
true
if the condition matched or
false
if not.
21.2.2.2
Pattern
The production
Pattern
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
with +1 as its
direction
argument to obtain a Matcher
Return an internal closure that takes two arguments, a String
str
and an integer
index
, and performs the following steps:
Assert
index
≤ the length of
str
If
Unicode
is
true
, let
Input
be a
List
consisting of the sequence of code points of
str
interpreted as a UTF-16 encoded (
6.1.4
) Unicode string. Otherwise, let
Input
be a
List
consisting of the sequence of code units that are the elements of
str
Input
will be used throughout the algorithms in
21.2.2
. Each element of
Input
is considered to be a character.
Let
InputLength
be the number of characters contained in
Input
. This variable will be used throughout the algorithms in
21.2.2
Let
listIndex
be the index into
Input
of the character that was obtained from element
index
of
str
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Let
cap
be a
List
of
NcapturingParens
undefined
values, indexed 1 through
NcapturingParens
Let
be the State (
listIndex
cap
).
Call
) and return its result.
Note
A Pattern evaluates (“compiles”) to an internal procedure value.
RegExpBuiltinExec
can then apply this procedure to a String and an offset within the
String to determine whether the pattern would match starting at exactly
that offset within the String, and, if it does match, what the values of
the capturing parentheses would be. The algorithms in
21.2.2
are designed so that compiling a pattern may throw a
SyntaxError
exception; on the other hand, once the pattern is successfully
compiled, applying the resulting internal procedure to find a match in a
String cannot throw an exception (except for any host-defined
exceptions that can occur anywhere such as out-of-memory).
21.2.2.3
Disjunction
With parameter
direction
The production
Disjunction
::
Alternative
evaluates as follows:
Evaluate
Alternative
with argument
direction
to obtain a Matcher
Return
The production
Disjunction
::
Alternative
Disjunction
evaluates as follows:
Evaluate
Alternative
with argument
direction
to obtain a Matcher
m1
Evaluate
Disjunction
with argument
direction
to obtain a Matcher
m2
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Call
m1
) and let
be its result.
If
is not
failure
, return
Call
m2
) and return its result.
Note
The
regular expression operator separates two alternatives. The pattern first tries to match the left
Alternative
(followed by the sequel of the regular expression); if it fails, it tries to match the right
Disjunction
(followed by the sequel of the regular expression). If the left
Alternative
, the right
Disjunction
, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left
Alternative
. If choices in the left
Alternative
are exhausted, the right
Disjunction
is tried instead of the left
Alternative
. Any capturing parentheses inside a portion of the pattern skipped by
produce
undefined
values instead of Strings. Thus, for example,
/a|ab/.exec(
"abc"
returns the result
"a"
and not
"ab"
. Moreover,
/((a)|(ab))((c)|(bc))/.exec(
"abc"
returns the array
"abc"
"a"
"a"
undefined
"bc"
undefined
"bc"
and not
"abc"
"ab"
undefined
"ab"
"c"
"c"
undefined
The order in which the two alternatives are tried is independent of the value of
direction
21.2.2.4
Alternative
With parameter
direction
The production
Alternative
::
[empty]
evaluates as follows:
Return a Matcher that takes two arguments, a State
and a Continuation
, and returns the result of calling
).
The production
Alternative
::
Alternative
Term
evaluates as follows:
Evaluate
Alternative
with argument
direction
to obtain a Matcher
m1
Evaluate
Term
with argument
direction
to obtain a Matcher
m2
If
direction
is equal to +1, then
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Let
be a Continuation that takes a State argument
and returns the result of calling
m2
).
Call
m1
) and return its result.
Else,
Assert
direction
is equal to -1.
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Let
be a Continuation that takes a State argument
and returns the result of calling
m1
).
Call
m2
) and return its result.
Note
Consecutive
Term
s try to simultaneously match consecutive portions of
Input
. When
direction
is equal to +1, if the left
Alternative
, the right
Term
and the sequel of the regular expression all have choice points, all
choices in the sequel are tried before moving on to the next choice in
the right
Term
, and all choices in the right
Term
are tried before moving on to the next choice in the left
Alternative
. When
direction
is equal to -1, the evaluation order of
Alternative
and
Term
are reversed.
21.2.2.5
Term
With parameter
direction
The production
Term
::
Assertion
evaluates as follows:
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Evaluate
Assertion
to obtain an AssertionTester
Call
) and let
be the resulting Boolean value.
If
is
false
, return
failure
Call
) and return its result.
Note
The AssertionTester is independent of
direction
The production
Term
::
Atom
evaluates as follows:
Return the Matcher that is the result of evaluating
Atom
with argument
direction
The production
Term
::
Atom
Quantifier
evaluates as follows:
Evaluate
Atom
with argument
direction
to obtain a Matcher
Evaluate
Quantifier
to obtain the three results: an integer
min
, an integer (or ∞)
max
, and Boolean
greedy
Assert
: If
max
is finite, then
max
is not less than
min
Let
parenIndex
be the number of left-capturing parentheses in the entire regular expression that occur to the left of this
Term
. This is the total number of
Atom
::
GroupSpecifier
Disjunction
Parse Nodes prior to or enclosing this
Term
Let
parenCount
be the number of left-capturing parentheses in
Atom
. This is the total number of
Atom
::
GroupSpecifier
Disjunction
Parse Nodes enclosed by
Atom
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Call
RepeatMatcher
min
max
greedy
parenIndex
parenCount
) and return its result.
21.2.2.5.1
Runtime Semantics: RepeatMatcher (
min
max
greedy
parenIndex
parenCount
The abstract operation RepeatMatcher takes eight parameters, a Matcher
, an integer
min
, an integer (or ∞)
max
, a Boolean
greedy
, a State
, a Continuation
, an integer
parenIndex
, and an integer
parenCount
, and performs the following steps:
If
max
is zero, return
).
Let
be an internal Continuation closure that takes one State argument
and performs the following steps when evaluated:
If
min
is zero and
's
endIndex
is equal to
's
endIndex
, return
failure
If
min
is zero, let
min2
be zero; otherwise let
min2
be
min
- 1.
If
max
is ∞, let
max2
be ∞; otherwise let
max2
be
max
- 1.
Call
RepeatMatcher
min2
max2
greedy
parenIndex
parenCount
) and return its result.
Let
cap
be a copy of
's
captures
List
For each integer
that satisfies
parenIndex
and
parenIndex
parenCount
, set
cap
] to
undefined
Let
be
's
endIndex
Let
xr
be the State (
cap
).
If
min
is not zero, return
xr
).
If
greedy
is
false
, then
Call
) and let
be its result.
If
is not
failure
, return
Call
xr
) and return its result.
Call
xr
) and let
be its result.
If
is not
failure
, return
Call
) and return its result.
Note 1
An
Atom
followed by a
Quantifier
is repeated the number of times specified by the
Quantifier
. A
Quantifier
can be non-greedy, in which case the
Atom
pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the
Atom
pattern is repeated as many times as possible while still matching the sequel. The
Atom
pattern is repeated rather than the input character sequence that it matches, so different repetitions of the
Atom
can match different input substrings.
Note 2
If the
Atom
and the sequel of the regular expression all have choice points, the
Atom
is first matched as many (or as few, if non-greedy) times as possible.
All choices in the sequel are tried before moving on to the next choice
in the last repetition of
Atom
. All choices in the last (n
th
) repetition of
Atom
are tried before moving on to the next choice in the next-to-last (n - 1)
st
repetition of
Atom
; at which point it may turn out that more or fewer repetitions of
Atom
are now possible; these are exhausted (again, starting with either as
few or as many as possible) before moving on to the next choice in the
(n - 1)
st
repetition of
Atom
and so on.
Compare
/a[a-z]{
}/.exec(
"abcdefghi"
which returns
"abcde"
with
/a[a-z]{
}?
/.exec("abcdefghi")
which returns
"abc"
Consider also
/(aa|aabaac|ba|b|c)*
/.exec("aabaac")
which, by the choice point ordering above, returns the array
"aaba"
"ba"
and not any of:
"aabaac"
"aabaac"
"aabaac"
"c"
The above ordering of choice points can be used to write a
regular expression that calculates the greatest common divisor of two
numbers (represented in unary notation). The following example
calculates the gcd of 10 and 15:
"aaaaaaaaaa,aaaaaaaaaaaaaaa"
.replace(
/^(a+)\1*,\1+$/
"$1"
which returns the gcd in unary notation
"aaaaa"
Note 3
Step 4 of the RepeatMatcher clears
Atom
's captures each time
Atom
is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*
/.exec("zaacbbbcac")
which returns the array
"zaacbbbcac"
"z"
"ac"
"a"
undefined
"c"
and not
"zaacbbbcac"
"z"
"ac"
"a"
"bbb"
"c"
because each iteration of the outermost
clears all captured Strings contained in the quantified
Atom
, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 1 of the RepeatMatcher's
closure states that, once the minimum number of repetitions has been satisfied, any more expansions of
Atom
that match the empty character sequence are not considered for further
repetitions. This prevents the regular expression engine from falling
into an infinite loop on patterns such as:
/(a*)*
/.exec("b")
or the slightly more complicated:
/(a*)b\
/.exec("baaaac")
which returns the array
"b"
""
21.2.2.6
Assertion
The production
Assertion
::
evaluates as follows:
Return an internal AssertionTester closure that takes a State argument
and performs the following steps when evaluated:
Let
be
's
endIndex
If
is zero, return
true
If
Multiline
is
false
, return
false
If the character
Input
- 1] is one of
LineTerminator
, return
true
Return
false
Note
Even when the
flag is used with a pattern,
always matches only at the beginning of
Input
, or (if
Multiline
is
true
) at the beginning of a line.
The production
Assertion
::
evaluates as follows:
Return an internal AssertionTester closure that takes a State argument
and performs the following steps when evaluated:
Let
be
's
endIndex
If
is equal to
InputLength
, return
true
If
Multiline
is
false
, return
false
If the character
Input
] is one of
LineTerminator
, return
true
Return
false
The production
Assertion
::
evaluates as follows:
Return an internal AssertionTester closure that takes a State argument
and performs the following steps when evaluated:
Let
be
's
endIndex
Call
IsWordChar
- 1) and let
be the Boolean result.
Call
IsWordChar
) and let
be the Boolean result.
If
is
true
and
is
false
, return
true
If
is
false
and
is
true
, return
true
Return
false
The production
Assertion
::
evaluates as follows:
Return an internal AssertionTester closure that takes a State argument
and performs the following steps when evaluated:
Let
be
's
endIndex
Call
IsWordChar
- 1) and let
be the Boolean result.
Call
IsWordChar
) and let
be the Boolean result.
If
is
true
and
is
false
, return
false
If
is
false
and
is
true
, return
false
Return
true
The production
Assertion
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
with +1 as its
direction
argument to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
is
failure
, return
failure
Let
be
's State.
Let
cap
be
's
captures
List
Let
xe
be
's
endIndex
Let
be the State (
xe
cap
).
Call
) and return its result.
The production
Assertion
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
with +1 as its
direction
argument to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
is not
failure
, return
failure
Call
) and return its result.
The production
Assertion
::
<=
Disjunction
evaluates as follows:
Evaluate
Disjunction
with -1 as its
direction
argument to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
is
failure
, return
failure
Let
be
's State.
Let
cap
be
's
captures
List
Let
xe
be
's
endIndex
Let
be the State (
xe
cap
).
Call
) and return its result.
The production
Assertion
::
Disjunction
evaluates as follows:
Evaluate
Disjunction
with -1 as its
direction
argument to obtain a Matcher
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
be a Continuation that always returns its State argument as a successful MatchResult.
Call
) and let
be its result.
If
is not
failure
, return
failure
Call
) and return its result.
21.2.2.6.1
Runtime Semantics: WordCharacters ( )
The abstract operation WordCharacters performs the following steps:
Let
be a set of characters containing the sixty-three characters:
Let
be an empty set.
For each character
not in set
where
Canonicalize
) is in
, add
to
Assert
: Unless
Unicode
and
IgnoreCase
are both
true
is empty.
Add the characters in set
to set
Return
21.2.2.6.2
Runtime Semantics: IsWordChar (
The abstract operation IsWordChar takes an integer parameter
and performs the following steps:
If
is -1 or
is
InputLength
, return
false
Let
be the character
Input
].
Let
wordChars
be the result of !
WordCharacters
().
If
is in
wordChars
, return
true
Return
false
21.2.2.7
Quantifier
The production
Quantifier
::
QuantifierPrefix
evaluates as follows:
Evaluate
QuantifierPrefix
to obtain the two results: an integer
min
and an integer (or ∞)
max
Return the three results
min
max
, and
true
The production
Quantifier
::
QuantifierPrefix
evaluates as follows:
Evaluate
QuantifierPrefix
to obtain the two results: an integer
min
and an integer (or ∞)
max
Return the three results
min
max
, and
false
The production
QuantifierPrefix
::
evaluates as follows:
Return the two results 0 and ∞.
The production
QuantifierPrefix
::
evaluates as follows:
Return the two results 1 and ∞.
The production
QuantifierPrefix
::
evaluates as follows:
Return the two results 0 and 1.
The production
QuantifierPrefix
::
DecimalDigits
evaluates as follows:
Let
be the MV of
DecimalDigits
(see
11.8.3
).
Return the two results
and
The production
QuantifierPrefix
::
DecimalDigits
evaluates as follows:
Let
be the MV of
DecimalDigits
Return the two results
and ∞.
The production
QuantifierPrefix
::
DecimalDigits
DecimalDigits
evaluates as follows:
Let
be the MV of the first
DecimalDigits
Let
be the MV of the second
DecimalDigits
Return the two results
and
21.2.2.8
Atom
With parameter
direction
The production
Atom
::
PatternCharacter
evaluates as follows:
Let
ch
be the character matched by
PatternCharacter
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
direction
) and return its Matcher result.
The production
Atom
::
evaluates as follows:
If
DotAll
is
true
, then
Let
be the set of all characters.
Otherwise, let
be the set of all characters except
LineTerminator
Call
CharacterSetMatcher
false
direction
) and return its Matcher result.
The production
Atom
::
AtomEscape
evaluates as follows:
Return the Matcher that is the result of evaluating
AtomEscape
with argument
direction
The production
Atom
::
CharacterClass
evaluates as follows:
Evaluate
CharacterClass
to obtain a CharSet
and a Boolean
invert
Call
CharacterSetMatcher
invert
direction
) and return its Matcher result.
The production
Atom
::
GroupSpecifier
Disjunction
evaluates as follows:
Evaluate
Disjunction
with argument
direction
to obtain a Matcher
Let
parenIndex
be the number of left-capturing parentheses in the entire regular expression that occur to the left of this
Atom
. This is the total number of
Atom
::
GroupSpecifier
Disjunction
Parse Nodes prior to or enclosing this
Atom
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
be an internal Continuation closure that takes one State argument
and performs the following steps:
Let
cap
be a copy of
's
captures
List
Let
xe
be
's
endIndex
Let
ye
be
's
endIndex
If
direction
is equal to +1, then
Assert
xe
ye
Let
be a new
List
whose elements are the characters of
Input
at indices
xe
(inclusive) through
ye
(exclusive).
Else,
Assert
direction
is equal to -1.
Assert
ye
xe
Let
be a new
List
whose elements are the characters of
Input
at indices
ye
(inclusive) through
xe
(exclusive).
Set
cap
parenIndex
+ 1] to
Let
be the State (
ye
cap
).
Call
) and return its result.
Call
) and return its result.
The production
Atom
::
Disjunction
evaluates as follows:
Return the Matcher that is the result of evaluating
Disjunction
with argument
direction
21.2.2.8.1
Runtime Semantics: CharacterSetMatcher (
invert
direction
The abstract operation CharacterSetMatcher takes three arguments, a CharSet
, a Boolean flag
invert
, and an integer
direction
, and performs the following steps:
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps when evaluated:
Let
be
's
endIndex
Let
be
direction
If
< 0 or
InputLength
, return
failure
Let
index
be
min
).
Let
ch
be the character
Input
index
].
Let
cc
be
Canonicalize
ch
).
If
invert
is
false
, then
If there does not exist a member
of set
such that
Canonicalize
) is
cc
, return
failure
Else,
Assert
invert
is
true
If there exists a member
of set
such that
Canonicalize
) is
cc
, return
failure
Let
cap
be
's
captures
List
Let
be the State (
cap
).
Call
) and return its result.
21.2.2.8.2
Runtime Semantics: Canonicalize (
ch
The abstract operation Canonicalize takes a character parameter
ch
and performs the following steps:
If
IgnoreCase
is
false
, return
ch
If
Unicode
is
true
, then
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for
ch
, return the result of applying that mapping to
ch
Return
ch
Else,
Assert
ch
is a UTF-16 code unit.
Let
be the String value consisting of the single code unit
ch
Let
be the same result produced as if by performing the algorithm for
String.prototype.toUpperCase
using
as the
this
value.
Assert
Type
) is String.
If
does not consist of a single code unit, return
ch
Let
cu
be
's single code unit element.
If the numeric value of
ch
≥ 128 and the numeric value of
cu
< 128, return
ch
Return
cu
Note 1
Parentheses of the form
Disjunction
serve both to group the components of the
Disjunction
pattern together and to save the result of the match. The result can be used either in a backreference (
followed by a nonzero decimal number), referenced in a replace String,
or returned as part of an array from the regular expression matching
internal procedure. To inhibit the capturing behaviour of parentheses,
use the form
(?:
Disjunction
instead.
Note 2
The form
(?=
Disjunction
specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside
Disjunction
must match at the current position, but the current position is not advanced before matching the sequel. If
Disjunction
can match at the current position in several ways, only the first one
is tried. Unlike other regular expression operators, there is no
backtracking into a
(?=
form (this unusual behaviour is inherited from Perl). This only matters when the
Disjunction
contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec(
"baaabac"
matches the empty String immediately after the first
and therefore returns the array:
""
"aaa"
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\
/.exec(
"baaabac"
This expression returns
"aba"
"a"
and not:
"aaaba"
"a"
Note 3
The form
(?!
Disjunction
specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside
Disjunction
must fail to match at the current position. The current position is not advanced before matching the sequel.
Disjunction
can contain capturing parentheses, but backreferences to them only make sense from within
Disjunction
itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return
undefined
because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\
c)\
(.*)/.exec(
"baaabaac"
looks for an
not immediately followed by some positive number n of
's, a
, another n
's (specified by the first
\2
) and a
. The second
\2
is outside the negative lookahead, so it matches against
undefined
and therefore always succeeds. The whole expression returns the array:
"baaabaac"
"ba"
undefined
"abaac"
Note 4
In case-insignificant matches when
Unicode
is
true
all characters are implicitly case-folded using the simple mapping
provided by the Unicode standard immediately before they are compared.
The simple mapping always maps to a single code point, so it does not
map, for example,
"ß"
(U+00DF) to
"SS"
. It may however map a code point outside the Basic Latin range to a character within, for example,
"ſ"
(U+017F) to
"s"
. Such characters are not mapped if
Unicode
is
false
. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as
/[a-z]/i
, but they will match
/[a-z]/ui
21.2.2.8.3
Runtime Semantics: UnicodeMatchProperty (
The abstract operation UnicodeMatchProperty takes a parameter
that is a
List
of Unicode code points and performs the following steps:
Assert
is a
List
of Unicode code points that is identical to a
List
of Unicode code points that is a Unicode
property name
or property alias listed in the “
Property name
and aliases” column of
Table 54
or
Table 55
Let
be the canonical
property name
of
as given in the “Canonical
property name
” column of the corresponding row.
Return the
List
of Unicode code points of
Implementations must support the Unicode property names and aliases listed in
Table 54
and
Table 55
. To ensure interoperability, implementations must not support any other property names or aliases.
Note 1
For example,
Script_Extensions
property name
) and
scx
(property alias) are valid, but
script_extensions
or
Scx
aren't.
Note 2
The listed properties form a superset of what
UTS18 RL1.2
requires.
Table 54: Non-binary Unicode property aliases and their canonical property names
Property name
and aliases
Canonical
property name
General_Category
gc
General_Category
Script
sc
Script
Script_Extensions
scx
Script_Extensions
Table 55: Binary Unicode property aliases and their canonical property names
Property name
and aliases
Canonical
property name
ASCII
ASCII
ASCII_Hex_Digit
AHex
ASCII_Hex_Digit
Alphabetic
Alpha
Alphabetic
Any
Any
Assigned
Assigned
Bidi_Control
Bidi_C
Bidi_Control
Bidi_Mirrored
Bidi_M
Bidi_Mirrored
Case_Ignorable
CI
Case_Ignorable
Cased
Cased
Changes_When_Casefolded
CWCF
Changes_When_Casefolded
Changes_When_Casemapped
CWCM
Changes_When_Casemapped
Changes_When_Lowercased
CWL
Changes_When_Lowercased
Changes_When_NFKC_Casefolded
CWKCF
Changes_When_NFKC_Casefolded
Changes_When_Titlecased
CWT
Changes_When_Titlecased
Changes_When_Uppercased
CWU
Changes_When_Uppercased
Dash
Dash
Default_Ignorable_Code_Point
DI
Default_Ignorable_Code_Point
Deprecated
Dep
Deprecated
Diacritic
Dia
Diacritic
Emoji
Emoji
Emoji_Component
Emoji_Component
Emoji_Modifier
Emoji_Modifier
Emoji_Modifier_Base
Emoji_Modifier_Base
Emoji_Presentation
Emoji_Presentation
Extended_Pictographic
Extended_Pictographic
Extender
Ext
Extender
Grapheme_Base
Gr_Base
Grapheme_Base
Grapheme_Extend
Gr_Ext
Grapheme_Extend
Hex_Digit
Hex
Hex_Digit
IDS_Binary_Operator
IDSB
IDS_Binary_Operator
IDS_Trinary_Operator
IDST
IDS_Trinary_Operator
ID_Continue
IDC
ID_Continue
ID_Start
IDS
ID_Start
Ideographic
Ideo
Ideographic
Join_Control
Join_C
Join_Control
Logical_Order_Exception
LOE
Logical_Order_Exception
Lowercase
Lower
Lowercase
Math
Math
Noncharacter_Code_Point
NChar
Noncharacter_Code_Point
Pattern_Syntax
Pat_Syn
Pattern_Syntax
Pattern_White_Space
Pat_WS
Pattern_White_Space
Quotation_Mark
QMark
Quotation_Mark
Radical
Radical
Regional_Indicator
RI
Regional_Indicator
Sentence_Terminal
STerm
Sentence_Terminal
Soft_Dotted
SD
Soft_Dotted
Terminal_Punctuation
Term
Terminal_Punctuation
Unified_Ideograph
UIdeo
Unified_Ideograph
Uppercase
Upper
Uppercase
Variation_Selector
VS
Variation_Selector
White_Space
space
White_Space
XID_Continue
XIDC
XID_Continue
XID_Start
XIDS
XID_Start
21.2.2.8.4
Runtime Semantics: UnicodeMatchPropertyValue (
The abstract operation UnicodeMatchPropertyValue takes two parameters
and
, each of which is a
List
of Unicode code points, and performs the following steps:
Assert
is a
List
of Unicode code points that is identical to a
List
of Unicode code points that is a canonical, unaliased Unicode
property name
listed in the “Canonical
property name
” column of
Table 54
Assert
is a
List
of Unicode code points that is identical to a
List
of Unicode code points that is a property value or property value alias for Unicode property
listed in the “Property value and aliases” column of
Table 56
or
Table 57
Let
value
be the canonical property value of
as given in the “Canonical property value” column of the corresponding row.
Return the
List
of Unicode code points of
value
Implementations must support the Unicode property value names and aliases listed in
Table 56
and
Table 57
. To ensure interoperability, implementations must not support any other property value names or aliases.
Note 1
For example,
Xpeo
and
Old_Persian
are valid
Script_Extensions
values, but
xpeo
and
Old Persian
aren't.
Note 2
This algorithm differs from
the matching rules for symbolic values listed in UAX44
: case,
white space
, U+002D (HYPHEN-MINUS), and U+005F (LOW LINE) are not ignored, and the
Is
prefix is not supported.
Table 56: Value aliases and canonical values for the Unicode property
General_Category
Property value and aliases
Canonical property value
Cased_Letter
LC
Cased_Letter
Close_Punctuation
Pe
Close_Punctuation
Connector_Punctuation
Pc
Connector_Punctuation
Control
Cc
cntrl
Control
Currency_Symbol
Sc
Currency_Symbol
Dash_Punctuation
Pd
Dash_Punctuation
Decimal_Number
Nd
digit
Decimal_Number
Enclosing_Mark
Me
Enclosing_Mark
Final_Punctuation
Pf
Final_Punctuation
Format
Cf
Format
Initial_Punctuation
Pi
Initial_Punctuation
Letter
Letter
Letter_Number
Nl
Letter_Number
Line_Separator
Zl
Line_Separator
Lowercase_Letter
Ll
Lowercase_Letter
Mark
Combining_Mark
Mark
Math_Symbol
Sm
Math_Symbol
Modifier_Letter
Lm
Modifier_Letter
Modifier_Symbol
Sk
Modifier_Symbol
Nonspacing_Mark
Mn
Nonspacing_Mark
Number
Number
Open_Punctuation
Ps
Open_Punctuation
Other
Other
Other_Letter
Lo
Other_Letter
Other_Number
No
Other_Number
Other_Punctuation
Po
Other_Punctuation
Other_Symbol
So
Other_Symbol
Paragraph_Separator
Zp
Paragraph_Separator
Private_Use
Co
Private_Use
Punctuation
punct
Punctuation
Separator
Separator
Space_Separator
Zs
Space_Separator
Spacing_Mark
Mc
Spacing_Mark
Surrogate
Cs
Surrogate
Symbol
Symbol
Titlecase_Letter
Lt
Titlecase_Letter
Unassigned
Cn
Unassigned
Uppercase_Letter
Lu
Uppercase_Letter
Table 57: Value aliases and canonical values for the Unicode properties
Script
and
Script_Extensions
Property value and aliases
Canonical property value
Adlam
Adlm
Adlam
Ahom
Ahom
Ahom
Anatolian_Hieroglyphs
Hluw
Anatolian_Hieroglyphs
Arabic
Arab
Arabic
Armenian
Armn
Armenian
Avestan
Avst
Avestan
Balinese
Bali
Balinese
Bamum
Bamu
Bamum
Bassa_Vah
Bass
Bassa_Vah
Batak
Batk
Batak
Bengali
Beng
Bengali
Bhaiksuki
Bhks
Bhaiksuki
Bopomofo
Bopo
Bopomofo
Brahmi
Brah
Brahmi
Braille
Brai
Braille
Buginese
Bugi
Buginese
Buhid
Buhd
Buhid
Canadian_Aboriginal
Cans
Canadian_Aboriginal
Carian
Cari
Carian
Caucasian_Albanian
Aghb
Caucasian_Albanian
Chakma
Cakm
Chakma
Cham
Cham
Cham
Cherokee
Cher
Cherokee
Common
Zyyy
Common
Coptic
Copt
Qaac
Coptic
Cuneiform
Xsux
Cuneiform
Cypriot
Cprt
Cypriot
Cyrillic
Cyrl
Cyrillic
Deseret
Dsrt
Deseret
Devanagari
Deva
Devanagari
Dogra
Dogr
Dogra
Duployan
Dupl
Duployan
Egyptian_Hieroglyphs
Egyp
Egyptian_Hieroglyphs
Elbasan
Elba
Elbasan
Ethiopic
Ethi
Ethiopic
Georgian
Geor
Georgian
Glagolitic
Glag
Glagolitic
Gothic
Goth
Gothic
Grantha
Gran
Grantha
Greek
Grek
Greek
Gujarati
Gujr
Gujarati
Gunjala_Gondi
Gong
Gunjala_Gondi
Gurmukhi
Guru
Gurmukhi
Han
Hani
Han
Hangul
Hang
Hangul
Hanifi_Rohingya
Rohg
Hanifi_Rohingya
Hanunoo
Hano
Hanunoo
Hatran
Hatr
Hatran
Hebrew
Hebr
Hebrew
Hiragana
Hira
Hiragana
Imperial_Aramaic
Armi
Imperial_Aramaic
Inherited
Zinh
Qaai
Inherited
Inscriptional_Pahlavi
Phli
Inscriptional_Pahlavi
Inscriptional_Parthian
Prti
Inscriptional_Parthian
Javanese
Java
Javanese
Kaithi
Kthi
Kaithi
Kannada
Knda
Kannada
Katakana
Kana
Katakana
Kayah_Li
Kali
Kayah_Li
Kharoshthi
Khar
Kharoshthi
Khmer
Khmr
Khmer
Khojki
Khoj
Khojki
Khudawadi
Sind
Khudawadi
Lao
Laoo
Lao
Latin
Latn
Latin
Lepcha
Lepc
Lepcha
Limbu
Limb
Limbu
Linear_A
Lina
Linear_A
Linear_B
Linb
Linear_B
Lisu
Lisu
Lisu
Lycian
Lyci
Lycian
Lydian
Lydi
Lydian
Mahajani
Mahj
Mahajani
Makasar
Maka
Makasar
Malayalam
Mlym
Malayalam
Mandaic
Mand
Mandaic
Manichaean
Mani
Manichaean
Marchen
Marc
Marchen
Medefaidrin
Medf
Medefaidrin
Masaram_Gondi
Gonm
Masaram_Gondi
Meetei_Mayek
Mtei
Meetei_Mayek
Mende_Kikakui
Mend
Mende_Kikakui
Meroitic_Cursive
Merc
Meroitic_Cursive
Meroitic_Hieroglyphs
Mero
Meroitic_Hieroglyphs
Miao
Plrd
Miao
Modi
Modi
Modi
Mongolian
Mong
Mongolian
Mro
Mroo
Mro
Multani
Mult
Multani
Myanmar
Mymr
Myanmar
Nabataean
Nbat
Nabataean
New_Tai_Lue
Talu
New_Tai_Lue
Newa
Newa
Newa
Nko
Nkoo
Nko
Nushu
Nshu
Nushu
Ogham
Ogam
Ogham
Ol_Chiki
Olck
Ol_Chiki
Old_Hungarian
Hung
Old_Hungarian
Old_Italic
Ital
Old_Italic
Old_North_Arabian
Narb
Old_North_Arabian
Old_Permic
Perm
Old_Permic
Old_Persian
Xpeo
Old_Persian
Old_Sogdian
Sogo
Old_Sogdian
Old_South_Arabian
Sarb
Old_South_Arabian
Old_Turkic
Orkh
Old_Turkic
Oriya
Orya
Oriya
Osage
Osge
Osage
Osmanya
Osma
Osmanya
Pahawh_Hmong
Hmng
Pahawh_Hmong
Palmyrene
Palm
Palmyrene
Pau_Cin_Hau
Pauc
Pau_Cin_Hau
Phags_Pa
Phag
Phags_Pa
Phoenician
Phnx
Phoenician
Psalter_Pahlavi
Phlp
Psalter_Pahlavi
Rejang
Rjng
Rejang
Runic
Runr
Runic
Samaritan
Samr
Samaritan
Saurashtra
Saur
Saurashtra
Sharada
Shrd
Sharada
Shavian
Shaw
Shavian
Siddham
Sidd
Siddham
SignWriting
Sgnw
SignWriting
Sinhala
Sinh
Sinhala
Sogdian
Sogd
Sogdian
Sora_Sompeng
Sora
Sora_Sompeng
Soyombo
Soyo
Soyombo
Sundanese
Sund
Sundanese
Syloti_Nagri
Sylo
Syloti_Nagri
Syriac
Syrc
Syriac
Tagalog
Tglg
Tagalog
Tagbanwa
Tagb
Tagbanwa
Tai_Le
Tale
Tai_Le
Tai_Tham
Lana
Tai_Tham
Tai_Viet
Tavt
Tai_Viet
Takri
Takr
Takri
Tamil
Taml
Tamil
Tangut
Tang
Tangut
Telugu
Telu
Telugu
Thaana
Thaa
Thaana
Thai
Thai
Thai
Tibetan
Tibt
Tibetan
Tifinagh
Tfng
Tifinagh
Tirhuta
Tirh
Tirhuta
Ugaritic
Ugar
Ugaritic
Vai
Vaii
Vai
Warang_Citi
Wara
Warang_Citi
Yi
Yiii
Yi
Zanabazar_Square
Zanb
Zanabazar_Square
21.2.2.9
AtomEscape
With parameter
direction
The production
AtomEscape
::
DecimalEscape
evaluates as follows:
Evaluate
DecimalEscape
to obtain an integer
Assert
NcapturingParens
Call
BackreferenceMatcher
direction
) and return its Matcher result.
The production
AtomEscape
::
CharacterEscape
evaluates as follows:
Evaluate
CharacterEscape
to obtain a character
ch
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
direction
) and return its Matcher result.
The production
AtomEscape
::
CharacterClassEscape
evaluates as follows:
Evaluate
CharacterClassEscape
to obtain a CharSet
Call
CharacterSetMatcher
false
direction
) and return its Matcher result.
Note
An escape sequence of the form
followed by a nonzero decimal number
matches the result of the
th set of capturing parentheses (
21.2.2.1
). It is an error if the regular expression has fewer than
capturing parentheses. If the regular expression has
or more capturing parentheses but the
th one is
undefined
because it has not captured anything, then the backreference always succeeds.
The production
AtomEscape
::
GroupName
evaluates as follows:
Search the enclosing
Pattern
for an instance of a
GroupSpecifier
for a
RegExpIdentifierName
which has a StringValue equal to the StringValue of the
RegExpIdentifierName
contained in
GroupName
Assert
: A unique such
GroupSpecifier
is found.
Let
parenIndex
be the number of left-capturing parentheses in the entire regular expression that occur to the left of the located
GroupSpecifier
. This is the total number of
Atom
::
GroupSpecifier
Disjunction
Parse Nodes prior to or enclosing the located
GroupSpecifier
Call
BackreferenceMatcher
parenIndex
direction
) and return its Matcher result.
21.2.2.9.1
Runtime Semantics: BackreferenceMatcher (
direction
The abstract operation BackreferenceMatcher takes two arguments, an integer
and an integer
direction
, and performs the following steps:
Return an internal Matcher closure that takes two arguments, a State
and a Continuation
, and performs the following steps:
Let
cap
be
's
captures
List
Let
be
cap
].
If
is
undefined
, return
).
Let
be
's
endIndex
Let
len
be the number of elements in
Let
be
direction
len
If
< 0 or
InputLength
, return
failure
Let
be
min
).
If there exists an integer
between 0 (inclusive) and
len
(exclusive) such that
Canonicalize
]) is not the same character value as
Canonicalize
Input
]), return
failure
Let
be the State (
cap
).
Call
) and return its result.
21.2.2.10
CharacterEscape
The
CharacterEscape
productions evaluate as follows:
CharacterEscape
::
ControlEscape
ControlLetter
[lookahead ∉
DecimalDigit
HexEscapeSequence
RegExpUnicodeEscapeSequence
IdentityEscape
Let
cv
be the CharacterValue of this
CharacterEscape
Return the character whose character value is
cv
21.2.2.11
DecimalEscape
The
DecimalEscape
productions evaluate as follows:
DecimalEscape
::
NonZeroDigit
DecimalDigits
opt
Return the CapturingGroupNumber of this
DecimalEscape
Note
If
is followed by a decimal number
whose first digit is not
, then the escape sequence is considered to be a backreference. It is an error if
is greater than the total number of left-capturing parentheses in the entire regular expression.
21.2.2.12
CharacterClassEscape
The production
CharacterClassEscape
::
evaluates as follows:
Return the ten-element set of characters containing the characters
through
inclusive.
The production
CharacterClassEscape
::
evaluates as follows:
Return the set of all characters not included in the set returned by
CharacterClassEscape
::
The production
CharacterClassEscape
::
evaluates as follows:
Return the set of characters containing the characters that are on the right-hand side of the
WhiteSpace
or
LineTerminator
productions.
The production
CharacterClassEscape
::
evaluates as follows:
Return the set of all characters not included in the set returned by
CharacterClassEscape
::
The production
CharacterClassEscape
::
evaluates as follows:
Return the set of all characters returned by
WordCharacters
().
The production
CharacterClassEscape
::
evaluates as follows:
Return the set of all characters not included in the set returned by
CharacterClassEscape
::
The production
CharacterClassEscape
::
p{
UnicodePropertyValueExpression
evaluates by returning the CharSet containing all Unicode code points included in the CharSet returned by
UnicodePropertyValueExpression
The production
CharacterClassEscape
::
P{
UnicodePropertyValueExpression
evaluates by returning the CharSet containing all Unicode code points not included in the CharSet returned by
UnicodePropertyValueExpression
The production
UnicodePropertyValueExpression
::
UnicodePropertyName
UnicodePropertyValue
evaluates as follows:
Let
ps
be SourceText of
UnicodePropertyName
Let
be !
UnicodeMatchProperty
ps
).
Assert
is a Unicode
property name
or property alias listed in the “
Property name
and aliases” column of
Table 54
Let
vs
be SourceText of
UnicodePropertyValue
Let
be !
UnicodeMatchPropertyValue
vs
).
Return the CharSet containing all Unicode code points whose character database definition includes the property
with value
The production
UnicodePropertyValueExpression
::
LoneUnicodePropertyNameOrValue
evaluates as follows:
Let
be SourceText of
LoneUnicodePropertyNameOrValue
If !
UnicodeMatchPropertyValue
"General_Category"
) is identical to a
List
of Unicode code points that is the name of a Unicode general category
or general category alias listed in the “Property value and aliases”
column of
Table 56
, then
Return
the CharSet containing all Unicode code points whose character database
definition includes the property “General_Category” with value
Let
be !
UnicodeMatchProperty
).
Assert
is a binary Unicode property or binary property alias listed in the “
Property name
and aliases” column of
Table 55
Return the CharSet containing all Unicode code points whose character database definition includes the property
with value “True”.
21.2.2.13
CharacterClass
The production
CharacterClass
::
ClassRanges
evaluates as follows:
Evaluate
ClassRanges
to obtain a CharSet
Return the two results
and
false
The production
CharacterClass
::
ClassRanges
evaluates as follows:
Evaluate
ClassRanges
to obtain a CharSet
Return the two results
and
true
21.2.2.14
ClassRanges
The production
ClassRanges
::
[empty]
evaluates as follows:
Return the empty CharSet.
The production
ClassRanges
::
NonemptyClassRanges
evaluates as follows:
Return the CharSet that is the result of evaluating
NonemptyClassRanges
21.2.2.15
NonemptyClassRanges
The production
NonemptyClassRanges
::
ClassAtom
evaluates as follows:
Return the CharSet that is the result of evaluating
ClassAtom
The production
NonemptyClassRanges
::
ClassAtom
NonemptyClassRangesNoDash
evaluates as follows:
Evaluate
ClassAtom
to obtain a CharSet
Evaluate
NonemptyClassRangesNoDash
to obtain a CharSet
Return the union of CharSets
and
The production
NonemptyClassRanges
::
ClassAtom
ClassAtom
ClassRanges
evaluates as follows:
Evaluate the first
ClassAtom
to obtain a CharSet
Evaluate the second
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRange
) and let
be the resulting CharSet.
Return the union of CharSets
and
21.2.2.15.1
Runtime Semantics: CharacterRange (
The abstract operation CharacterRange takes two CharSet parameters
and
and performs the following steps:
Assert
and
each contain exactly one character.
Let
be the one character in CharSet
Let
be the one character in CharSet
Let
be the character value of character
Let
be the character value of character
Assert
Return the set containing all characters numbered
through
, inclusive.
21.2.2.16
NonemptyClassRangesNoDash
The production
NonemptyClassRangesNoDash
::
ClassAtom
evaluates as follows:
Return the CharSet that is the result of evaluating
ClassAtom
The production
NonemptyClassRangesNoDash
::
ClassAtomNoDash
NonemptyClassRangesNoDash
evaluates as follows:
Evaluate
ClassAtomNoDash
to obtain a CharSet
Evaluate
NonemptyClassRangesNoDash
to obtain a CharSet
Return the union of CharSets
and
The production
NonemptyClassRangesNoDash
::
ClassAtomNoDash
ClassAtom
ClassRanges
evaluates as follows:
Evaluate
ClassAtomNoDash
to obtain a CharSet
Evaluate
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRange
) and let
be the resulting CharSet.
Return the union of CharSets
and
Note 1
ClassRanges
can expand into a single
ClassAtom
and/or ranges of two
ClassAtom
separated by dashes. In the latter case the
ClassRanges
includes all characters between the first
ClassAtom
and the second
ClassAtom
, inclusive; an error occurs if either
ClassAtom
does not represent a single character (for example, if one is \w) or if the first
ClassAtom
's character value is greater than the second
ClassAtom
's character value.
Note 2
Even if the pattern ignores case, the case of the two ends
of a range is significant in determining which characters belong to the
range. Thus, for example, the pattern
/[E-F]/i
matches only the letters
, and
, while the pattern
/[E-f]/i
matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols
, and
Note 3
character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of
ClassRanges
, the beginning or end limit of a range specification, or immediately follows a range specification.
21.2.2.17
ClassAtom
The production
ClassAtom
::
evaluates as follows:
Return the CharSet containing the single character
U+002D (HYPHEN-MINUS).
The production
ClassAtom
::
ClassAtomNoDash
evaluates as follows:
Return the CharSet that is the result of evaluating
ClassAtomNoDash
21.2.2.18
ClassAtomNoDash
The production
ClassAtomNoDash
::
SourceCharacter
but not one of
or
or
evaluates as follows:
Return the CharSet containing the character matched by
SourceCharacter
The production
ClassAtomNoDash
::
ClassEscape
evaluates as follows:
Return the CharSet that is the result of evaluating
ClassEscape
21.2.2.19
ClassEscape
The
ClassEscape
productions evaluate as follows:
ClassEscape
::
ClassEscape
::
ClassEscape
::
CharacterEscape
Let
cv
be the CharacterValue of this
ClassEscape
Let
be the character whose character value is
cv
Return the CharSet containing the single character
ClassEscape
::
CharacterClassEscape
Return the CharSet that is the result of evaluating
CharacterClassEscape
Note
ClassAtom
can use any of the escape sequences that are allowed in the rest of the regular expression except for
\b
\B
, and backreferences. Inside a
CharacterClass
\b
means the backspace character, while
\B
and backreferences raise errors. Using a backreference inside a
ClassAtom
causes an error.
21.2.3
The RegExp Constructor
The RegExp
constructor
is the intrinsic object
%RegExp%
is the initial value of the
RegExp
property of the
global object
creates and initializes a new RegExp object when called as a function rather than as a
constructor
. Thus the function call
RegExp(…)
is equivalent to the object creation expression
new RegExp(…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
RegExp
behaviour must include a
super
call to the
RegExp
constructor
to create and initialize subclass instances with the necessary internal slots.
21.2.3.1
RegExp (
pattern
flags
The following steps are taken:
Let
patternIsRegExp
be ?
IsRegExp
pattern
).
If NewTarget is
undefined
, then
Let
newTarget
be the
active function object
If
patternIsRegExp
is
true
and
flags
is
undefined
, then
Let
patternConstructor
be ?
Get
pattern
"constructor"
).
If
SameValue
newTarget
patternConstructor
) is
true
, return
pattern
Else, let
newTarget
be NewTarget.
If
Type
pattern
) is Object and
pattern
has a [[RegExpMatcher]] internal slot, then
Let
be
pattern
.[[OriginalSource]].
If
flags
is
undefined
, let
be
pattern
.[[OriginalFlags]].
Else, let
be
flags
Else if
patternIsRegExp
is
true
, then
Let
be ?
Get
pattern
"source"
).
If
flags
is
undefined
, then
Let
be ?
Get
pattern
"flags"
).
Else, let
be
flags
Else,
Let
be
pattern
Let
be
flags
Let
be ?
RegExpAlloc
newTarget
).
Return ?
RegExpInitialize
).
Note
If pattern is supplied using a
StringLiteral
the usual escape sequence substitutions are performed before the String
is processed by RegExp. If pattern must contain an escape sequence to
be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must
be escaped within the
StringLiteral
to prevent them being removed when the contents of the
StringLiteral
are formed.
21.2.3.2
Abstract Operations for the RegExp Constructor
21.2.3.2.1
Runtime Semantics: RegExpAlloc (
newTarget
When the abstract operation RegExpAlloc with argument
newTarget
is called, the following steps are taken:
Let
obj
be ?
OrdinaryCreateFromConstructor
newTarget
"%RegExpPrototype%"
, « [[RegExpMatcher]], [[OriginalSource]], [[OriginalFlags]] »).
Perform !
DefinePropertyOrThrow
obj
"lastIndex"
, PropertyDescriptor { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}).
Return
obj
21.2.3.2.2
Runtime Semantics: RegExpInitialize (
obj
pattern
flags
When the abstract operation RegExpInitialize with arguments
obj
pattern
, and
flags
is called, the following steps are taken:
If
pattern
is
undefined
, let
be the empty String.
Else, let
be ?
ToString
pattern
).
If
flags
is
undefined
, let
be the empty String.
Else, let
be ?
ToString
flags
).
If
contains any code unit other than
"g"
"i"
"m"
"s"
"u"
, or
"y"
or if it contains the same code unit more than once, throw a
SyntaxError
exception.
If
contains
"u"
, let
BMP
be
false
; else let
BMP
be
true
If
BMP
is
true
, then
Parse
using the grammars in
21.2.1
and interpreting each of its 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements. The
goal symbol
for the parse is
Pattern
[~U, ~N]
. If the result of parsing contains a
GroupName
, reparse with the
goal symbol
Pattern
[~U, +N]
and use this result instead. Throw a
SyntaxError
exception if
did not conform to the grammar, if any elements of
were not matched by the parse, or if any Early Error conditions exist.
Let
patternCharacters
be a
List
whose elements are the code unit elements of
Else,
Parse
using the grammars in
21.2.1
and interpreting
as UTF-16 encoded Unicode code points (
6.1.4
). The
goal symbol
for the parse is
Pattern
[+U, +N]
. Throw a
SyntaxError
exception if
did not conform to the grammar, if any elements of
were not matched by the parse, or if any Early Error conditions exist.
Let
patternCharacters
be a
List
whose elements are the code points resulting from applying UTF-16 decoding to
's sequence of elements.
Set
obj
.[[OriginalSource]] to
Set
obj
.[[OriginalFlags]] to
Set
obj
.[[RegExpMatcher]] to the internal procedure that evaluates the above parse of
by applying the semantics provided in
21.2.2
using
patternCharacters
as the pattern's
List
of
SourceCharacter
values and
as the flag parameters.
Perform ?
Set
obj
"lastIndex"
, 0,
true
).
Return
obj
21.2.3.2.3
Runtime Semantics: RegExpCreate (
When the abstract operation RegExpCreate with arguments
and
is called, the following steps are taken:
Let
obj
be ?
RegExpAlloc
%RegExp%
).
Return ?
RegExpInitialize
obj
).
21.2.3.2.4
Runtime Semantics: EscapeRegExpPattern (
When the abstract operation EscapeRegExpPattern with arguments
and
is called, the following occurs:
Let
be a String in the form of a
Pattern
[~U]
Pattern
[+U]
if
contains
"u"
) equivalent to
interpreted as UTF-16 encoded Unicode code points (
6.1.4
), in which certain code points are escaped as described below.
may or may not be identical to
; however, the internal procedure that would result from evaluating
as a
Pattern
[~U]
Pattern
[+U]
if
contains
"u"
must behave identically to the internal procedure given by the
constructed object's [[RegExpMatcher]] internal slot. Multiple calls to
this abstract operation using the same values for
and
must produce identical results.
The code points
or any
LineTerminator
occurring in the pattern shall be escaped in
as necessary to ensure that the
string-concatenation
of
"/"
"/"
, and
can be parsed (in an appropriate lexical context) as a
RegularExpressionLiteral
that behaves identically to the constructed regular expression. For example, if
is
"/"
, then
could be
"\/"
or
"\u002F"
, among other possibilities, but not
"/"
, because
///
followed by
would be parsed as a
SingleLineComment
rather than a
RegularExpressionLiteral
. If
is the empty String, this specification can be met by letting
be
"(?:)"
Return
21.2.4
Properties of the RegExp Constructor
The RegExp
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
21.2.4.1
RegExp.prototype
The initial value of
RegExp.prototype
is the intrinsic object
%RegExpPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
21.2.4.2
get RegExp [ @@species ]
RegExp[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
RegExp prototype methods normally use their
this
object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its @@species property.
21.2.5
Properties of the RegExp Prototype Object
The RegExp prototype object:
is the intrinsic object
%RegExpPrototype%
is an ordinary object.
is not a RegExp instance and does not have a
[[RegExpMatcher]] internal slot or any of the other internal slots of
RegExp instance objects.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
Note
The RegExp prototype object does not have a
valueOf
property of its own; however, it inherits the
valueOf
property from the Object prototype object.
21.2.5.1
RegExp.prototype.constructor
The initial value of
RegExp.prototype.constructor
is the intrinsic object
%RegExp%
21.2.5.2
RegExp.prototype.exec (
string
Performs a regular expression match of
string
against the regular expression and returns an Array object containing the results of the match, or
null
if
string
did not match.
The String
ToString
string
) is searched for an occurrence of the regular expression pattern as follows:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[RegExpMatcher]] internal slot, throw a
TypeError
exception.
Let
be ?
ToString
string
).
Return ?
RegExpBuiltinExec
).
21.2.5.2.1
Runtime Semantics: RegExpExec (
The abstract operation RegExpExec with arguments
and
performs the following steps:
Assert
Type
) is Object.
Assert
Type
) is String.
Let
exec
be ?
Get
"exec"
).
If
IsCallable
exec
) is
true
, then
Let
result
be ?
Call
exec
, «
»).
If
Type
result
) is neither Object or Null, throw a
TypeError
exception.
Return
result
If
does not have a [[RegExpMatcher]] internal slot, throw a
TypeError
exception.
Return ?
RegExpBuiltinExec
).
Note
If a callable
exec
property is not found
this algorithm falls back to attempting to use the built-in RegExp
matching algorithm. This provides compatible behaviour for code written
for prior editions where most built-in algorithms that use regular
expressions did not perform a dynamic property lookup of
exec
21.2.5.2.2
Runtime Semantics: RegExpBuiltinExec (
The abstract operation RegExpBuiltinExec with arguments
and
performs the following steps:
Assert
is an initialized RegExp instance.
Assert
Type
) is String.
Let
length
be the number of code units in
Let
lastIndex
be ?
ToLength
(?
Get
"lastIndex"
)).
Let
flags
be
.[[OriginalFlags]].
If
flags
contains
"g"
, let
global
be
true
, else let
global
be
false
If
flags
contains
"y"
, let
sticky
be
true
, else let
sticky
be
false
If
global
is
false
and
sticky
is
false
, set
lastIndex
to 0.
Let
matcher
be
.[[RegExpMatcher]].
If
flags
contains
"u"
, let
fullUnicode
be
true
, else let
fullUnicode
be
false
Let
matchSucceeded
be
false
Repeat, while
matchSucceeded
is
false
If
lastIndex
length
, then
If
global
is
true
or
sticky
is
true
, then
Perform ?
Set
"lastIndex"
, 0,
true
).
Return
null
Let
be
matcher
lastIndex
).
If
is
failure
, then
If
sticky
is
true
, then
Perform ?
Set
"lastIndex"
, 0,
true
).
Return
null
Set
lastIndex
to
AdvanceStringIndex
lastIndex
fullUnicode
).
Else,
Assert
is a State.
Set
matchSucceeded
to
true
Let
be
's
endIndex
value.
If
fullUnicode
is
true
, then
is an index into the
Input
character list, derived from
, matched by
matcher
. Let
eUTF
be the smallest index into
that corresponds to the character at element
of
Input
. If
is greater than or equal to the number of elements in
Input
, then
eUTF
is the number of code units in
Set
to
eUTF
If
global
is
true
or
sticky
is
true
, then
Perform ?
Set
"lastIndex"
true
).
Let
be the number of elements in
's
captures
List
. (This is the same value as
21.2.2.1
's
NcapturingParens
.)
Assert
< 2
32
- 1.
Let
be !
ArrayCreate
+ 1).
Assert
: The value of
's
"length"
property is
+ 1.
Perform !
CreateDataProperty
"index"
lastIndex
).
Perform !
CreateDataProperty
"input"
).
Let
matchedSubstr
be the matched substring (i.e. the portion of
between offset
lastIndex
inclusive and offset
exclusive).
Perform !
CreateDataProperty
"0"
matchedSubstr
).
If
contains any
GroupName
, then
Let
groups
be
ObjectCreate
null
).
Else,
Let
groups
be
undefined
Perform !
CreateDataProperty
"groups"
groups
).
For each integer
such that
> 0 and
, do
Let
captureI
be
th
element of
's
captures
List
If
captureI
is
undefined
, let
capturedValue
be
undefined
Else if
fullUnicode
is
true
, then
Assert
captureI
is a
List
of code points.
Let
capturedValue
be the String value whose code units are the
UTF16Encoding
of the code points of
captureI
Else
fullUnicode
is
false
Assert
captureI
is a
List
of code units.
Let
capturedValue
be the String value consisting of the code units of
captureI
Perform !
CreateDataProperty
, !
ToString
),
capturedValue
).
If the
th capture of
was defined with a
GroupName
, then
Let
be the StringValue of the corresponding
RegExpIdentifierName
Perform !
CreateDataProperty
groups
capturedValue
).
Return
21.2.5.2.3
AdvanceStringIndex (
index
unicode
The abstract operation AdvanceStringIndex with arguments
index
, and
unicode
performs the following steps:
Assert
Type
) is String.
Assert
index
is an integer such that 0 ≤
index
≤ 2
53
- 1.
Assert
Type
unicode
) is Boolean.
If
unicode
is
false
, return
index
+ 1.
Let
length
be the number of code units in
If
index
+ 1 ≥
length
, return
index
+ 1.
Let
first
be the numeric value of the code unit at index
index
within
If
first
< 0xD800 or
first
> 0xDBFF, return
index
+ 1.
Let
second
be the numeric value of the code unit at index
index
+ 1 within
If
second
< 0xDC00 or
second
> 0xDFFF, return
index
+ 1.
Return
index
+ 2.
21.2.5.3
get RegExp.prototype.dotAll
RegExp.prototype.dotAll
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x0073 (LATIN SMALL LETTER S), return
true
Return
false
21.2.5.4
get RegExp.prototype.flags
RegExp.prototype.flags
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
result
be the empty String.
Let
global
be
ToBoolean
(?
Get
"global"
)).
If
global
is
true
, append the code unit 0x0067 (LATIN SMALL LETTER G) as the last code unit of
result
Let
ignoreCase
be
ToBoolean
(?
Get
"ignoreCase"
)).
If
ignoreCase
is
true
, append the code unit 0x0069 (LATIN SMALL LETTER I) as the last code unit of
result
Let
multiline
be
ToBoolean
(?
Get
"multiline"
)).
If
multiline
is
true
, append the code unit 0x006D (LATIN SMALL LETTER M) as the last code unit of
result
Let
dotAll
be
ToBoolean
(?
Get
"dotAll"
)).
If
dotAll
is
true
, append the code unit 0x0073 (LATIN SMALL LETTER S) as the last code unit of
result
Let
unicode
be
ToBoolean
(?
Get
"unicode"
)).
If
unicode
is
true
, append the code unit 0x0075 (LATIN SMALL LETTER U) as the last code unit of
result
Let
sticky
be
ToBoolean
(?
Get
"sticky"
)).
If
sticky
is
true
, append the code unit 0x0079 (LATIN SMALL LETTER Y) as the last code unit of
result
Return
result
21.2.5.5
get RegExp.prototype.global
RegExp.prototype.global
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x0067 (LATIN SMALL LETTER G), return
true
Return
false
21.2.5.6
get RegExp.prototype.ignoreCase
RegExp.prototype.ignoreCase
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x0069 (LATIN SMALL LETTER I), return
true
Return
false
21.2.5.7
RegExp.prototype [ @@match ] (
string
When the
@@match
method is called with argument
string
, the following steps are taken:
Let
rx
be the
this
value.
If
Type
rx
) is not Object, throw a
TypeError
exception.
Let
be ?
ToString
string
).
Let
global
be
ToBoolean
(?
Get
rx
"global"
)).
If
global
is
false
, then
Return ?
RegExpExec
rx
).
Else
global
is
true
Let
fullUnicode
be
ToBoolean
(?
Get
rx
"unicode"
)).
Perform ?
Set
rx
"lastIndex"
, 0,
true
).
Let
be !
ArrayCreate
(0).
Let
be 0.
Repeat,
Let
result
be ?
RegExpExec
rx
).
If
result
is
null
, then
If
= 0, return
null
Return
Else
result
is not
null
Let
matchStr
be ?
ToString
(?
Get
result
"0"
)).
Let
status
be
CreateDataProperty
, !
ToString
),
matchStr
).
Assert
status
is
true
If
matchStr
is the empty String, then
Let
thisIndex
be ?
ToLength
(?
Get
rx
"lastIndex"
)).
Let
nextIndex
be
AdvanceStringIndex
thisIndex
fullUnicode
).
Perform ?
Set
rx
"lastIndex"
nextIndex
true
).
Increment
The value of the
name
property of this function is
"[Symbol.match]"
Note
The @@match property is used by the
IsRegExp
abstract operation to identify objects that have the basic behaviour of
regular expressions. The absence of a @@match property or the existence
of such a property whose value does not Boolean coerce to
true
indicates that the object is not intended to be used as a regular expression object.
21.2.5.8
get RegExp.prototype.multiline
RegExp.prototype.multiline
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x006D (LATIN SMALL LETTER M), return
true
Return
false
21.2.5.9
RegExp.prototype [ @@replace ] (
string
replaceValue
When the
@@replace
method is called with arguments
string
and
replaceValue
, the following steps are taken:
Let
rx
be the
this
value.
If
Type
rx
) is not Object, throw a
TypeError
exception.
Let
be ?
ToString
string
).
Let
lengthS
be the number of code unit elements in
Let
functionalReplace
be
IsCallable
replaceValue
).
If
functionalReplace
is
false
, then
Set
replaceValue
to ?
ToString
replaceValue
).
Let
global
be
ToBoolean
(?
Get
rx
"global"
)).
If
global
is
true
, then
Let
fullUnicode
be
ToBoolean
(?
Get
rx
"unicode"
)).
Perform ?
Set
rx
"lastIndex"
, 0,
true
).
Let
results
be a new empty
List
Let
done
be
false
Repeat, while
done
is
false
Let
result
be ?
RegExpExec
rx
).
If
result
is
null
, set
done
to
true
Else
result
is not
null
Append
result
to the end of
results
If
global
is
false
, set
done
to
true
Else,
Let
matchStr
be ?
ToString
(?
Get
result
"0"
)).
If
matchStr
is the empty String, then
Let
thisIndex
be ?
ToLength
(?
Get
rx
"lastIndex"
)).
Let
nextIndex
be
AdvanceStringIndex
thisIndex
fullUnicode
).
Perform ?
Set
rx
"lastIndex"
nextIndex
true
).
Let
accumulatedResult
be the empty String value.
Let
nextSourcePosition
be 0.
For each
result
in
results
, do
Let
nCaptures
be ?
ToLength
(?
Get
result
"length"
)).
Set
nCaptures
to
max
nCaptures
- 1, 0).
Let
matched
be ?
ToString
(?
Get
result
"0"
)).
Let
matchLength
be the number of code units in
matched
Let
position
be ?
ToInteger
(?
Get
result
"index"
)).
Set
position
to
max
min
position
lengthS
), 0).
Let
be 1.
Let
captures
be a new empty
List
Repeat, while
nCaptures
Let
capN
be ?
Get
result
, !
ToString
)).
If
capN
is not
undefined
, then
Set
capN
to ?
ToString
capN
).
Append
capN
as the last element of
captures
Increase
by 1.
Let
namedCaptures
be ?
Get
result
"groups"
).
If
functionalReplace
is
true
, then
Let
replacerArgs
be «
matched
».
Append in list order the elements of
captures
to the end of the
List
replacerArgs
Append
position
and
to
replacerArgs
If
namedCaptures
is not
undefined
, then
Append
namedCaptures
as the last element of
replacerArgs
Let
replValue
be ?
Call
replaceValue
undefined
replacerArgs
).
Let
replacement
be ?
ToString
replValue
).
Else,
Let
replacement
be
GetSubstitution
matched
position
captures
namedCaptures
replaceValue
).
If
position
nextSourcePosition
, then
NOTE:
position
should not normally move backwards. If it does, it is an indication of
an ill-behaving RegExp subclass or use of an access triggered
side-effect to change the global flag or other characteristics of
rx
. In such cases, the corresponding substitution is ignored.
Set
accumulatedResult
to the
string-concatenation
of the current value of
accumulatedResult
, the substring of
consisting of the code units from
nextSourcePosition
(inclusive) up to
position
(exclusive), and
replacement
Set
nextSourcePosition
to
position
matchLength
If
nextSourcePosition
lengthS
, return
accumulatedResult
Return the
string-concatenation
of
accumulatedResult
and the substring of
consisting of the code units from
nextSourcePosition
(inclusive) up through the final code unit of
(inclusive).
The value of the
name
property of this function is
"[Symbol.replace]"
21.2.5.10
RegExp.prototype [ @@search ] (
string
When the
@@search
method is called with argument
string
, the following steps are taken:
Let
rx
be the
this
value.
If
Type
rx
) is not Object, throw a
TypeError
exception.
Let
be ?
ToString
string
).
Let
previousLastIndex
be ?
Get
rx
"lastIndex"
).
If
SameValue
previousLastIndex
, 0) is
false
, then
Perform ?
Set
rx
"lastIndex"
, 0,
true
).
Let
result
be ?
RegExpExec
rx
).
Let
currentLastIndex
be ?
Get
rx
"lastIndex"
).
If
SameValue
currentLastIndex
previousLastIndex
) is
false
, then
Perform ?
Set
rx
"lastIndex"
previousLastIndex
true
).
If
result
is
null
, return -1.
Return ?
Get
result
"index"
).
The value of the
name
property of this function is
"[Symbol.search]"
Note
The
lastIndex
and
global
properties of this RegExp object are ignored when performing the search. The
lastIndex
property is left unchanged.
21.2.5.11
get RegExp.prototype.source
RegExp.prototype.source
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalSource]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
"(?:)"
Otherwise, throw a
TypeError
exception.
Assert
has an [[OriginalFlags]] internal slot.
Let
src
be
.[[OriginalSource]].
Let
flags
be
.[[OriginalFlags]].
Return
EscapeRegExpPattern
src
flags
).
21.2.5.12
RegExp.prototype [ @@split ] (
string
limit
Note 1
Returns an Array object into which substrings of the result of converting
string
to a String have been stored. The substrings are determined by searching from left to right for matches of the
this
value regular expression; these occurrences are not part of any
substring in the returned array, but serve to divide up the String
value.
The
this
value may be an empty regular
expression or a regular expression that can match an empty String. In
this case, the regular expression does not match the empty substring at
the beginning or end of the input String, nor does it match the empty
substring at the end of the previous separator match. (For example, if
the regular expression matches the empty String, the String is split up
into individual code unit elements; the length of the result array
equals the length of the String, and each substring contains one code
unit.) Only the first match at a given index of the String is
considered, even if backtracking could yield a non-empty-substring match
at that index. (For example,
/a*?/[Symbol.split]("ab")
evaluates to the array
["a", "b"]
, while
/a*/[Symbol.split]("ab")
evaluates to the array
["","b"]
.)
If the
string
is (or converts to) the empty
String, the result depends on whether the regular expression can match
the empty String. If it can, the result array contains no elements.
Otherwise, the result array contains one element, which is the empty
String.
If the regular expression contains capturing parentheses, then each time
separator
is matched the results (including any
undefined
results) of the capturing parentheses are spliced into the output array. For example,
(\
/)?([^<>
]+)>/[Symbol.split]("A
bold
and
CODE
coded
CODE
")
evaluates to the array
"A"
undefined
"B"
"bold"
"/"
"B"
"and"
undefined
"CODE"
"coded"
"/"
"CODE"
""
If
limit
is not
undefined
, then the output array is truncated so that it contains no more than
limit
elements.
When the
@@split
method is called, the following steps are taken:
Let
rx
be the
this
value.
If
Type
rx
) is not Object, throw a
TypeError
exception.
Let
be ?
ToString
string
).
Let
be ?
SpeciesConstructor
rx
%RegExp%
).
Let
flags
be ?
ToString
(?
Get
rx
"flags"
)).
If
flags
contains
"u"
, let
unicodeMatching
be
true
Else, let
unicodeMatching
be
false
If
flags
contains
"y"
, let
newFlags
be
flags
Else, let
newFlags
be the
string-concatenation
of
flags
and
"y"
Let
splitter
be ?
Construct
, «
rx
newFlags
»).
Let
be !
ArrayCreate
(0).
Let
lengthA
be 0.
If
limit
is
undefined
, let
lim
be 2
32
- 1; else let
lim
be ?
ToUint32
limit
).
Let
size
be the length of
Let
be 0.
If
lim
= 0, return
If
size
= 0, then
Let
be ?
RegExpExec
splitter
).
If
is not
null
, return
Perform !
CreateDataProperty
"0"
).
Return
Let
be
Repeat, while
size
Perform ?
Set
splitter
"lastIndex"
true
).
Let
be ?
RegExpExec
splitter
).
If
is
null
, set
to
AdvanceStringIndex
unicodeMatching
).
Else
is not
null
Let
be ?
ToLength
(?
Get
splitter
"lastIndex"
)).
Set
to
min
size
).
If
, set
to
AdvanceStringIndex
unicodeMatching
).
Else
Let
be the String value equal to the substring of
consisting of the code units at indices
(inclusive) through
(exclusive).
Perform !
CreateDataProperty
, !
ToString
lengthA
),
).
Increase
lengthA
by 1.
If
lengthA
lim
, return
Set
to
Let
numberOfCaptures
be ?
ToLength
(?
Get
"length"
)).
Set
numberOfCaptures
to
max
numberOfCaptures
- 1, 0).
Let
be 1.
Repeat, while
numberOfCaptures
Let
nextCapture
be ?
Get
, !
ToString
)).
Perform !
CreateDataProperty
, !
ToString
lengthA
),
nextCapture
).
Increase
by 1.
Increase
lengthA
by 1.
If
lengthA
lim
, return
Set
to
Let
be the String value equal to the substring of
consisting of the code units at indices
(inclusive) through
size
(exclusive).
Perform !
CreateDataProperty
, !
ToString
lengthA
),
).
Return
The value of the
name
property of this function is
"[Symbol.split]"
Note 2
The
@@split
method ignores the value of the
global
and
sticky
properties of this RegExp object.
21.2.5.13
get RegExp.prototype.sticky
RegExp.prototype.sticky
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x0079 (LATIN SMALL LETTER Y), return
true
Return
false
21.2.5.14
RegExp.prototype.test (
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
string
be ?
ToString
).
Let
match
be ?
RegExpExec
string
).
If
match
is not
null
, return
true
; else return
false
21.2.5.15
RegExp.prototype.toString ( )
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
pattern
be ?
ToString
(?
Get
"source"
)).
Let
flags
be ?
ToString
(?
Get
"flags"
)).
Let
result
be the
string-concatenation
of
"/"
pattern
"/"
, and
flags
Return
result
Note
The returned String has the form of a
RegularExpressionLiteral
that evaluates to another RegExp object with the same behaviour as this object.
21.2.5.16
get RegExp.prototype.unicode
RegExp.prototype.unicode
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[OriginalFlags]] internal slot, then
If
SameValue
%RegExpPrototype%
) is
true
, return
undefined
Otherwise, throw a
TypeError
exception.
Let
flags
be
.[[OriginalFlags]].
If
flags
contains the code unit 0x0075 (LATIN SMALL LETTER U), return
true
Return
false
21.2.6
Properties of RegExp Instances
RegExp instances are ordinary objects that inherit properties
from the RegExp prototype object. RegExp instances have internal slots
[[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value
of the [[RegExpMatcher]] internal slot is an implementation-dependent
representation of the
Pattern
of the RegExp object.
Note
Prior to ECMAScript 2015,
RegExp
instances were specified as having the own data properties
source
global
ignoreCase
, and
multiline
. Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
21.2.6.1
lastIndex
The value of the
lastIndex
property specifies the String index at which to start the next match. It is coerced to an integer when used (see
21.2.5.2.2
). This property shall have the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22
Indexed Collections
22.1
Array Objects
Array objects are exotic objects that give special treatment to a certain class of property names. See
9.4.2
for a definition of this special treatment.
22.1.1
The Array Constructor
The Array
constructor
is the intrinsic object
%Array%
is the initial value of the
Array
property of the
global object
creates and initializes a new Array
exotic object
when called as a
constructor
also creates and initializes a new Array object when called as a function rather than as a
constructor
. Thus the function call
Array(…)
is equivalent to the object creation expression
new Array(…)
with the same arguments.
is a single function whose behaviour is overloaded based upon the number and types of its arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the exotic
Array
behaviour must include a
super
call to the
Array
constructor
to initialize subclass instances that are Array exotic objects. However, most of the
Array.prototype
methods are generic methods that are not dependent upon their
this
value being an Array
exotic object
has a
"length"
property whose value is 1.
22.1.1.1
Array ( )
This description applies if and only if the Array
constructor
is called with no arguments.
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
= 0.
If NewTarget is
undefined
, let
newTarget
be the
active function object
, else let
newTarget
be NewTarget.
Let
proto
be ?
GetPrototypeFromConstructor
newTarget
"%ArrayPrototype%"
).
Return !
ArrayCreate
(0,
proto
).
22.1.1.2
Array (
len
This description applies if and only if the Array
constructor
is called with exactly one argument.
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
= 1.
If NewTarget is
undefined
, let
newTarget
be the
active function object
, else let
newTarget
be NewTarget.
Let
proto
be ?
GetPrototypeFromConstructor
newTarget
"%ArrayPrototype%"
).
Let
array
be !
ArrayCreate
(0,
proto
).
If
Type
len
) is not Number, then
Let
defineStatus
be
CreateDataProperty
array
"0"
len
).
Assert
defineStatus
is
true
Let
intLen
be 1.
Else,
Let
intLen
be
ToUint32
len
).
If
intLen
len
, throw a
RangeError
exception.
Perform !
Set
array
"length"
intLen
true
).
Return
array
22.1.1.3
Array ( ...
items
This description applies if and only if the Array
constructor
is called with at least two arguments.
When the
Array
function is called, the following steps are taken:
Let
numberOfArgs
be the number of arguments passed to this function call.
Assert
numberOfArgs
≥ 2.
If NewTarget is
undefined
, let
newTarget
be the
active function object
, else let
newTarget
be NewTarget.
Let
proto
be ?
GetPrototypeFromConstructor
newTarget
"%ArrayPrototype%"
).
Let
array
be ?
ArrayCreate
numberOfArgs
proto
).
Let
be 0.
Let
items
be a zero-origined
List
containing the argument items in order.
Repeat, while
numberOfArgs
Let
Pk
be !
ToString
).
Let
itemK
be
items
].
Let
defineStatus
be
CreateDataProperty
array
Pk
itemK
).
Assert
defineStatus
is
true
Increase
by 1.
Assert
: The value of
array
's
"length"
property is
numberOfArgs
Return
array
22.1.2
Properties of the Array Constructor
The Array
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
22.1.2.1
Array.from (
items
[ ,
mapfn
[ ,
thisArg
] ] )
When the
from
method is called with argument
items
and optional arguments
mapfn
and
thisArg
, the following steps are taken:
Let
be the
this
value.
If
mapfn
is
undefined
, let
mapping
be
false
Else,
If
IsCallable
mapfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
mapping
be
true
Let
usingIterator
be ?
GetMethod
items
, @@iterator).
If
usingIterator
is not
undefined
, then
If
IsConstructor
) is
true
, then
Let
be ?
Construct
).
Else,
Let
be !
ArrayCreate
(0).
Let
iteratorRecord
be ?
GetIterator
items
sync
usingIterator
).
Let
be 0.
Repeat,
If
≥ 2
53
- 1, then
Let
error
be
ThrowCompletion
(a newly created
TypeError
object).
Return ?
IteratorClose
iteratorRecord
error
).
Let
Pk
be !
ToString
).
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, then
Perform ?
Set
"length"
true
).
Return
Let
nextValue
be ?
IteratorValue
next
).
If
mapping
is
true
, then
Let
mappedValue
be
Call
mapfn
, «
nextValue
»).
If
mappedValue
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
mappedValue
).
Set
mappedValue
to
mappedValue
.[[Value]].
Else, let
mappedValue
be
nextValue
Let
defineStatus
be
CreateDataPropertyOrThrow
Pk
mappedValue
).
If
defineStatus
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
defineStatus
).
Increase
by 1.
NOTE:
items
is not an Iterable so assume it is an array-like object.
Let
arrayLike
be !
ToObject
items
).
Let
len
be ?
ToLength
(?
Get
arrayLike
"length"
)).
If
IsConstructor
) is
true
, then
Let
be ?
Construct
, «
len
»).
Else,
Let
be ?
ArrayCreate
len
).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
arrayLike
Pk
).
If
mapping
is
true
, then
Let
mappedValue
be ?
Call
mapfn
, «
kValue
»).
Else, let
mappedValue
be
kValue
Perform ?
CreateDataPropertyOrThrow
Pk
mappedValue
).
Increase
by 1.
Perform ?
Set
"length"
len
true
).
Return
Note
The
from
function is an intentionally generic factory method; it does not require that its
this
value be the Array
constructor
. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.
22.1.2.2
Array.isArray (
arg
The
isArray
function takes one argument
arg
, and performs the following steps:
Return ?
IsArray
arg
).
22.1.2.3
Array.of ( ...
items
When the
of
method is called with any number of arguments, the following steps are taken:
Let
len
be the actual number of arguments passed to this function.
Let
items
be the
List
of arguments passed to this function.
Let
be the
this
value.
If
IsConstructor
) is
true
, then
Let
be ?
Construct
, «
len
»).
Else,
Let
be ?
ArrayCreate
len
).
Let
be 0.
Repeat, while
len
Let
kValue
be
items
].
Let
Pk
be !
ToString
).
Perform ?
CreateDataPropertyOrThrow
Pk
kValue
).
Increase
by 1.
Perform ?
Set
"length"
len
true
).
Return
Note 1
The
items
argument is assumed to be a well-formed rest argument value.
Note 2
The
of
function is an intentionally generic factory method; it does not require that its
this
value be the Array
constructor
. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.
22.1.2.4
Array.prototype
The value of
Array.prototype
is
%ArrayPrototype%
, the intrinsic Array prototype object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22.1.2.5
get Array [ @@species ]
Array[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
Array prototype methods normally use their
this
object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its @@species property.
22.1.3
Properties of the Array Prototype Object
The Array prototype object:
is the intrinsic object
%ArrayPrototype%
is an Array
exotic object
and has the internal methods specified for such objects.
has a
"length"
property whose initial value is 0 and whose attributes are { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
Note
The Array prototype object is specified to be an Array
exotic object
to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
22.1.3.1
Array.prototype.concat ( ...
arguments
When the
concat
method is called with zero or
more arguments, it returns an array containing the array elements of the
object followed by the array elements of each argument in order.
The following steps are taken:
Let
be ?
ToObject
this
value).
Let
be ?
ArraySpeciesCreate
, 0).
Let
be 0.
Let
items
be a
List
whose first element is
and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of the element.
Let
spreadable
be ?
IsConcatSpreadable
).
If
spreadable
is
true
, then
Let
be 0.
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
> 2
53
- 1, throw a
TypeError
exception.
Repeat, while
len
Let
be !
ToString
).
Let
exists
be ?
HasProperty
).
If
exists
is
true
, then
Let
subElement
be ?
Get
).
Perform ?
CreateDataPropertyOrThrow
, !
ToString
),
subElement
).
Increase
by 1.
Increase
by 1.
Else
is added as a single item rather than spread,
If
≥ 2
53
- 1, throw a
TypeError
exception.
Perform ?
CreateDataPropertyOrThrow
, !
ToString
),
).
Increase
by 1.
Perform ?
Set
"length"
true
).
Return
The
"length"
property of the
concat
method is 1.
Note 1
The explicit setting of the
"length"
property
in step 6 is necessary to ensure that its value is correct in situations
where the trailing elements of the result Array are not present.
Note 2
The
concat
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.1.1
Runtime Semantics: IsConcatSpreadable (
The abstract operation IsConcatSpreadable with argument
performs the following steps:
If
Type
) is not Object, return
false
Let
spreadable
be ?
Get
, @@isConcatSpreadable).
If
spreadable
is not
undefined
, return
ToBoolean
spreadable
).
Return ?
IsArray
).
22.1.3.2
Array.prototype.constructor
The initial value of
Array.prototype.constructor
is the intrinsic object
%Array%
22.1.3.3
Array.prototype.copyWithin (
target
start
[ ,
end
] )
The
copyWithin
method takes up to three arguments
target
start
and
end
Note 1
The
end
argument is optional with the length of the
this
object as its default value. If
target
is negative, it is treated as
length
target
where
length
is the length of the array. If
start
is negative, it is treated as
length
start
. If
end
is negative, it is treated as
length
end
The following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
relativeTarget
be ?
ToInteger
target
).
If
relativeTarget
< 0, let
to
be
max
((
len
relativeTarget
), 0); else let
to
be
min
relativeTarget
len
).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
from
be
max
((
len
relativeStart
), 0); else let
from
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
count
be
min
final
from
len
to
).
If
from
to
and
to
from
count
, then
Let
direction
be -1.
Set
from
to
from
count
- 1.
Set
to
to
to
count
- 1.
Else,
Let
direction
be 1.
Repeat, while
count
> 0
Let
fromKey
be !
ToString
from
).
Let
toKey
be !
ToString
to
).
Let
fromPresent
be ?
HasProperty
fromKey
).
If
fromPresent
is
true
, then
Let
fromVal
be ?
Get
fromKey
).
Perform ?
Set
toKey
fromVal
true
).
Else
fromPresent
is
false
Perform ?
DeletePropertyOrThrow
toKey
).
Set
from
to
from
direction
Set
to
to
to
direction
Decrease
count
by 1.
Return
Note 2
The
copyWithin
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.4
Array.prototype.entries ( )
The following steps are taken:
Let
be ?
ToObject
this
value).
Return
CreateArrayIterator
"key+value"
).
This function is the
%ArrayProto_entries%
intrinsic object.
22.1.3.5
Array.prototype.every (
callbackfn
[ ,
thisArg
] )
Note 1
callbackfn
should be a function that accepts three arguments and returns a value that is coercible to the Boolean value
true
or
false
every
calls
callbackfn
once for each element present in the array, in ascending order, until it finds one where
callbackfn
returns
false
. If such an element is found,
every
immediately returns
false
. Otherwise, if
callbackfn
returned
true
for all elements,
every
will return
true
callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
every
is set before the first call to
callbackfn
. Elements which are appended to the array after the call to
every
begins will not be visited by
callbackfn
. If existing elements of the array are changed, their value as passed to
callbackfn
will be the value at the time
every
visits them; elements that are deleted after the call to
every
begins and before being visited are not visited.
every
acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns
true
When the
every
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Let
testResult
be
ToBoolean
(?
Call
callbackfn
, «
kValue
»)).
If
testResult
is
false
, return
false
Increase
by 1.
Return
true
Note 2
The
every
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.6
Array.prototype.fill (
value
[ ,
start
[ ,
end
] ] )
The
fill
method takes up to three arguments
value
start
and
end
Note 1
The
start
and
end
arguments are optional with default values of 0 and the length of the
this
object. If
start
is negative, it is treated as
length
start
where
length
is the length of the array. If
end
is negative, it is treated as
length
end
The following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
be
max
((
len
relativeStart
), 0); else let
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Repeat, while
final
Let
Pk
be !
ToString
).
Perform ?
Set
Pk
value
true
).
Increase
by 1.
Return
Note 2
The
fill
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.7
Array.prototype.filter (
callbackfn
[ ,
thisArg
] )
Note 1
callbackfn
should be a function that accepts three arguments and returns a value that is coercible to the Boolean value
true
or
false
filter
calls
callbackfn
once for each element in the array, in ascending order, and constructs a new array of all the values for which
callbackfn
returns
true
callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
filter
is set before the first call to
callbackfn
. Elements which are appended to the array after the call to
filter
begins will not be visited by
callbackfn
. If existing elements of the array are changed their value as passed to
callbackfn
will be the value at the time
filter
visits them; elements that are deleted after the call to
filter
begins and before being visited are not visited.
When the
filter
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be ?
ArraySpeciesCreate
, 0).
Let
be 0.
Let
to
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Let
selected
be
ToBoolean
(?
Call
callbackfn
, «
kValue
»)).
If
selected
is
true
, then
Perform ?
CreateDataPropertyOrThrow
, !
ToString
to
),
kValue
).
Increase
to
by 1.
Increase
by 1.
Return
Note 2
The
filter
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.8
Array.prototype.find (
predicate
[ ,
thisArg
] )
The
find
method is called with one or two arguments,
predicate
and
thisArg
Note 1
predicate
should be a function that accepts three arguments and returns a value that is coercible to a Boolean value.
find
calls
predicate
once for each element of the array, in ascending order, until it finds one where
predicate
returns
true
. If such an element is found,
find
immediately returns that element value. Otherwise,
find
returns
undefined
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
predicate
. If it is not provided,
undefined
is used instead.
predicate
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
find
does not directly mutate the object on which it is called but the object may be mutated by the calls to
predicate
The range of elements processed by
find
is set before the first call to
predicate
. Elements that are appended to the array after the call to
find
begins will not be visited by
predicate
. If existing elements of the array are changed, their value as passed to
predicate
will be the value at the time that
find
visits them.
When the
find
method is called, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
predicate
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
Pk
).
Let
testResult
be
ToBoolean
(?
Call
predicate
, «
kValue
»)).
If
testResult
is
true
, return
kValue
Increase
by 1.
Return
undefined
Note 2
The
find
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.9
Array.prototype.findIndex (
predicate
[ ,
thisArg
] )
Note 1
predicate
should be a function that accepts three arguments and returns a value that is coercible to the Boolean value
true
or
false
findIndex
calls
predicate
once for each element of the array, in ascending order, until it finds one where
predicate
returns
true
. If such an element is found,
findIndex
immediately returns the index of that element value. Otherwise,
findIndex
returns -1.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
predicate
. If it is not provided,
undefined
is used instead.
predicate
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
findIndex
does not directly mutate the object on which it is called but the object may be mutated by the calls to
predicate
The range of elements processed by
findIndex
is set before the first call to
predicate
. Elements that are appended to the array after the call to
findIndex
begins will not be visited by
predicate
. If existing elements of the array are changed, their value as passed to
predicate
will be the value at the time that
findIndex
visits them.
When the
findIndex
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
predicate
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
Pk
).
Let
testResult
be
ToBoolean
(?
Call
predicate
, «
kValue
»)).
If
testResult
is
true
, return
Increase
by 1.
Return -1.
Note 2
The
findIndex
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.10
Array.prototype.flat( [
depth
] )
When the
flat
method is called with zero or one arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
sourceLen
be ?
ToLength
(?
Get
"length"
)).
Let
depthNum
be 1.
If
depth
is not
undefined
, then
Set
depthNum
to ?
ToInteger
depth
).
Let
be ?
ArraySpeciesCreate
, 0).
Perform ?
FlattenIntoArray
sourceLen
, 0,
depthNum
).
Return
22.1.3.10.1
FlattenIntoArray(
target
source
sourceLen
start
depth
[ ,
mapperFunction
thisArg
])
Let
targetIndex
be
start
Let
sourceIndex
be 0.
Repeat, while
sourceIndex
sourceLen
Let
be !
ToString
sourceIndex
).
Let
exists
be ?
HasProperty
source
).
If
exists
is
true
, then
Let
element
be ?
Get
source
).
If
mapperFunction
is present, then
Assert
thisArg
is present.
Set
element
to ?
Call
mapperFunction
thisArg
, «
element
sourceIndex
source
»).
Let
shouldFlatten
be
false
If
depth
> 0, then
Set
shouldFlatten
to ?
IsArray
element
).
If
shouldFlatten
is
true
, then
Let
elementLen
be ?
ToLength
(?
Get
element
"length"
)).
Set
targetIndex
to ?
FlattenIntoArray
target
element
elementLen
targetIndex
depth
- 1).
Else,
If
targetIndex
≥ 2
53
-1, throw a
TypeError
exception.
Perform ?
CreateDataPropertyOrThrow
target
, !
ToString
targetIndex
),
element
).
Increase
targetIndex
by 1.
Increase
sourceIndex
by 1.
Return
targetIndex
22.1.3.11
Array.prototype.flatMap (
mapperFunction
[ ,
thisArg
] )
When the
flatMap
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
sourceLen
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
mapperFunction
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be ?
ArraySpeciesCreate
, 0).
Perform ?
FlattenIntoArray
sourceLen
, 0, 1,
mapperFunction
).
Return
22.1.3.12
Array.prototype.forEach (
callbackfn
[ ,
thisArg
] )
Note 1
callbackfn
should be a function that accepts three arguments.
forEach
calls
callbackfn
once for each element present in the array, in ascending order.
callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
When the
forEach
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Perform ?
Call
callbackfn
, «
kValue
»).
Increase
by 1.
Return
undefined
This function is the
%ArrayProto_forEach%
intrinsic object.
Note 2
The
forEach
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.13
Array.prototype.includes (
searchElement
[ ,
fromIndex
] )
Note 1
includes
compares
searchElement
to the elements of the array, in ascending order, using the
SameValueZero
algorithm, and if found at any position, returns
true
; otherwise,
false
is returned.
The optional second argument
fromIndex
defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array,
false
is returned, i.e. the array will not be searched. If it is negative, it
is used as the offset from the end of the array to compute
fromIndex
. If the computed index is less than 0, the whole array will be searched.
When the
includes
method is called, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
is 0, return
false
Let
be ?
ToInteger
fromIndex
).
Assert
: If
fromIndex
is
undefined
, then
is 0.
If
≥ 0, then
Let
be
Else
< 0,
Let
be
len
If
< 0, set
to 0.
Repeat, while
len
Let
elementK
be the result of ?
Get
, !
ToString
)).
If
SameValueZero
searchElement
elementK
) is
true
, return
true
Increase
by 1.
Return
false
Note 2
The
includes
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
The
includes
method intentionally differs from the similar
indexOf
method in two ways. First, it uses the
SameValueZero
algorithm, instead of
Strict Equality Comparison
, allowing it to detect
NaN
array elements. Second, it does not skip missing array elements, instead treating them as
undefined
22.1.3.14
Array.prototype.indexOf (
searchElement
[ ,
fromIndex
] )
Note 1
indexOf
compares
searchElement
to the elements of the array, in ascending order, using the
Strict Equality Comparison
algorithm, and if found at one or more indices, returns the smallest such index; otherwise, -1 is returned.
The optional second argument
fromIndex
defaults
to 0 (i.e. the whole array is searched). If it is greater than or equal
to the length of the array, -1 is returned, i.e. the array will not be
searched. If it is negative, it is used as the offset from the end of
the array to compute
fromIndex
. If the computed index is less than 0, the whole array will be searched.
When the
indexOf
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
is 0, return -1.
Let
be ?
ToInteger
fromIndex
).
Assert
: If
fromIndex
is
undefined
, then
is 0.
If
len
, return -1.
If
≥ 0, then
If
is
-0
, let
be
+0
; else let
be
Else
< 0,
Let
be
len
If
< 0, set
to 0.
Repeat, while
len
Let
kPresent
be ?
HasProperty
, !
ToString
)).
If
kPresent
is
true
, then
Let
elementK
be ?
Get
, !
ToString
)).
Let
same
be the result of performing
Strict Equality Comparison
searchElement
===
elementK
If
same
is
true
, return
Increase
by 1.
Return -1.
Note 2
The
indexOf
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.15
Array.prototype.join (
separator
Note 1
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the
separator
. If no separator is provided, a single comma is used as the separator.
The
join
method takes one argument,
separator
, and performs the following steps:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
separator
is
undefined
, let
sep
be the single-element String
","
Else, let
sep
be ?
ToString
separator
).
Let
be the empty String.
Let
be 0.
Repeat, while
len
If
> 0, set
to the
string-concatenation
of
and
sep
Let
element
be ?
Get
, !
ToString
)).
If
element
is
undefined
or
null
, let
next
be the empty String; otherwise, let
next
be ?
ToString
element
).
Set
to the
string-concatenation
of
and
next
Increase
by 1.
Return
Note 2
The
join
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.16
Array.prototype.keys ( )
The following steps are taken:
Let
be ?
ToObject
this
value).
Return
CreateArrayIterator
"key"
).
This function is the
%ArrayProto_keys%
intrinsic object.
22.1.3.17
Array.prototype.lastIndexOf (
searchElement
[ ,
fromIndex
] )
Note 1
lastIndexOf
compares
searchElement
to the elements of the array in descending order using the
Strict Equality Comparison
algorithm, and if found at one or more indices, returns the largest such index; otherwise, -1 is returned.
The optional second argument
fromIndex
defaults
to the array's length minus one (i.e. the whole array is searched). If
it is greater than or equal to the length of the array, the whole array
will be searched. If it is negative, it is used as the offset from the
end of the array to compute
fromIndex
. If the computed index is less than 0, -1 is returned.
When the
lastIndexOf
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
is 0, return -1.
If
fromIndex
is present, let
be ?
ToInteger
fromIndex
); else let
be
len
- 1.
If
≥ 0, then
If
is
-0
, let
be
+0
; else let
be
min
len
- 1).
Else
< 0,
Let
be
len
Repeat, while
≥ 0
Let
kPresent
be ?
HasProperty
, !
ToString
)).
If
kPresent
is
true
, then
Let
elementK
be ?
Get
, !
ToString
)).
Let
same
be the result of performing
Strict Equality Comparison
searchElement
===
elementK
If
same
is
true
, return
Decrease
by 1.
Return -1.
Note 2
The
lastIndexOf
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.18
Array.prototype.map (
callbackfn
[ ,
thisArg
] )
Note 1
callbackfn
should be a function that accepts three arguments.
map
calls
callbackfn
once for each element in the array, in ascending order, and constructs a new Array from the results.
callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
map
is set before the first call to
callbackfn
. Elements which are appended to the array after the call to
map
begins will not be visited by
callbackfn
. If existing elements of the array are changed, their value as passed to
callbackfn
will be the value at the time
map
visits them; elements that are deleted after the call to
map
begins and before being visited are not visited.
When the
map
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be ?
ArraySpeciesCreate
len
).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Let
mappedValue
be ?
Call
callbackfn
, «
kValue
»).
Perform ?
CreateDataPropertyOrThrow
Pk
mappedValue
).
Increase
by 1.
Return
Note 2
The
map
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.19
Array.prototype.pop ( )
Note 1
The last element of the array is removed from the array and returned.
When the
pop
method is called, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
is zero, then
Perform ?
Set
"length"
, 0,
true
).
Return
undefined
Else
len
> 0,
Let
newLen
be
len
- 1.
Let
index
be !
ToString
newLen
).
Let
element
be ?
Get
index
).
Perform ?
DeletePropertyOrThrow
index
).
Perform ?
Set
"length"
newLen
true
).
Return
element
Note 2
The
pop
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.20
Array.prototype.push ( ...
items
Note 1
The arguments are appended to the end of the array, in the
order in which they appear. The new length of the array is returned as
the result of the call.
When the
push
method is called with zero or more arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
items
be a
List
whose elements are, in left to right order, the arguments that were passed to this function invocation.
Let
argCount
be the number of elements in
items
If
len
argCount
> 2
53
- 1, throw a
TypeError
exception.
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of the element.
Perform ?
Set
, !
ToString
len
),
true
).
Increase
len
by 1.
Perform ?
Set
"length"
len
true
).
Return
len
The
"length"
property of the
push
method is 1.
Note 2
The
push
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.21
Array.prototype.reduce (
callbackfn
[ ,
initialValue
] )
Note 1
callbackfn
should be a function that takes four arguments.
reduce
calls the callback, as a function, once for each element after the first element present in the array, in ascending order.
callbackfn
is called with four arguments: the
previousValue
(value from the previous call to
callbackfn
), the
currentValue
(value of the current element), the
currentIndex
, and the object being traversed. The first time that callback is called, the
previousValue
and
currentValue
can be one of two values. If an
initialValue
was supplied in the call to
reduce
, then
previousValue
will be equal to
initialValue
and
currentValue
will be equal to the first value in the array. If no
initialValue
was supplied, then
previousValue
will be equal to the first value in the array and
currentValue
will be equal to the second. It is a
TypeError
if the array contains no elements and
initialValue
is not provided.
reduce
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
reduce
is set before the first call to
callbackfn
. Elements that are appended to the array after the call to
reduce
begins will not be visited by
callbackfn
. If existing elements of the array are changed, their value as passed to
callbackfn
will be the value at the time
reduce
visits them; elements that are deleted after the call to
reduce
begins and before being visited are not visited.
When the
reduce
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
len
is 0 and
initialValue
is not present, throw a
TypeError
exception.
Let
be 0.
Let
accumulator
be
undefined
If
initialValue
is present, then
Set
accumulator
to
initialValue
Else
initialValue
is not present,
Let
kPresent
be
false
Repeat, while
kPresent
is
false
and
len
Let
Pk
be !
ToString
).
Set
kPresent
to ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Set
accumulator
to ?
Get
Pk
).
Increase
by 1.
If
kPresent
is
false
, throw a
TypeError
exception.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Set
accumulator
to ?
Call
callbackfn
undefined
, «
accumulator
kValue
»).
Increase
by 1.
Return
accumulator
Note 2
The
reduce
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.22
Array.prototype.reduceRight (
callbackfn
[ ,
initialValue
] )
Note 1
callbackfn
should be a function that takes four arguments.
reduceRight
calls the callback, as a function, once for each element after the first element present in the array, in descending order.
callbackfn
is called with four arguments: the
previousValue
(value from the previous call to
callbackfn
), the
currentValue
(value of the current element), the
currentIndex
, and the object being traversed. The first time the function is called, the
previousValue
and
currentValue
can be one of two values. If an
initialValue
was supplied in the call to
reduceRight
, then
previousValue
will be equal to
initialValue
and
currentValue
will be equal to the last value in the array. If no
initialValue
was supplied, then
previousValue
will be equal to the last value in the array and
currentValue
will be equal to the second-to-last value. It is a
TypeError
if the array contains no elements and
initialValue
is not provided.
reduceRight
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
reduceRight
is set before the first call to
callbackfn
. Elements that are appended to the array after the call to
reduceRight
begins will not be visited by
callbackfn
. If existing elements of the array are changed by
callbackfn
, their value as passed to
callbackfn
will be the value at the time
reduceRight
visits them; elements that are deleted after the call to
reduceRight
begins and before being visited are not visited.
When the
reduceRight
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
len
is 0 and
initialValue
is not present, throw a
TypeError
exception.
Let
be
len
- 1.
Let
accumulator
be
undefined
If
initialValue
is present, then
Set
accumulator
to
initialValue
Else
initialValue
is not present,
Let
kPresent
be
false
Repeat, while
kPresent
is
false
and
≥ 0
Let
Pk
be !
ToString
).
Set
kPresent
to ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Set
accumulator
to ?
Get
Pk
).
Decrease
by 1.
If
kPresent
is
false
, throw a
TypeError
exception.
Repeat, while
≥ 0
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Set
accumulator
to ?
Call
callbackfn
undefined
, «
accumulator
kValue
»).
Decrease
by 1.
Return
accumulator
Note 2
The
reduceRight
function is intentionally
generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a
method.
22.1.3.23
Array.prototype.reverse ( )
Note 1
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.
When the
reverse
method is called, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
middle
be
floor
len
/ 2).
Let
lower
be 0.
Repeat, while
lower
middle
Let
upper
be
len
lower
- 1.
Let
upperP
be !
ToString
upper
).
Let
lowerP
be !
ToString
lower
).
Let
lowerExists
be ?
HasProperty
lowerP
).
If
lowerExists
is
true
, then
Let
lowerValue
be ?
Get
lowerP
).
Let
upperExists
be ?
HasProperty
upperP
).
If
upperExists
is
true
, then
Let
upperValue
be ?
Get
upperP
).
If
lowerExists
is
true
and
upperExists
is
true
, then
Perform ?
Set
lowerP
upperValue
true
).
Perform ?
Set
upperP
lowerValue
true
).
Else if
lowerExists
is
false
and
upperExists
is
true
, then
Perform ?
Set
lowerP
upperValue
true
).
Perform ?
DeletePropertyOrThrow
upperP
).
Else if
lowerExists
is
true
and
upperExists
is
false
, then
Perform ?
DeletePropertyOrThrow
lowerP
).
Perform ?
Set
upperP
lowerValue
true
).
Else both
lowerExists
and
upperExists
are
false
No action is required.
Increase
lower
by 1.
Return
Note 2
The
reverse
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.24
Array.prototype.shift ( )
Note 1
The first element of the array is removed from the array and returned.
When the
shift
method is called, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
len
is zero, then
Perform ?
Set
"length"
, 0,
true
).
Return
undefined
Let
first
be ?
Get
"0"
).
Let
be 1.
Repeat, while
len
Let
from
be !
ToString
).
Let
to
be !
ToString
- 1).
Let
fromPresent
be ?
HasProperty
from
).
If
fromPresent
is
true
, then
Let
fromVal
be ?
Get
from
).
Perform ?
Set
to
fromVal
true
).
Else
fromPresent
is
false
Perform ?
DeletePropertyOrThrow
to
).
Increase
by 1.
Perform ?
DeletePropertyOrThrow
, !
ToString
len
- 1)).
Perform ?
Set
"length"
len
- 1,
true
).
Return
first
Note 2
The
shift
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.25
Array.prototype.slice (
start
end
Note 1
The
slice
method takes two arguments,
start
and
end
, and returns an array containing the elements of the array from element
start
up to, but not including, element
end
(or through the end of the array if
end
is
undefined
). If
start
is negative, it is treated as
length
start
where
length
is the length of the array. If
end
is negative, it is treated as
length
end
where
length
is the length of the array.
The following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
be
max
((
len
relativeStart
), 0); else let
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
count
be
max
final
, 0).
Let
be ?
ArraySpeciesCreate
count
).
Let
be 0.
Repeat, while
final
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Perform ?
CreateDataPropertyOrThrow
, !
ToString
),
kValue
).
Increase
by 1.
Increase
by 1.
Perform ?
Set
"length"
true
).
Return
Note 2
The explicit setting of the
"length"
property
of the result Array in step 11 was necessary in previous editions of
ECMAScript to ensure that its length was correct in situations where the
trailing elements of the result Array were not present. Setting
"length"
became unnecessary starting in ES2015 when the result Array was
initialized to its proper length rather than an empty Array but is
carried forward to preserve backward compatibility.
Note 3
The
slice
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.26
Array.prototype.some (
callbackfn
[ ,
thisArg
] )
Note 1
callbackfn
should be a function that accepts three arguments and returns a value that is coercible to the Boolean value
true
or
false
some
calls
callbackfn
once for each element present in the array, in ascending order, until it finds one where
callbackfn
returns
true
. If such an element is found,
some
immediately returns
true
. Otherwise,
some
returns
false
callbackfn
is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
The range of elements processed by
some
is set before the first call to
callbackfn
. Elements that are appended to the array after the call to
some
begins will not be visited by
callbackfn
. If existing elements of the array are changed, their value as passed to
callbackfn
will be the value at the time that
some
visits them; elements that are deleted after the call to
some
begins and before being visited are not visited.
some
acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns
false
When the
some
method is called with one or two arguments, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kPresent
be ?
HasProperty
Pk
).
If
kPresent
is
true
, then
Let
kValue
be ?
Get
Pk
).
Let
testResult
be
ToBoolean
(?
Call
callbackfn
, «
kValue
»)).
If
testResult
is
true
, return
true
Increase
by 1.
Return
false
Note 2
The
some
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.27
Array.prototype.sort (
comparefn
The elements of this array are sorted. The sort must be
stable (that is, elements that compare equal must remain in their
original order). If
comparefn
is not
undefined
, it should be a function that accepts two arguments
and
and returns a negative value if
, zero if
, or a positive value if
Upon entry, the following steps are performed to initialize evaluation of the
sort
function:
If
comparefn
is not
undefined
and
IsCallable
comparefn
) is
false
, throw a
TypeError
exception.
Let
obj
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
obj
"length"
)).
Within this specification of the
sort
method, an object,
obj
, is said to be
sparse
if the following algorithm returns
true
For each integer
in the range 0 ≤
len
, do
Let
elem
be
obj
.[[GetOwnProperty]](!
ToString
)).
If
elem
is
undefined
, return
true
Return
false
The
sort order
is the ordering, after completion of this function, of the
integer-indexed
property values of
obj
whose integer indexes are less than
len
. The result of the
sort
function is then determined as follows:
If
comparefn
is not
undefined
and is not a consistent comparison function for the elements of this
array (see below), the sort order is implementation-defined. The sort
order is also implementation-defined if
comparefn
is
undefined
and
SortCompare
does not act as a consistent comparison function.
Let
proto
be
obj
.[[GetPrototypeOf]](). If
proto
is not
null
and there exists an integer
such that all of the conditions below are satisfied then the sort order is implementation-defined:
obj
is sparse
0 ≤
len
HasProperty
proto
ToString
)) is
true
The sort order is also implementation-defined if
obj
is sparse and any of the following conditions are true:
IsExtensible
obj
) is
false
Any
integer index
property of
obj
whose name is a nonnegative integer less than
len
is a
data property
whose [[Configurable]] attribute is
false
The sort order is also implementation-defined if any of the following conditions are true:
If
obj
is an
exotic object
(including Proxy exotic objects) whose behaviour for [[Get]], [[Set]],
[[Delete]], and [[GetOwnProperty]] is not the ordinary object
implementation of these internal methods.
If any index property of
obj
whose name is a nonnegative integer less than
len
is an
accessor property
or is a
data property
whose [[Writable]] attribute is
false
If
comparefn
is
undefined
and the application of
ToString
to any value passed as an argument to
SortCompare
modifies
obj
or any object on
obj
's prototype chain.
If
comparefn
is
undefined
and all applications of
ToString
, to any specific value passed as an argument to
SortCompare
, do not produce the same result.
The following steps are taken:
Perform an implementation-dependent sequence of calls to the [[Get]] and [[Set]] internal methods of
obj
, to the
DeletePropertyOrThrow
and
HasOwnProperty
abstract operation with
obj
as the first argument, and to
SortCompare
(described below), such that:
The property key argument for each call to [[Get]], [[Set]],
HasOwnProperty
, or
DeletePropertyOrThrow
is the string representation of a nonnegative integer less than
len
The arguments for calls to
SortCompare
are values returned by a previous call to the [[Get]] internal method,
unless the properties accessed by those previous calls did not exist
according to
HasOwnProperty
. If both prospective arguments to
SortCompare
correspond to non-existent properties, use
+0
instead of calling
SortCompare
. If only the first prospective argument is non-existent use +1. If only the second prospective argument is non-existent use -1.
If
obj
is not sparse then
DeletePropertyOrThrow
must not be called.
If any [[Set]] call returns
false
TypeError
exception is thrown.
If an
abrupt completion
is returned from any of these operations, it is immediately returned as the value of this function.
Return
obj
Unless the sort order is specified above to be
implementation-defined, the returned object must have the following two
characteristics:
There must be some mathematical permutation π of the nonnegative integers less than
len
, such that for every nonnegative integer
less than
len
, if property
old[
existed, then
new[π(
)]
is exactly the same value as
old[
. But if property
old[
did not exist, then
new[π(
)]
does not exist.
Then for all nonnegative integers
and
, each less than
len
, if
SortCompare
(old[
], old[
]) < 0
(see
SortCompare
below), then
new[π(
)] < new[π(
)]
Here the notation
old[
is used to refer to the hypothetical result of calling
obj
.[[Get]](
) before this function is executed, and the notation
new[
to refer to the hypothetical result of calling
obj
.[[Get]](
) after this function has been executed.
A function
comparefn
is a consistent comparison function for a set of values
if all of the requirements below are met for all values
, and
(possibly the same value) in the set
: The notation
CF
means
comparefn
) < 0
CF
means
comparefn
) = 0
(of either sign); and
CF
means
comparefn
) > 0
Calling
comparefn
) always returns the same value
when given a specific pair of values
and
as its two arguments. Furthermore,
Type
) is Number, and
is not
NaN
. Note that this implies that exactly one of
CF
CF
, and
CF
will be true for a given pair of
and
Calling
comparefn
) does not modify
obj
or any object on
obj
's prototype chain.
CF
(reflexivity)
If
CF
, then
CF
(symmetry)
If
CF
and
CF
, then
CF
(transitivity of =
CF
If
CF
and
CF
, then
CF
(transitivity of <
CF
If
CF
and
CF
, then
CF
(transitivity of >
CF
Note 1
The above conditions are necessary and sufficient to ensure that
comparefn
divides the set
into equivalence classes and that these equivalence classes are totally ordered.
Note 2
The
sort
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.27.1
Runtime Semantics: SortCompare (
The SortCompare abstract operation is called with two arguments
and
. It also has access to the
comparefn
argument passed to the current invocation of the
sort
method. The following steps are taken:
If
and
are both
undefined
, return
+0
If
is
undefined
, return 1.
If
is
undefined
, return -1.
If
comparefn
is not
undefined
, then
Let
be ?
ToNumber
(?
Call
comparefn
undefined
, «
»)).
If
is
NaN
, return
+0
Return
Let
xString
be ?
ToString
).
Let
yString
be ?
ToString
).
Let
xSmaller
be the result of performing
Abstract Relational Comparison
xString
yString
If
xSmaller
is
true
, return -1.
Let
ySmaller
be the result of performing
Abstract Relational Comparison
yString
xString
If
ySmaller
is
true
, return 1.
Return
+0
Note 1
Because non-existent property values always compare greater than
undefined
property values, and
undefined
always compares greater than any other value,
undefined
property values always sort to the end of the result, followed by non-existent property values.
Note 2
Method calls performed by the
ToString
abstract operations
in steps 5 and 7 have the potential to cause SortCompare to not behave as a consistent comparison function.
22.1.3.28
Array.prototype.splice (
start
deleteCount
, ...
items
Note 1
When the
splice
method is called with two or more arguments
start
deleteCount
and zero or more
items
, the
deleteCount
elements of the array starting at
integer index
start
are replaced by the arguments
items
. An Array object containing the deleted elements (if any) is returned.
The following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
actualStart
be
max
((
len
relativeStart
), 0); else let
actualStart
be
min
relativeStart
len
).
If the number of actual arguments is 0, then
Let
insertCount
be 0.
Let
actualDeleteCount
be 0.
Else if the number of actual arguments is 1, then
Let
insertCount
be 0.
Let
actualDeleteCount
be
len
actualStart
Else,
Let
insertCount
be the number of actual arguments minus 2.
Let
dc
be ?
ToInteger
deleteCount
).
Let
actualDeleteCount
be
min
max
dc
, 0),
len
actualStart
).
If
len
insertCount
actualDeleteCount
> 2
53
- 1, throw a
TypeError
exception.
Let
be ?
ArraySpeciesCreate
actualDeleteCount
).
Let
be 0.
Repeat, while
actualDeleteCount
Let
from
be !
ToString
actualStart
).
Let
fromPresent
be ?
HasProperty
from
).
If
fromPresent
is
true
, then
Let
fromValue
be ?
Get
from
).
Perform ?
CreateDataPropertyOrThrow
, !
ToString
),
fromValue
).
Increment
by 1.
Perform ?
Set
"length"
actualDeleteCount
true
).
Let
items
be a
List
whose elements are, in left to right order, the portion of the actual
argument list starting with the third argument. The list is empty if
fewer than three arguments were passed.
Let
itemCount
be the number of elements in
items
If
itemCount
actualDeleteCount
, then
Set
to
actualStart
Repeat, while
< (
len
actualDeleteCount
Let
from
be !
ToString
actualDeleteCount
).
Let
to
be !
ToString
itemCount
).
Let
fromPresent
be ?
HasProperty
from
).
If
fromPresent
is
true
, then
Let
fromValue
be ?
Get
from
).
Perform ?
Set
to
fromValue
true
).
Else
fromPresent
is
false
Perform ?
DeletePropertyOrThrow
to
).
Increase
by 1.
Set
to
len
Repeat, while
> (
len
actualDeleteCount
itemCount
Perform ?
DeletePropertyOrThrow
, !
ToString
- 1)).
Decrease
by 1.
Else if
itemCount
actualDeleteCount
, then
Set
to (
len
actualDeleteCount
).
Repeat, while
actualStart
Let
from
be !
ToString
actualDeleteCount
- 1).
Let
to
be !
ToString
itemCount
- 1).
Let
fromPresent
be ?
HasProperty
from
).
If
fromPresent
is
true
, then
Let
fromValue
be ?
Get
from
).
Perform ?
Set
to
fromValue
true
).
Else
fromPresent
is
false
Perform ?
DeletePropertyOrThrow
to
).
Decrease
by 1.
Set
to
actualStart
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of that element.
Perform ?
Set
, !
ToString
),
true
).
Increase
by 1.
Perform ?
Set
"length"
len
actualDeleteCount
itemCount
true
).
Return
Note 2
The explicit setting of the
"length"
property
of the result Array in step 19 was necessary in previous editions of
ECMAScript to ensure that its length was correct in situations where the
trailing elements of the result Array were not present. Setting
"length"
became unnecessary starting in ES2015 when the result Array was
initialized to its proper length rather than an empty Array but is
carried forward to preserve backward compatibility.
Note 3
The
splice
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.29
Array.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the
Array.prototype.toLocaleString
method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following
specification of the
toLocaleString
method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for the
Array.prototype.toLocaleString
method.
The meanings of the optional parameters to this method are
defined in the ECMA-402 specification; implementations that do not
include ECMA-402 support must not use those parameter positions for
anything else.
The following steps are taken:
Let
array
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
array
"length"
)).
Let
separator
be the String value for the list-separator String appropriate for the
host environment's current locale (this is derived in an
implementation-defined way).
Let
be the empty String.
Let
be 0.
Repeat, while
len
If
> 0, then
Set
to the
string-concatenation
of
and
separator
Let
nextElement
be ?
Get
array
, !
ToString
)).
If
nextElement
is not
undefined
or
null
, then
Let
be ?
ToString
(?
Invoke
nextElement
"toLocaleString"
)).
Set
to the
string-concatenation
of
and
Increase
by 1.
Return
Note 2
The elements of the array are converted to Strings using their
toLocaleString
methods, and these Strings are then concatenated, separated by
occurrences of a separator String that has been derived in an
implementation-defined locale-specific way. The result of calling this
function is intended to be analogous to the result of
toString
, except that the result of this function is intended to be locale-specific.
Note 3
The
toLocaleString
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.30
Array.prototype.toString ( )
When the
toString
method is called, the following steps are taken:
Let
array
be ?
ToObject
this
value).
Let
func
be ?
Get
array
"join"
).
If
IsCallable
func
) is
false
, set
func
to the intrinsic function
%ObjProto_toString%
Return ?
Call
func
array
).
Note
The
toString
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.31
Array.prototype.unshift ( ...
items
Note 1
The arguments are prepended to the start of the array, such
that their order within the array is the same as the order in which
they appear in the argument list.
When the
unshift
method is called with zero or more arguments
item1
item2
, etc., the following steps are taken:
Let
be ?
ToObject
this
value).
Let
len
be ?
ToLength
(?
Get
"length"
)).
Let
argCount
be the number of actual arguments.
If
argCount
> 0, then
If
len
argCount
> 2
53
- 1, throw a
TypeError
exception.
Let
be
len
Repeat, while
> 0,
Let
from
be !
ToString
- 1).
Let
to
be !
ToString
argCount
- 1).
Let
fromPresent
be ?
HasProperty
from
).
If
fromPresent
is
true
, then
Let
fromValue
be ?
Get
from
).
Perform ?
Set
to
fromValue
true
).
Else
fromPresent
is
false
Perform ?
DeletePropertyOrThrow
to
).
Decrease
by 1.
Let
be 0.
Let
items
be a
List
whose elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while
items
is not empty
Remove the first element from
items
and let
be the value of that element.
Perform ?
Set
, !
ToString
),
true
).
Increase
by 1.
Perform ?
Set
"length"
len
argCount
true
).
Return
len
argCount
The
"length"
property of the
unshift
method is 1.
Note 2
The
unshift
function is intentionally generic; it does not require that its
this
value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.32
Array.prototype.values ( )
The following steps are taken:
Let
be ?
ToObject
this
value).
Return
CreateArrayIterator
"value"
).
This function is the
%ArrayProto_values%
intrinsic object.
22.1.3.33
Array.prototype [ @@iterator ] ( )
The initial value of the @@iterator property is the same
function object
as the initial value of the
Array.prototype.values
property.
22.1.3.34
Array.prototype [ @@unscopables ]
The initial value of the @@unscopables
data property
is an object created by the following steps:
Let
unscopableList
be
ObjectCreate
null
).
Perform
CreateDataProperty
unscopableList
"copyWithin"
true
).
Perform
CreateDataProperty
unscopableList
"entries"
true
).
Perform
CreateDataProperty
unscopableList
"fill"
true
).
Perform
CreateDataProperty
unscopableList
"find"
true
).
Perform
CreateDataProperty
unscopableList
"findIndex"
true
).
Perform
CreateDataProperty
unscopableList
"flat"
true
).
Perform
CreateDataProperty
unscopableList
"flatMap"
true
).
Perform
CreateDataProperty
unscopableList
"includes"
true
).
Perform
CreateDataProperty
unscopableList
"keys"
true
).
Perform
CreateDataProperty
unscopableList
"values"
true
).
Assert
: Each of the above calls returns
true
Return
unscopableList
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
Note
The own property names of this object are property names that were not included as standard properties of
Array.prototype
prior to the ECMAScript 2015 specification. These names are ignored for
with
statement binding purposes in order to preserve the behaviour of
existing code that might use one of these names as a binding in an outer
scope that is shadowed by a
with
statement whose binding object is an Array object.
22.1.4
Properties of Array Instances
Array instances are Array exotic objects and have the internal
methods specified for such objects. Array instances inherit properties
from the Array prototype object.
Array instances have a
"length"
property, and a set of enumerable properties with
array index
names.
22.1.4.1
length
The
"length"
property of an Array instance is a
data property
whose value is always numerically greater than the name of every configurable own property whose name is an
array index
The
"length"
property initially has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
Reducing the value of the
"length"
property has the side-effect of deleting own array elements whose
array index
is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the
"length"
property of an Array object to a value that is numerically less than or equal to the largest numeric own
property name
of an existing non-configurable
array-indexed
property of the array will result in the length being set to a numeric
value that is one greater than that non-configurable numeric own
property name
. See
9.4.2.1
22.1.5
Array Iterator Objects
An Array Iterator is an object, that represents a specific
iteration over some specific Array instance object. There is not a named
constructor
for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.
22.1.5.1
CreateArrayIterator (
array
kind
Several methods of Array objects return Iterator objects. The abstract operation CreateArrayIterator with arguments
array
and
kind
is used to create such iterator objects. It performs the following steps:
Assert
Type
array
) is Object.
Let
iterator
be
ObjectCreate
%ArrayIteratorPrototype%
, « [[IteratedObject]], [[ArrayIteratorNextIndex]], [[ArrayIterationKind]] »).
Set
iterator
.[[IteratedObject]] to
array
Set
iterator
.[[ArrayIteratorNextIndex]] to 0.
Set
iterator
.[[ArrayIterationKind]] to
kind
Return
iterator
22.1.5.2
The %ArrayIteratorPrototype% Object
The
%ArrayIteratorPrototype%
object:
has properties that are inherited by all Array Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%IteratorPrototype%
has the following properties:
22.1.5.2.1
%ArrayIteratorPrototype%.next ( )
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have all of the internal slots of an Array Iterator Instance (
22.1.5.3
), throw a
TypeError
exception.
Let
be
.[[IteratedObject]].
If
is
undefined
, return
CreateIterResultObject
undefined
true
).
Let
index
be
.[[ArrayIteratorNextIndex]].
Let
itemKind
be
.[[ArrayIterationKind]].
If
has a [[TypedArrayName]] internal slot, then
If
IsDetachedBuffer
.[[ViewedArrayBuffer]]) is
true
, throw a
TypeError
exception.
Let
len
be
.[[ArrayLength]].
Else,
Let
len
be ?
ToLength
(?
Get
"length"
)).
If
index
len
, then
Set
.[[IteratedObject]] to
undefined
Return
CreateIterResultObject
undefined
true
).
Set
.[[ArrayIteratorNextIndex]] to
index
+ 1.
If
itemKind
is
"key"
, return
CreateIterResultObject
index
false
).
Let
elementKey
be !
ToString
index
).
Let
elementValue
be ?
Get
elementKey
).
If
itemKind
is
"value"
, let
result
be
elementValue
Else,
Assert
itemKind
is
"key+value"
Let
result
be
CreateArrayFromList
(«
index
elementValue
»).
Return
CreateIterResultObject
result
false
).
22.1.5.2.2
%ArrayIteratorPrototype% [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Array Iterator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
22.1.5.3
Properties of Array Iterator Instances
Array Iterator instances are ordinary objects that inherit properties from the
%ArrayIteratorPrototype%
intrinsic object. Array Iterator instances are initially created with the internal slots listed in
Table 58
Table 58: Internal Slots of Array Iterator Instances
Internal Slot
Description
[[IteratedObject]]
The object whose array elements are being iterated.
[[ArrayIteratorNextIndex]]
The
integer index
of the next
integer index
to be examined by this iteration.
[[ArrayIterationKind]]
A String value that identifies what is returned for each element of the iteration. The possible values are:
"key"
"value"
"key+value"
22.2
TypedArray Objects
TypedArray
objects present an array-like view of an underlying binary data buffer (
24.1
). Each element of a
TypedArray
instance has the same underlying binary scalar data type. There is a distinct
TypedArray
constructor
, listed in
Table 59
, for each of the nine supported element types. Each
constructor
in
Table 59
has a corresponding distinct prototype object.
Table 59: The TypedArray Constructors
Constructor
Name and Intrinsic
Element Type
Element Size
Conversion Operation
Description
Equivalent C Type
Int8Array
%Int8Array%
Int8
ToInt8
8-bit 2's complement signed integer
signed char
Uint8Array
%Uint8Array%
Uint8
ToUint8
8-bit unsigned integer
unsigned char
Uint8ClampedArray
%Uint8ClampedArray%
Uint8C
ToUint8Clamp
8-bit unsigned integer (clamped conversion)
unsigned char
Int16Array
%Int16Array%
Int16
ToInt16
16-bit 2's complement signed integer
short
Uint16Array
%Uint16Array%
Uint16
ToUint16
16-bit unsigned integer
unsigned short
Int32Array
%Int32Array%
Int32
ToInt32
32-bit 2's complement signed integer
int
Uint32Array
%Uint32Array%
Uint32
ToUint32
32-bit unsigned integer
unsigned int
Float32Array
%Float32Array%
Float32
32-bit IEEE floating point
float
Float64Array
%Float64Array%
Float64
64-bit IEEE floating point
double
In the definitions below, references to
TypedArray
should be replaced with the appropriate
constructor
name from the above table. The phrase “the element size in bytes”
refers to the value in the Element Size column of the table in the row
corresponding to the
constructor
. The phrase “element Type” refers to the value in the Element Type column for that row.
22.2.1
The %TypedArray% Intrinsic Object
The
%TypedArray%
intrinsic object:
is a
constructor
function object
that all of the
TypedArray
constructor
objects inherit from.
along with its corresponding prototype object, provides common properties that are inherited by all
TypedArray
constructors and their instances.
does not have a global name or appear as a property of the
global object
acts as the abstract superclass of the various
TypedArray
constructors.
will throw an error when invoked, because it is an abstract class
constructor
. The
TypedArray
constructors do not perform a
super
call to it.
22.2.1.1
%TypedArray% ( )
The
%TypedArray%
constructor
performs the following steps:
Throw a
TypeError
exception.
The
"length"
property of the
%TypedArray%
constructor
function is 0.
22.2.2
Properties of the %TypedArray% Intrinsic Object
The
%TypedArray%
intrinsic object:
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has a
name
property whose value is
"TypedArray"
has the following properties:
22.2.2.1
%TypedArray%.from (
source
[ ,
mapfn
[ ,
thisArg
] ] )
When the
from
method is called with argument
source
, and optional arguments
mapfn
and
thisArg
, the following steps are taken:
Let
be the
this
value.
If
IsConstructor
) is
false
, throw a
TypeError
exception.
If
mapfn
is present and
mapfn
is not
undefined
, then
If
IsCallable
mapfn
) is
false
, throw a
TypeError
exception.
Let
mapping
be
true
Else, let
mapping
be
false
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
usingIterator
be ?
GetMethod
source
, @@iterator).
If
usingIterator
is not
undefined
, then
Let
values
be ?
IterableToList
source
usingIterator
).
Let
len
be the number of elements in
values
Let
targetObj
be ?
TypedArrayCreate
, «
len
»).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be the first element of
values
and remove that element from
values
If
mapping
is
true
, then
Let
mappedValue
be ?
Call
mapfn
, «
kValue
»).
Else, let
mappedValue
be
kValue
Perform ?
Set
targetObj
Pk
mappedValue
true
).
Increase
by 1.
Assert
values
is now an empty
List
Return
targetObj
NOTE:
source
is not an Iterable so assume it is already an array-like object.
Let
arrayLike
be !
ToObject
source
).
Let
len
be ?
ToLength
(?
Get
arrayLike
"length"
)).
Let
targetObj
be ?
TypedArrayCreate
, «
len
»).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
arrayLike
Pk
).
If
mapping
is
true
, then
Let
mappedValue
be ?
Call
mapfn
, «
kValue
»).
Else, let
mappedValue
be
kValue
Perform ?
Set
targetObj
Pk
mappedValue
true
).
Increase
by 1.
Return
targetObj
22.2.2.1.1
Runtime Semantics: IterableToList (
items
method
The abstract operation IterableToList performs the following steps:
Let
iteratorRecord
be ?
GetIterator
items
sync
method
).
Let
values
be a new empty
List
Let
next
be
true
Repeat, while
next
is not
false
Set
next
to ?
IteratorStep
iteratorRecord
).
If
next
is not
false
, then
Let
nextValue
be ?
IteratorValue
next
).
Append
nextValue
to the end of the
List
values
Return
values
22.2.2.2
%TypedArray%.of ( ...
items
When the
of
method is called with any number of arguments, the following steps are taken:
Let
len
be the actual number of arguments passed to this function.
Let
items
be the
List
of arguments passed to this function.
Let
be the
this
value.
If
IsConstructor
) is
false
, throw a
TypeError
exception.
Let
newObj
be ?
TypedArrayCreate
, «
len
»).
Let
be 0.
Repeat, while
len
Let
kValue
be
items
].
Let
Pk
be !
ToString
).
Perform ?
Set
newObj
Pk
kValue
true
).
Increase
by 1.
Return
newObj
Note
The
items
argument is assumed to be a well-formed rest argument value.
22.2.2.3
%TypedArray%.prototype
The initial value of
%TypedArray%
.prototype
is the
%TypedArrayPrototype%
intrinsic object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22.2.2.4
get %TypedArray% [ @@species ]
%TypedArray%
[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
%TypedArrayPrototype%
methods normally use their
this
object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its @@species property.
22.2.3
Properties of the %TypedArrayPrototype% Object
The
%TypedArrayPrototype%
object:
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to
TypedArray
instance objects.
22.2.3.1
get %TypedArray%.prototype.buffer
%TypedArray%
.prototype.buffer
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
Return
buffer
22.2.3.2
get %TypedArray%.prototype.byteLength
%TypedArray%
.prototype.byteLength
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, return 0.
Let
size
be
.[[ByteLength]].
Return
size
22.2.3.3
get %TypedArray%.prototype.byteOffset
%TypedArray%
.prototype.byteOffset
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, return 0.
Let
offset
be
.[[ByteOffset]].
Return
offset
22.2.3.4
%TypedArray%.prototype.constructor
The initial value of
%TypedArray%
.prototype.constructor
is the
%TypedArray%
intrinsic object.
22.2.3.5
%TypedArray%.prototype.copyWithin (
target
start
[ ,
end
] )
The interpretation and use of the arguments of
%TypedArray%
.prototype.copyWithin
are the same as for
Array.prototype.copyWithin
as defined in
22.1.3.3
The following steps are taken:
Let
be
this
value.
Perform ?
ValidateTypedArray
).
Let
len
be
.[[ArrayLength]].
Let
relativeTarget
be ?
ToInteger
target
).
If
relativeTarget
< 0, let
to
be
max
((
len
relativeTarget
), 0); else let
to
be
min
relativeTarget
len
).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
from
be
max
((
len
relativeStart
), 0); else let
from
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
count
be
min
final
from
len
to
).
If
count
> 0, then
NOTE: The copying must be performed in a manner that preserves the bit-level encoding of the source data.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
typedArrayName
be the String value of
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
typedArrayName
Let
byteOffset
be
.[[ByteOffset]].
Let
toByteIndex
be
to
elementSize
byteOffset
Let
fromByteIndex
be
from
elementSize
byteOffset
Let
countBytes
be
count
elementSize
If
fromByteIndex
toByteIndex
and
toByteIndex
fromByteIndex
countBytes
, then
Let
direction
be -1.
Set
fromByteIndex
to
fromByteIndex
countBytes
- 1.
Set
toByteIndex
to
toByteIndex
countBytes
- 1.
Else,
Let
direction
be 1.
Repeat, while
countBytes
> 0
Let
value
be
GetValueFromBuffer
buffer
fromByteIndex
"Uint8"
true
"Unordered"
).
Perform
SetValueInBuffer
buffer
toByteIndex
"Uint8"
value
true
"Unordered"
).
Set
fromByteIndex
to
fromByteIndex
direction
Set
toByteIndex
to
toByteIndex
direction
Decrease
countBytes
by 1.
Return
22.2.3.5.1
Runtime Semantics: ValidateTypedArray (
When called with argument
, the following steps are taken:
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Return
buffer
22.2.3.6
%TypedArray%.prototype.entries ( )
The following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Return
CreateArrayIterator
"key+value"
).
22.2.3.7
%TypedArray%.prototype.every (
callbackfn
[ ,
thisArg
] )
%TypedArray%
.prototype.every
is a distinct function that implements the same algorithm as
Array.prototype.every
as defined in
22.1.3.5
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose integer-indexed
properties are not sparse. However, such optimization must not introduce
any observable changes in the specified behaviour of the algorithm and
must take into account the possibility that calls to
callbackfn
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.8
%TypedArray%.prototype.fill (
value
[ ,
start
[ ,
end
] ] )
The interpretation and use of the arguments of
%TypedArray%
.prototype.fill
are the same as for
Array.prototype.fill
as defined in
22.1.3.6
The following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Let
len
be
.[[ArrayLength]].
Set
value
to ?
ToNumber
value
).
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
be
max
((
len
relativeStart
), 0); else let
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
If
IsDetachedBuffer
.[[ViewedArrayBuffer]]) is
true
, throw a
TypeError
exception.
Repeat, while
final
Let
Pk
be !
ToString
).
Perform !
Set
Pk
value
true
).
Increase
by 1.
Return
22.2.3.9
%TypedArray%.prototype.filter (
callbackfn
[ ,
thisArg
] )
The interpretation and use of the arguments of
%TypedArray%
.prototype.filter
are the same as for
Array.prototype.filter
as defined in
22.1.3.7
When the
filter
method is called with one or two arguments, the following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Let
len
be
.[[ArrayLength]].
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
kept
be a new empty
List
Let
be 0.
Let
captured
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
Pk
).
Let
selected
be
ToBoolean
(?
Call
callbackfn
, «
kValue
»)).
If
selected
is
true
, then
Append
kValue
to the end of
kept
Increase
captured
by 1.
Increase
by 1.
Let
be ?
TypedArraySpeciesCreate
, «
captured
»).
Let
be 0.
For each element
of
kept
, do
Perform !
Set
, !
ToString
),
true
).
Increment
by 1.
Return
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.10
%TypedArray%.prototype.find (
predicate
[ ,
thisArg
] )
%TypedArray%
.prototype.find
is a distinct function that implements the same algorithm as
Array.prototype.find
as defined in
22.1.3.8
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
predicate
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.11
%TypedArray%.prototype.findIndex (
predicate
[ ,
thisArg
] )
%TypedArray%
.prototype.findIndex
is a distinct function that implements the same algorithm as
Array.prototype.findIndex
as defined in
22.1.3.9
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
predicate
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.12
%TypedArray%.prototype.forEach (
callbackfn
[ ,
thisArg
] )
%TypedArray%
.prototype.forEach
is a distinct function that implements the same algorithm as
Array.prototype.forEach
as defined in
22.1.3.12
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.13
%TypedArray%.prototype.includes (
searchElement
[ ,
fromIndex
] )
%TypedArray%
.prototype.includes
is a distinct function that implements the same algorithm as
Array.prototype.includes
as defined in
22.1.3.13
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.14
%TypedArray%.prototype.indexOf (
searchElement
[ ,
fromIndex
] )
%TypedArray%
.prototype.indexOf
is a distinct function that implements the same algorithm as
Array.prototype.indexOf
as defined in
22.1.3.14
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.15
%TypedArray%.prototype.join (
separator
%TypedArray%
.prototype.join
is a distinct function that implements the same algorithm as
Array.prototype.join
as defined in
22.1.3.15
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.16
%TypedArray%.prototype.keys ( )
The following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Return
CreateArrayIterator
"key"
).
22.2.3.17
%TypedArray%.prototype.lastIndexOf (
searchElement
[ ,
fromIndex
] )
%TypedArray%
.prototype.lastIndexOf
is a distinct function that implements the same algorithm as
Array.prototype.lastIndexOf
as defined in
22.1.3.17
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.18
get %TypedArray%.prototype.length
%TypedArray%
.prototype.length
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, return 0.
Let
length
be
.[[ArrayLength]].
Return
length
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.19
%TypedArray%.prototype.map (
callbackfn
[ ,
thisArg
] )
The interpretation and use of the arguments of
%TypedArray%
.prototype.map
are the same as for
Array.prototype.map
as defined in
22.1.3.18
When the
map
method is called with one or two arguments, the following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Let
len
be
.[[ArrayLength]].
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
be ?
TypedArraySpeciesCreate
, «
len
»).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
Pk
).
Let
mappedValue
be ?
Call
callbackfn
, «
kValue
»).
Perform ?
Set
Pk
mappedValue
true
).
Increase
by 1.
Return
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.20
%TypedArray%.prototype.reduce (
callbackfn
[ ,
initialValue
] )
%TypedArray%
.prototype.reduce
is a distinct function that implements the same algorithm as
Array.prototype.reduce
as defined in
22.1.3.21
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.21
%TypedArray%.prototype.reduceRight (
callbackfn
[ ,
initialValue
] )
%TypedArray%
.prototype.reduceRight
is a distinct function that implements the same algorithm as
Array.prototype.reduceRight
as defined in
22.1.3.22
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.22
%TypedArray%.prototype.reverse ( )
%TypedArray%
.prototype.reverse
is a distinct function that implements the same algorithm as
Array.prototype.reverse
as defined in
22.1.3.23
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.23
%TypedArray%.prototype.set (
overloaded
[ ,
offset
] )
%TypedArray%
.prototype.set
is a single function whose behaviour is overloaded based upon the type of its first argument.
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.23.1
%TypedArray%.prototype.set (
array
[ ,
offset
] )
Sets multiple values in this
TypedArray
, reading the values from the object
array
. The optional
offset
value indicates the first element index in this
TypedArray
where values are written. If omitted, it is assumed to be 0.
Assert
array
is any
ECMAScript language value
other than an Object with a [[TypedArrayName]] internal slot. If it is such an Object, the definition in
22.2.3.23.2
applies.
Let
target
be the
this
value.
If
Type
target
) is not Object, throw a
TypeError
exception.
If
target
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
target
has a [[ViewedArrayBuffer]] internal slot.
Let
targetOffset
be ?
ToInteger
offset
).
If
targetOffset
< 0, throw a
RangeError
exception.
Let
targetBuffer
be
target
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
targetBuffer
) is
true
, throw a
TypeError
exception.
Let
targetLength
be
target
.[[ArrayLength]].
Let
targetName
be the String value of
target
.[[TypedArrayName]].
Let
targetElementSize
be the Number value of the Element Size value specified in
Table 59
for
targetName
Let
targetType
be the String value of the Element Type value in
Table 59
for
targetName
Let
targetByteOffset
be
target
.[[ByteOffset]].
Let
src
be ?
ToObject
array
).
Let
srcLength
be ?
ToLength
(?
Get
src
"length"
)).
If
srcLength
targetOffset
targetLength
, throw a
RangeError
exception.
Let
targetByteIndex
be
targetOffset
targetElementSize
targetByteOffset
Let
be 0.
Let
limit
be
targetByteIndex
targetElementSize
srcLength
Repeat, while
targetByteIndex
limit
Let
Pk
be !
ToString
).
Let
kNumber
be ?
ToNumber
(?
Get
src
Pk
)).
If
IsDetachedBuffer
targetBuffer
) is
true
, throw a
TypeError
exception.
Perform
SetValueInBuffer
targetBuffer
targetByteIndex
targetType
kNumber
true
"Unordered"
).
Increase
by 1.
Set
targetByteIndex
to
targetByteIndex
targetElementSize
Return
undefined
22.2.3.23.2
%TypedArray%.prototype.set (
typedArray
[ ,
offset
] )
Sets multiple values in this
TypedArray
, reading the values from the
typedArray
argument object. The optional
offset
value indicates the first element index in this
TypedArray
where values are written. If omitted, it is assumed to be 0.
Assert
typedArray
has a [[TypedArrayName]] internal slot. If it does not, the definition in
22.2.3.23.1
applies.
Let
target
be the
this
value.
If
Type
target
) is not Object, throw a
TypeError
exception.
If
target
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
target
has a [[ViewedArrayBuffer]] internal slot.
Let
targetOffset
be ?
ToInteger
offset
).
If
targetOffset
< 0, throw a
RangeError
exception.
Let
targetBuffer
be
target
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
targetBuffer
) is
true
, throw a
TypeError
exception.
Let
targetLength
be
target
.[[ArrayLength]].
Let
srcBuffer
be
typedArray
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
srcBuffer
) is
true
, throw a
TypeError
exception.
Let
targetName
be the String value of
target
.[[TypedArrayName]].
Let
targetType
be the String value of the Element Type value in
Table 59
for
targetName
Let
targetElementSize
be the Number value of the Element Size value specified in
Table 59
for
targetName
Let
targetByteOffset
be
target
.[[ByteOffset]].
Let
srcName
be the String value of
typedArray
.[[TypedArrayName]].
Let
srcType
be the String value of the Element Type value in
Table 59
for
srcName
Let
srcElementSize
be the Number value of the Element Size value specified in
Table 59
for
srcName
Let
srcLength
be
typedArray
.[[ArrayLength]].
Let
srcByteOffset
be
typedArray
.[[ByteOffset]].
If
srcLength
targetOffset
targetLength
, throw a
RangeError
exception.
If both
IsSharedArrayBuffer
srcBuffer
) and
IsSharedArrayBuffer
targetBuffer
) are
true
, then
If
srcBuffer
.[[ArrayBufferData]] and
targetBuffer
.[[ArrayBufferData]] are the same
Shared Data Block
values, let
same
be
true
; else let
same
be
false
Else, let
same
be
SameValue
srcBuffer
targetBuffer
).
If
same
is
true
, then
Let
srcByteLength
be
typedArray
.[[ByteLength]].
Set
srcBuffer
to ?
CloneArrayBuffer
srcBuffer
srcByteOffset
srcByteLength
%ArrayBuffer%
).
NOTE:
%ArrayBuffer%
is used to clone
srcBuffer
because is it known to not have any observable side-effects.
Let
srcByteIndex
be 0.
Else, let
srcByteIndex
be
srcByteOffset
Let
targetByteIndex
be
targetOffset
targetElementSize
targetByteOffset
Let
limit
be
targetByteIndex
targetElementSize
srcLength
If
SameValue
srcType
targetType
) is
true
, then
NOTE: If
srcType
and
targetType
are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
Repeat, while
targetByteIndex
limit
Let
value
be
GetValueFromBuffer
srcBuffer
srcByteIndex
"Uint8"
true
"Unordered"
).
Perform
SetValueInBuffer
targetBuffer
targetByteIndex
"Uint8"
value
true
"Unordered"
).
Increase
srcByteIndex
by 1.
Increase
targetByteIndex
by 1.
Else,
Repeat, while
targetByteIndex
limit
Let
value
be
GetValueFromBuffer
srcBuffer
srcByteIndex
srcType
true
"Unordered"
).
Perform
SetValueInBuffer
targetBuffer
targetByteIndex
targetType
value
true
"Unordered"
).
Set
srcByteIndex
to
srcByteIndex
srcElementSize
Set
targetByteIndex
to
targetByteIndex
targetElementSize
Return
undefined
22.2.3.24
%TypedArray%.prototype.slice (
start
end
The interpretation and use of the arguments of
%TypedArray%
.prototype.slice
are the same as for
Array.prototype.slice
as defined in
22.1.3.25
. The following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Let
len
be
.[[ArrayLength]].
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
be
max
((
len
relativeStart
), 0); else let
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
count
be
max
final
, 0).
Let
be ?
TypedArraySpeciesCreate
, «
count
»).
Let
srcName
be the String value of
.[[TypedArrayName]].
Let
srcType
be the String value of the Element Type value in
Table 59
for
srcName
Let
targetName
be the String value of
.[[TypedArrayName]].
Let
targetType
be the String value of the Element Type value in
Table 59
for
targetName
If
SameValue
srcType
targetType
) is
false
, then
Let
be 0.
Repeat, while
final
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
Pk
).
Perform !
Set
, !
ToString
),
kValue
true
).
Increase
by 1.
Increase
by 1.
Else if
count
> 0, then
Let
srcBuffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
srcBuffer
) is
true
, throw a
TypeError
exception.
Let
targetBuffer
be
.[[ViewedArrayBuffer]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
srcType
NOTE: If
srcType
and
targetType
are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
Let
srcByteOffet
be
.[[ByteOffset]].
Let
targetByteIndex
be
.[[ByteOffset]].
Let
srcByteIndex
be (
elementSize
) +
srcByteOffet
Let
limit
be
targetByteIndex
count
elementSize
Repeat, while
targetByteIndex
limit
Let
value
be
GetValueFromBuffer
srcBuffer
srcByteIndex
"Uint8"
true
"Unordered"
).
Perform
SetValueInBuffer
targetBuffer
targetByteIndex
"Uint8"
value
true
"Unordered"
).
Increase
srcByteIndex
by 1.
Increase
targetByteIndex
by 1.
Return
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.25
%TypedArray%.prototype.some (
callbackfn
[ ,
thisArg
] )
%TypedArray%
.prototype.some
is a distinct function that implements the same algorithm as
Array.prototype.some
as defined in
22.1.3.26
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm and must take into account the possibility that calls to
callbackfn
may cause the
this
value to become detached.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
22.2.3.26
%TypedArray%.prototype.sort (
comparefn
%TypedArray%
.prototype.sort
is a distinct function that, except as described below, implements the same requirements as those of
Array.prototype.sort
as defined in
22.1.3.27
. The implementation of the
%TypedArray%
.prototype.sort
specification may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. The only internal methods of the
this
object that the algorithm may call are [[Get]] and [[Set]].
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize evaluation of the
sort
function. These steps are used instead of the entry steps in
22.1.3.27
If
comparefn
is not
undefined
and
IsCallable
comparefn
) is
false
, throw a
TypeError
exception.
Let
obj
be the
this
value.
Let
buffer
be ?
ValidateTypedArray
obj
).
Let
len
be
obj
.[[ArrayLength]].
The implementation-defined sort order condition for exotic objects is not applied by
%TypedArray%
.prototype.sort
The following version of
SortCompare
is used by
%TypedArray%
.prototype.sort
. It performs a numeric comparison rather than the string comparison used in
22.1.3.27
SortCompare
has access to the
comparefn
and
buffer
values of the current invocation of the
sort
method.
When the TypedArray
SortCompare
abstract operation is called with two arguments
and
, the following steps are taken:
Assert
: Both
Type
) and
Type
) is Number.
If
comparefn
is not
undefined
, then
Let
be ?
ToNumber
(?
Call
comparefn
undefined
, «
»)).
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
If
is
NaN
, return
+0
Return
If
and
are both
NaN
, return
+0
If
is
NaN
, return 1.
If
is
NaN
, return -1.
If
, return -1.
If
, return 1.
If
is
-0
and
is
+0
, return -1.
If
is
+0
and
is
-0
, return 1.
Return
+0
Note
Because
NaN
always compares greater than any other value,
NaN
property values always sort to the end of the result when
comparefn
is not provided.
22.2.3.27
%TypedArray%.prototype.subarray (
begin
end
Returns a new
TypedArray
object whose element type is the same as this
TypedArray
and whose ArrayBuffer is the same as the ArrayBuffer of this
TypedArray
, referencing the elements at
begin
, inclusive, up to
end
, exclusive. If either
begin
or
end
is negative, it refers to an index from the end of the array, as opposed to from the beginning.
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
Let
srcLength
be
.[[ArrayLength]].
Let
relativeBegin
be ?
ToInteger
begin
).
If
relativeBegin
< 0, let
beginIndex
be
max
((
srcLength
relativeBegin
), 0); else let
beginIndex
be
min
relativeBegin
srcLength
).
If
end
is
undefined
, let
relativeEnd
be
srcLength
; else, let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
endIndex
be
max
((
srcLength
relativeEnd
), 0); else let
endIndex
be
min
relativeEnd
srcLength
).
Let
newLength
be
max
endIndex
beginIndex
, 0).
Let
constructorName
be the String value of
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
constructorName
Let
srcByteOffset
be
.[[ByteOffset]].
Let
beginByteOffset
be
srcByteOffset
beginIndex
elementSize
Let
argumentsList
be «
buffer
beginByteOffset
newLength
».
Return ?
TypedArraySpeciesCreate
argumentsList
).
This function is not generic. The
this
value must be an object with a [[TypedArrayName]] internal slot.
22.2.3.28
%TypedArray%.prototype.toLocaleString ( [
reserved1
[ ,
reserved2
] ] )
%TypedArray%
.prototype.toLocaleString
is a distinct function that implements the same algorithm as
Array.prototype.toLocaleString
as defined in
22.1.3.29
except that the
this
object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of
"length"
. The implementation of the algorithm may be optimized with the knowledge that the
this
value is an object that has a fixed length and whose
integer-indexed
properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the
algorithm.
This function is not generic.
ValidateTypedArray
is applied to the
this
value prior to evaluating the algorithm. If its result is an
abrupt completion
that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for
Array.prototype.toLocaleString
that is in the ECMA-402 specification.
22.2.3.29
%TypedArray%.prototype.toString ( )
The initial value of the
%TypedArray%
.prototype.toString
data property
is the same built-in
function object
as the
Array.prototype.toString
method defined in
22.1.3.30
22.2.3.30
%TypedArray%.prototype.values ( )
The following steps are taken:
Let
be the
this
value.
Perform ?
ValidateTypedArray
).
Return
CreateArrayIterator
"value"
).
22.2.3.31
%TypedArray%.prototype [ @@iterator ] ( )
The initial value of the @@iterator property is the same
function object
as the initial value of the
%TypedArray%
.prototype.values
property.
22.2.3.32
get %TypedArray%.prototype [ @@toStringTag ]
%TypedArray%
.prototype[@@toStringTag]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, return
undefined
If
does not have a [[TypedArrayName]] internal slot, return
undefined
Let
name
be
.[[TypedArrayName]].
Assert
Type
name
) is String.
Return
name
This property has the attributes { [[Enumerable]]:
false
, [[Configurable]]:
true
}.
The initial value of the
name
property of this function is
"get [Symbol.toStringTag]"
22.2.4
The
TypedArray
Constructors
Each
TypedArray
constructor
is an intrinsic object that has the structure described below, differing only in the name used as the
constructor
name instead of
TypedArray
, in
Table 59
is a single function whose behaviour is overloaded based
upon the number and types of its arguments. The actual behaviour of a
call of
TypedArray
depends upon the number and kind of arguments that are passed to it.
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
TypedArray
behaviour must include a
super
call to the
TypedArray
constructor
to create and initialize the subclass instance with the internal state necessary to support the
%TypedArray%
.prototype
built-in methods.
has a
"length"
property whose value is 3.
22.2.4.1
TypedArray
( )
This description applies only if the
TypedArray
function is called with no arguments.
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
constructorName
be the String value of the
Constructor
Name value specified in
Table 59
for this
TypedArray
constructor
Return ?
AllocateTypedArray
constructorName
, NewTarget,
"%
TypedArray
Prototype%"
, 0).
22.2.4.2
TypedArray
length
This description applies only if the
TypedArray
function is called with at least one argument and the Type of the first argument is not Object.
TypedArray
called with argument
length
performs the following steps:
Assert
Type
length
) is not Object.
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
elementLength
be ?
ToIndex
length
).
Let
constructorName
be the String value of the
Constructor
Name value specified in
Table 59
for this
TypedArray
constructor
Return ?
AllocateTypedArray
constructorName
, NewTarget,
"%
TypedArray
Prototype%"
elementLength
).
22.2.4.2.1
Runtime Semantics: AllocateTypedArray (
constructorName
newTarget
defaultProto
[ ,
length
] )
The abstract operation AllocateTypedArray with arguments
constructorName
newTarget
defaultProto
and optional argument
length
is used to validate and create an instance of a TypedArray
constructor
constructorName
is required to be the name of a TypedArray
constructor
in
Table 59
. If the
length
argument is passed, an ArrayBuffer of that length is also allocated and
associated with the new TypedArray instance. AllocateTypedArray
provides common semantics that is used by all of the
TypedArray
overloads. AllocateTypedArray performs the following steps:
Let
proto
be ?
GetPrototypeFromConstructor
newTarget
defaultProto
).
Let
obj
be
IntegerIndexedObjectCreate
proto
, « [[ViewedArrayBuffer]], [[TypedArrayName]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] »).
Assert
obj
.[[ViewedArrayBuffer]] is
undefined
Set
obj
.[[TypedArrayName]] to
constructorName
If
length
is not present, then
Set
obj
.[[ByteLength]] to 0.
Set
obj
.[[ByteOffset]] to 0.
Set
obj
.[[ArrayLength]] to 0.
Else,
Perform ?
AllocateTypedArrayBuffer
obj
length
).
Return
obj
22.2.4.2.2
Runtime Semantics: AllocateTypedArrayBuffer (
length
The abstract operation AllocateTypedArrayBuffer with arguments
and
length
allocates and associates an ArrayBuffer with the TypedArray instance
. It performs the following steps:
Assert
is an Object that has a [[ViewedArrayBuffer]] internal slot.
Assert
.[[ViewedArrayBuffer]] is
undefined
Assert
length
≥ 0.
Let
constructorName
be the String value of
.[[TypedArrayName]].
Let
elementSize
be the Element Size value in
Table 59
for
constructorName
Let
byteLength
be
elementSize
length
Let
data
be ?
AllocateArrayBuffer
%ArrayBuffer%
byteLength
).
Set
.[[ViewedArrayBuffer]] to
data
Set
.[[ByteLength]] to
byteLength
Set
.[[ByteOffset]] to 0.
Set
.[[ArrayLength]] to
length
Return
22.2.4.3
TypedArray
typedArray
This description applies only if the
TypedArray
function is called with at least one argument and the Type of the first
argument is Object and that object has a [[TypedArrayName]] internal
slot.
TypedArray
called with argument
typedArray
performs the following steps:
Assert
Type
typedArray
) is Object and
typedArray
has a [[TypedArrayName]] internal slot.
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
constructorName
be the String value of the
Constructor
Name value specified in
Table 59
for this
TypedArray
constructor
Let
be ?
AllocateTypedArray
constructorName
, NewTarget,
"%
TypedArray
Prototype%"
).
Let
srcArray
be
typedArray
Let
srcData
be
srcArray
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
srcData
) is
true
, throw a
TypeError
exception.
Let
elementType
be the String value of the Element Type value in
Table 59
for
constructorName
Let
elementLength
be
srcArray
.[[ArrayLength]].
Let
srcName
be the String value of
srcArray
.[[TypedArrayName]].
Let
srcType
be the String value of the Element Type value in
Table 59
for
srcName
Let
srcElementSize
be the Element Size value in
Table 59
for
srcName
Let
srcByteOffset
be
srcArray
.[[ByteOffset]].
Let
elementSize
be the Element Size value in
Table 59
for
constructorName
Let
byteLength
be
elementSize
elementLength
If
IsSharedArrayBuffer
srcData
) is
false
, then
Let
bufferConstructor
be ?
SpeciesConstructor
srcData
%ArrayBuffer%
).
Else,
Let
bufferConstructor
be
%ArrayBuffer%
If
SameValue
elementType
srcType
) is
true
, then
Let
data
be ?
CloneArrayBuffer
srcData
srcByteOffset
byteLength
bufferConstructor
).
Else,
Let
data
be ?
AllocateArrayBuffer
bufferConstructor
byteLength
).
If
IsDetachedBuffer
srcData
) is
true
, throw a
TypeError
exception.
Let
srcByteIndex
be
srcByteOffset
Let
targetByteIndex
be 0.
Let
count
be
elementLength
Repeat, while
count
> 0
Let
value
be
GetValueFromBuffer
srcData
srcByteIndex
srcType
true
"Unordered"
).
Perform
SetValueInBuffer
data
targetByteIndex
elementType
value
true
"Unordered"
).
Set
srcByteIndex
to
srcByteIndex
srcElementSize
Set
targetByteIndex
to
targetByteIndex
elementSize
Decrement
count
by 1.
Set
.[[ViewedArrayBuffer]] to
data
Set
.[[ByteLength]] to
byteLength
Set
.[[ByteOffset]] to 0.
Set
.[[ArrayLength]] to
elementLength
Return
22.2.4.4
TypedArray
object
This description applies only if the
TypedArray
function is called with at least one argument and the Type of the first
argument is Object and that object does not have either a
[[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
TypedArray
called with argument
object
performs the following steps:
Assert
Type
object
) is Object and
object
does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
constructorName
be the String value of the
Constructor
Name value specified in
Table 59
for this
TypedArray
constructor
Let
be ?
AllocateTypedArray
constructorName
, NewTarget,
"%
TypedArray
Prototype%"
).
Let
usingIterator
be ?
GetMethod
object
, @@iterator).
If
usingIterator
is not
undefined
, then
Let
values
be ?
IterableToList
object
usingIterator
).
Let
len
be the number of elements in
values
Perform ?
AllocateTypedArrayBuffer
len
).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be the first element of
values
and remove that element from
values
Perform ?
Set
Pk
kValue
true
).
Increase
by 1.
Assert
values
is now an empty
List
Return
NOTE:
object
is not an Iterable so assume it is already an array-like object.
Let
arrayLike
be
object
Let
len
be ?
ToLength
(?
Get
arrayLike
"length"
)).
Perform ?
AllocateTypedArrayBuffer
len
).
Let
be 0.
Repeat, while
len
Let
Pk
be !
ToString
).
Let
kValue
be ?
Get
arrayLike
Pk
).
Perform ?
Set
Pk
kValue
true
).
Increase
by 1.
Return
22.2.4.5
TypedArray
buffer
[ ,
byteOffset
[ ,
length
] ] )
This description applies only if the
TypedArray
function is called with at least one argument and the Type of the first
argument is Object and that object has an [[ArrayBufferData]] internal
slot.
TypedArray
called with at least one argument
buffer
performs the following steps:
Assert
Type
buffer
) is Object and
buffer
has an [[ArrayBufferData]] internal slot.
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
constructorName
be the String value of the
Constructor
Name value specified in
Table 59
for this
TypedArray
constructor
Let
be ?
AllocateTypedArray
constructorName
, NewTarget,
"%
TypedArray
Prototype%"
).
Let
elementSize
be the Number value of the Element Size value in
Table 59
for
constructorName
Let
offset
be ?
ToIndex
byteOffset
).
If
offset
modulo
elementSize
≠ 0, throw a
RangeError
exception.
If
length
is present and
length
is not
undefined
, then
Let
newLength
be ?
ToIndex
length
).
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
bufferByteLength
be
buffer
.[[ArrayBufferByteLength]].
If
length
is either not present or
undefined
, then
If
bufferByteLength
modulo
elementSize
≠ 0, throw a
RangeError
exception.
Let
newByteLength
be
bufferByteLength
offset
If
newByteLength
< 0, throw a
RangeError
exception.
Else,
Let
newByteLength
be
newLength
elementSize
If
offset
newByteLength
bufferByteLength
, throw a
RangeError
exception.
Set
.[[ViewedArrayBuffer]] to
buffer
Set
.[[ByteLength]] to
newByteLength
Set
.[[ByteOffset]] to
offset
Set
.[[ArrayLength]] to
newByteLength
elementSize
Return
22.2.4.6
TypedArrayCreate (
constructor
argumentList
The abstract operation TypedArrayCreate with arguments
constructor
and
argumentList
is used to specify the creation of a new TypedArray object using a
constructor
function. It performs the following steps:
Let
newTypedArray
be ?
Construct
constructor
argumentList
).
Perform ?
ValidateTypedArray
newTypedArray
).
If
argumentList
is a
List
of a single Number, then
If
newTypedArray
.[[ArrayLength]] <
argumentList
[0], throw a
TypeError
exception.
Return
newTypedArray
22.2.4.7
TypedArraySpeciesCreate (
exemplar
argumentList
The abstract operation TypedArraySpeciesCreate with arguments
exemplar
and
argumentList
is used to specify the creation of a new TypedArray object using a
constructor
function that is derived from
exemplar
. It performs the following steps:
Assert
exemplar
is an Object that has a [[TypedArrayName]] internal slot.
Let
defaultConstructor
be the intrinsic object listed in column one of
Table 59
for
exemplar
.[[TypedArrayName]].
Let
constructor
be ?
SpeciesConstructor
exemplar
defaultConstructor
).
Return ?
TypedArrayCreate
constructor
argumentList
).
22.2.5
Properties of the
TypedArray
Constructors
Each
TypedArray
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%TypedArray%
has a
name
property whose value is the String value of the
constructor
name specified for it in
Table 59
has the following properties:
22.2.5.1
TypedArray
.BYTES_PER_ELEMENT
The value of
TypedArray
.BYTES_PER_ELEMENT is the Number value of the Element Size value specified in
Table 59
for
TypedArray
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22.2.5.2
TypedArray
.prototype
The initial value of
TypedArray
.prototype
is the corresponding
TypedArray
prototype intrinsic object (
22.2.6
).
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22.2.6
Properties of the
TypedArray
Prototype Objects
Each
TypedArray
prototype object:
has a [[Prototype]] internal slot whose value is the intrinsic object
%TypedArrayPrototype%
is an ordinary object.
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to
TypedArray
instance objects.
22.2.6.1
TypedArray
.prototype.BYTES_PER_ELEMENT
The value of
TypedArray
.prototype.BYTES_PER_ELEMENT
is the Number value of the Element Size value specified in
Table 59
for
TypedArray
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
22.2.6.2
TypedArray
.prototype.constructor
The initial value of a
TypedArray
.prototype.constructor
is the corresponding %
TypedArray
% intrinsic object.
22.2.7
Properties of
TypedArray
Instances
TypedArray
instances are
Integer-Indexed exotic objects
. Each
TypedArray
instance inherits properties from the corresponding
TypedArray
prototype object. Each
TypedArray
instance has the following internal slots: [[TypedArrayName]],
[[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and
[[ArrayLength]].
23
Keyed Collections
23.1
Map Objects
Map objects are collections of key/value pairs where both the
keys and values may be arbitrary ECMAScript language values. A distinct
key value may only occur in one key/value pair within the Map's
collection. Distinct key values are discriminated using the
SameValueZero
comparison algorithm.
Map object must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear on
the number of elements in the collection. The data structures used in
this Map objects specification is only intended to describe the required
observable semantics of Map objects. It is not intended to be a viable
implementation model.
23.1.1
The Map Constructor
The Map
constructor
is the intrinsic object
%Map%
is the initial value of the
Map
property of the
global object
creates and initializes a new Map object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Map
behaviour must include a
super
call to the
Map
constructor
to create and initialize the subclass instance with the internal state necessary to support the
Map.prototype
built-in methods.
23.1.1.1
Map ( [
iterable
] )
When the
Map
function is called with optional argument
iterable
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
map
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%MapPrototype%"
, « [[MapData]] »).
Set
map
.[[MapData]] to a new empty
List
If
iterable
is not present, or is either
undefined
or
null
, return
map
Let
adder
be ?
Get
map
"set"
).
Return ?
AddEntriesFromIterable
map
iterable
adder
).
Note
If the parameter
iterable
is present, it is
expected to be an object that implements an @@iterator method that
returns an iterator object that produces a two element array-like object
whose first element is a value that will be used as a Map key and whose
second element is the value to associate with that key.
23.1.1.2
AddEntriesFromIterable (
target
iterable
adder
The abstract operation AddEntriesFromIterable accepts a
target
object, an
iterable
of entries, and an
adder
function to be invoked, with
target
as the receiver.
If
IsCallable
adder
) is
false
, throw a
TypeError
exception.
Assert
iterable
is present, and is neither
undefined
nor
null
Let
iteratorRecord
be ?
GetIterator
iterable
).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
target
Let
nextItem
be ?
IteratorValue
next
).
If
Type
nextItem
) is not Object, then
Let
error
be
ThrowCompletion
(a newly created
TypeError
object).
Return ?
IteratorClose
iteratorRecord
error
).
Let
be
Get
nextItem
"0"
).
If
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
).
Let
be
Get
nextItem
"1"
).
If
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
).
Let
status
be
Call
adder
target
, «
.[[Value]],
.[[Value]] »).
If
status
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
status
).
Note
The parameter
iterable
is expected to be an
object that implements an @@iterator method that returns an iterator
object that produces a two element array-like object whose first element
is a value that will be used as a Map key and whose second element is
the value to associate with that key.
23.1.2
Properties of the Map Constructor
The Map
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
23.1.2.1
Map.prototype
The initial value of
Map.prototype
is the intrinsic object
%MapPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
23.1.2.2
get Map [ @@species ]
Map[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
Methods that create derived collection objects should call @@species to determine the
constructor
to use to create the derived objects. Subclass
constructor
may over-ride @@species to change the default
constructor
assignment.
23.1.3
Properties of the Map Prototype Object
The Map prototype object:
is the intrinsic object
%MapPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[MapData]] internal slot.
23.1.3.1
Map.prototype.clear ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
Set
.[[Key]] to
empty
Set
.[[Value]] to
empty
Return
undefined
Note
The existing [[MapData]]
List
is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that
List
23.1.3.2
Map.prototype.constructor
The initial value of
Map.prototype.constructor
is the intrinsic object
%Map%
23.1.3.3
Map.prototype.delete (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValueZero
.[[Key]],
key
) is
true
, then
Set
.[[Key]] to
empty
Set
.[[Value]] to
empty
Return
true
Return
false
Note
The value
empty
is used as a
specification device to indicate that an entry has been deleted. Actual
implementations may take other actions such as physically removing the
entry from internal data structures.
23.1.3.4
Map.prototype.entries ( )
The following steps are taken:
Let
be the
this
value.
Return ?
CreateMapIterator
"key+value"
).
23.1.3.5
Map.prototype.forEach (
callbackfn
[ ,
thisArg
] )
When the
forEach
method is called with one or two arguments, the following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, in original key insertion order, do
If
.[[Key]] is not
empty
, then
Perform ?
Call
callbackfn
, «
.[[Value]],
.[[Key]],
»).
Return
undefined
Note
callbackfn
should be a function that accepts three arguments.
forEach
calls
callbackfn
once for each key/value pair present in the map object, in key insertion order.
callbackfn
is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.
forEach
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to
forEach
begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the
forEach
call completes. Keys that are deleted after the call to
forEach
begins and before being visited are not visited unless the key is added again before the
forEach
call completes.
23.1.3.6
Map.prototype.get (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValueZero
.[[Key]],
key
) is
true
, return
.[[Value]].
Return
undefined
23.1.3.7
Map.prototype.has (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValueZero
.[[Key]],
key
) is
true
, return
true
Return
false
23.1.3.8
Map.prototype.keys ( )
The following steps are taken:
Let
be the
this
value.
Return ?
CreateMapIterator
"key"
).
23.1.3.9
Map.prototype.set (
key
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValueZero
.[[Key]],
key
) is
true
, then
Set
.[[Value]] to
value
Return
If
key
is
-0
, set
key
to
+0
Let
be the
Record
{ [[Key]]:
key
, [[Value]]:
value
}.
Append
as the last element of
entries
Return
23.1.3.10
get Map.prototype.size
Map.prototype.size
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[MapData]].
Let
count
be 0.
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
, increase
count
by 1.
Return
count
23.1.3.11
Map.prototype.values ( )
The following steps are taken:
Let
be the
this
value.
Return ?
CreateMapIterator
"value"
).
23.1.3.12
Map.prototype [ @@iterator ] ( )
The initial value of the @@iterator property is the same
function object
as the initial value of the
entries
property.
23.1.3.13
Map.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Map"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.1.4
Properties of Map Instances
Map instances are ordinary objects that inherit properties from
the Map prototype. Map instances also have a [[MapData]] internal slot.
23.1.5
Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named
constructor
for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.
23.1.5.1
CreateMapIterator (
map
kind
Several methods of Map objects return Iterator objects. The abstract operation CreateMapIterator with arguments
map
and
kind
is used to create such iterator objects. It performs the following steps:
If
Type
map
) is not Object, throw a
TypeError
exception.
If
map
does not have a [[MapData]] internal slot, throw a
TypeError
exception.
Let
iterator
be
ObjectCreate
%MapIteratorPrototype%
, « [[Map]], [[MapNextIndex]], [[MapIterationKind]] »).
Set
iterator
.[[Map]] to
map
Set
iterator
.[[MapNextIndex]] to 0.
Set
iterator
.[[MapIterationKind]] to
kind
Return
iterator
23.1.5.2
The %MapIteratorPrototype% Object
The
%MapIteratorPrototype%
object:
has properties that are inherited by all Map Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%IteratorPrototype%
has the following properties:
23.1.5.2.1
%MapIteratorPrototype%.next ( )
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have all of the internal slots of a Map Iterator Instance (
23.1.5.3
), throw a
TypeError
exception.
Let
be
.[[Map]].
Let
index
be
.[[MapNextIndex]].
Let
itemKind
be
.[[MapIterationKind]].
If
is
undefined
, return
CreateIterResultObject
undefined
true
).
Assert
has a [[MapData]] internal slot.
Let
entries
be the
List
that is
.[[MapData]].
Let
numEntries
be the number of elements of
entries
NOTE:
numEntries
must be redetermined each time this method is evaluated.
Repeat, while
index
is less than
numEntries
Let
be the
Record
{ [[Key]], [[Value]] } that is the value of
entries
index
].
Increase
index
by 1.
Set
.[[MapNextIndex]] to
index
If
.[[Key]] is not
empty
, then
If
itemKind
is
"key"
, let
result
be
.[[Key]].
Else if
itemKind
is
"value"
, let
result
be
.[[Value]].
Else,
Assert
itemKind
is
"key+value"
Let
result
be
CreateArrayFromList
(«
.[[Key]],
.[[Value]] »).
Return
CreateIterResultObject
result
false
).
Set
.[[Map]] to
undefined
Return
CreateIterResultObject
undefined
true
).
23.1.5.2.2
%MapIteratorPrototype% [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Map Iterator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.1.5.3
Properties of Map Iterator Instances
Map Iterator instances are ordinary objects that inherit properties from the
%MapIteratorPrototype%
intrinsic object. Map Iterator instances are initially created with the internal slots described in
Table 60
Table 60: Internal Slots of Map Iterator Instances
Internal Slot
Description
[[Map]]
The Map object that is being iterated.
[[MapNextIndex]]
The
integer index
of the next Map data element to be examined by this iterator.
[[MapIterationKind]]
A String value that identifies what is to be returned
for each element of the iteration. The possible values are:
"key"
"value"
"key+value"
23.2
Set Objects
Set objects are collections of ECMAScript language values. A
distinct value may only occur once as an element of a Set's collection.
Distinct values are discriminated using the
SameValueZero
comparison algorithm.
Set objects must be implemented using either hash tables or other
mechanisms that, on average, provide access times that are sublinear on
the number of elements in the collection. The data structures used in
this Set objects specification is only intended to describe the required
observable semantics of Set objects. It is not intended to be a viable
implementation model.
23.2.1
The Set Constructor
The Set
constructor
is the intrinsic object
%Set%
is the initial value of the
Set
property of the
global object
creates and initializes a new Set object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Set
behaviour must include a
super
call to the
Set
constructor
to create and initialize the subclass instance with the internal state necessary to support the
Set.prototype
built-in methods.
23.2.1.1
Set ( [
iterable
] )
When the
Set
function is called with optional argument
iterable
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
set
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%SetPrototype%"
, « [[SetData]] »).
Set
set
.[[SetData]] to a new empty
List
If
iterable
is not present, set
iterable
to
undefined
If
iterable
is either
undefined
or
null
, return
set
Let
adder
be ?
Get
set
"add"
).
If
IsCallable
adder
) is
false
, throw a
TypeError
exception.
Let
iteratorRecord
be ?
GetIterator
iterable
).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
set
Let
nextValue
be ?
IteratorValue
next
).
Let
status
be
Call
adder
set
, «
nextValue
»).
If
status
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
status
).
23.2.2
Properties of the Set Constructor
The Set
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
23.2.2.1
Set.prototype
The initial value of
Set.prototype
is the intrinsic
%SetPrototype%
object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
23.2.2.2
get Set [ @@species ]
Set[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
Methods that create derived collection objects should call @@species to determine the
constructor
to use to create the derived objects. Subclass
constructor
may over-ride @@species to change the default
constructor
assignment.
23.2.3
Properties of the Set Prototype Object
The Set prototype object:
is the intrinsic object
%SetPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[SetData]] internal slot.
23.2.3.1
Set.prototype.add (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[SetData]].
For each
that is an element of
entries
, do
If
is not
empty
and
SameValueZero
value
) is
true
, then
Return
If
value
is
-0
, set
value
to
+0
Append
value
as the last element of
entries
Return
23.2.3.2
Set.prototype.clear ( )
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[SetData]].
For each
that is an element of
entries
, do
Replace the element of
entries
whose value is
with an element whose value is
empty
Return
undefined
Note
The existing [[SetData]]
List
is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that
List
23.2.3.3
Set.prototype.constructor
The initial value of
Set.prototype.constructor
is the intrinsic object
%Set%
23.2.3.4
Set.prototype.delete (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[SetData]].
For each
that is an element of
entries
, do
If
is not
empty
and
SameValueZero
value
) is
true
, then
Replace the element of
entries
whose value is
with an element whose value is
empty
Return
true
Return
false
Note
The value
empty
is used as a
specification device to indicate that an entry has been deleted. Actual
implementations may take other actions such as physically removing the
entry from internal data structures.
23.2.3.5
Set.prototype.entries ( )
The following steps are taken:
Let
be the
this
value.
Return ?
CreateSetIterator
"key+value"
).
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.
23.2.3.6
Set.prototype.forEach (
callbackfn
[ ,
thisArg
] )
When the
forEach
method is called with one or two arguments, the following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
If
IsCallable
callbackfn
) is
false
, throw a
TypeError
exception.
If
thisArg
is present, let
be
thisArg
; else let
be
undefined
Let
entries
be the
List
that is
.[[SetData]].
For each
that is an element of
entries
, in original insertion order, do
If
is not
empty
, then
Perform ?
Call
callbackfn
, «
»).
Return
undefined
Note
callbackfn
should be a function that accepts three arguments.
forEach
calls
callbackfn
once for each value present in the set object, in value insertion order.
callbackfn
is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a
thisArg
parameter is provided, it will be used as the
this
value for each invocation of
callbackfn
. If it is not provided,
undefined
is used instead.
callbackfn
is called with three arguments: the
first two arguments are a value contained in the Set. The same value is
passed for both arguments. The Set object being traversed is passed as
the third argument.
The
callbackfn
is called with three arguments to be consistent with the call back functions used by
forEach
methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach
does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn
Each value is normally visited only once. However, a value
will be revisited if it is deleted after it has been visited and then
re-added before the
forEach
call completes. Values that are deleted after the call to
forEach
begins and before being visited are not visited unless the value is added again before the
forEach
call completes. New values added after the call to
forEach
begins are visited.
23.2.3.7
Set.prototype.has (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[SetData]].
For each
that is an element of
entries
, do
If
is not
empty
and
SameValueZero
value
) is
true
, return
true
Return
false
23.2.3.8
Set.prototype.keys ( )
The initial value of the
keys
property is the same
function object
as the initial value of the
values
property.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.
23.2.3.9
get Set.prototype.size
Set.prototype.size
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[SetData]].
Let
count
be 0.
For each
that is an element of
entries
, do
If
is not
empty
, increase
count
by 1.
Return
count
23.2.3.10
Set.prototype.values ( )
The following steps are taken:
Let
be the
this
value.
Return ?
CreateSetIterator
"value"
).
23.2.3.11
Set.prototype [ @@iterator ] ( )
The initial value of the @@iterator property is the same
function object
as the initial value of the
values
property.
23.2.3.12
Set.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Set"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.2.4
Properties of Set Instances
Set instances are ordinary objects that inherit properties from
the Set prototype. Set instances also have a [[SetData]] internal slot.
23.2.5
Set Iterator Objects
A Set Iterator is an ordinary object, with the structure
defined below, that represents a specific iteration over some specific
Set instance object. There is not a named
constructor
for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.
23.2.5.1
CreateSetIterator (
set
kind
Several methods of Set objects return Iterator objects. The abstract operation CreateSetIterator with arguments
set
and
kind
is used to create such iterator objects. It performs the following steps:
If
Type
set
) is not Object, throw a
TypeError
exception.
If
set
does not have a [[SetData]] internal slot, throw a
TypeError
exception.
Let
iterator
be
ObjectCreate
%SetIteratorPrototype%
, « [[IteratedSet]], [[SetNextIndex]], [[SetIterationKind]] »).
Set
iterator
.[[IteratedSet]] to
set
Set
iterator
.[[SetNextIndex]] to 0.
Set
iterator
.[[SetIterationKind]] to
kind
Return
iterator
23.2.5.2
The %SetIteratorPrototype% Object
The
%SetIteratorPrototype%
object:
has properties that are inherited by all Set Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%IteratorPrototype%
has the following properties:
23.2.5.2.1
%SetIteratorPrototype%.next ( )
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have all of the internal slots of a Set Iterator Instance (
23.2.5.3
), throw a
TypeError
exception.
Let
be
.[[IteratedSet]].
Let
index
be
.[[SetNextIndex]].
Let
itemKind
be
.[[SetIterationKind]].
If
is
undefined
, return
CreateIterResultObject
undefined
true
).
Assert
has a [[SetData]] internal slot.
Let
entries
be the
List
that is
.[[SetData]].
Let
numEntries
be the number of elements of
entries
NOTE:
numEntries
must be redetermined each time this method is evaluated.
Repeat, while
index
is less than
numEntries
Let
be
entries
index
].
Increase
index
by 1.
Set
.[[SetNextIndex]] to
index
If
is not
empty
, then
If
itemKind
is
"key+value"
, then
Return
CreateIterResultObject
CreateArrayFromList
(«
»),
false
).
Return
CreateIterResultObject
false
).
Set
.[[IteratedSet]] to
undefined
Return
CreateIterResultObject
undefined
true
).
23.2.5.2.2
%SetIteratorPrototype% [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Set Iterator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.2.5.3
Properties of Set Iterator Instances
Set Iterator instances are ordinary objects that inherit properties from the
%SetIteratorPrototype%
intrinsic object. Set Iterator instances are initially created with the internal slots specified in
Table 61
Table 61: Internal Slots of Set Iterator Instances
Internal Slot
Description
[[IteratedSet]]
The Set object that is being iterated.
[[SetNextIndex]]
The
integer index
of the next Set data element to be examined by this iterator
[[SetIterationKind]]
A String value that identifies what is to be returned
for each element of the iteration. The possible values are:
"key"
"value"
"key+value"
"key"
and
"value"
have the same meaning.
23.3
WeakMap Objects
WeakMap objects are collections of key/value pairs where the keys
are objects and values may be arbitrary ECMAScript language values. A
WeakMap may be queried to see if it contains a key/value pair with a
specific key, but no mechanism is provided for enumerating the objects
it holds as keys. If an object that is being used as the key of a
WeakMap key/value pair is only reachable by following a chain of
references that start within that WeakMap, then that key/value pair is
inaccessible and is automatically removed from the WeakMap. WeakMap
implementations must detect and remove such key/value pairs and any
associated resources.
An implementation may impose an arbitrarily determined latency
between the time a key/value pair of a WeakMap becomes inaccessible and
the time when the key/value pair is removed from the WeakMap. If this
latency was observable to ECMAScript program, it would be a source of
indeterminacy that could impact program execution. For that reason, an
ECMAScript implementation must not provide any means to observe a key of
a WeakMap that does not require the observer to present the observed
key.
WeakMap objects must be implemented using either hash tables or
other mechanisms that, on average, provide access times that are
sublinear on the number of key/value pairs in the collection. The data
structure used in this WeakMap objects specification are only intended
to describe the required observable semantics of WeakMap objects. It is
not intended to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for
dynamically associating state with an object in a manner that does not
“leak” memory resources if, in the absence of the WeakMap or WeakSet,
the object otherwise became inaccessible and subject to resource
reclamation by the implementation's garbage collection mechanisms. This
characteristic can be achieved by using an inverted per-object mapping
of weak map instances to keys. Alternatively each weak map may
internally store its key to value mappings but this approach requires
coordination between the WeakMap or WeakSet implementation and the
garbage collector. The following references describe mechanism that may
be useful to implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In
Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97)
, A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183,
Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in
Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no.
21, pp. 3481-3497, 2008,
23.3.1
The WeakMap Constructor
The WeakMap
constructor
is the intrinsic object
%WeakMap%
is the initial value of the
WeakMap
property of the
global object
creates and initializes a new WeakMap object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
WeakMap
behaviour must include a
super
call to the
WeakMap
constructor
to create and initialize the subclass instance with the internal state necessary to support the
WeakMap.prototype
built-in methods.
23.3.1.1
WeakMap ( [
iterable
] )
When the
WeakMap
function is called with optional argument
iterable
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
map
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%WeakMapPrototype%"
, « [[WeakMapData]] »).
Set
map
.[[WeakMapData]] to a new empty
List
If
iterable
is not present, or is either
undefined
or
null
, return
map
Let
adder
be ?
Get
map
"set"
).
Return ?
AddEntriesFromIterable
map
iterable
adder
).
Note
If the parameter
iterable
is present, it is
expected to be an object that implements an @@iterator method that
returns an iterator object that produces a two element array-like object
whose first element is a value that will be used as a WeakMap key and
whose second element is the value to associate with that key.
23.3.2
Properties of the WeakMap Constructor
The WeakMap
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
23.3.2.1
WeakMap.prototype
The initial value of
WeakMap.prototype
is the intrinsic object
%WeakMapPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
23.3.3
Properties of the WeakMap Prototype Object
The WeakMap prototype object:
is the intrinsic object
%WeakMapPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[WeakMapData]] internal slot.
23.3.3.1
WeakMap.prototype.constructor
The initial value of
WeakMap.prototype.constructor
is the intrinsic object
%WeakMap%
23.3.3.2
WeakMap.prototype.delete (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakMapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakMapData]].
If
Type
key
) is not Object, return
false
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValue
.[[Key]],
key
) is
true
, then
Set
.[[Key]] to
empty
Set
.[[Value]] to
empty
Return
true
Return
false
Note
The value
empty
is used as a
specification device to indicate that an entry has been deleted. Actual
implementations may take other actions such as physically removing the
entry from internal data structures.
23.3.3.3
WeakMap.prototype.get (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakMapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakMapData]].
If
Type
key
) is not Object, return
undefined
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValue
.[[Key]],
key
) is
true
, return
.[[Value]].
Return
undefined
23.3.3.4
WeakMap.prototype.has (
key
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakMapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakMapData]].
If
Type
key
) is not Object, return
false
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValue
.[[Key]],
key
) is
true
, return
true
Return
false
23.3.3.5
WeakMap.prototype.set (
key
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakMapData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakMapData]].
If
Type
key
) is not Object, throw a
TypeError
exception.
For each
Record
{ [[Key]], [[Value]] }
that is an element of
entries
, do
If
.[[Key]] is not
empty
and
SameValue
.[[Key]],
key
) is
true
, then
Set
.[[Value]] to
value
Return
Let
be the
Record
{ [[Key]]:
key
, [[Value]]:
value
}.
Append
as the last element of
entries
Return
23.3.3.6
WeakMap.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"WeakMap"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.3.4
Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties
from the WeakMap prototype. WeakMap instances also have a
[[WeakMapData]] internal slot.
23.4
WeakSet Objects
WeakSet objects are collections of objects. A distinct object may
only occur once as an element of a WeakSet's collection. A WeakSet may
be queried to see if it contains a specific object, but no mechanism is
provided for enumerating the objects it holds. If an object that is
contained by a WeakSet is only reachable by following a chain of
references that start within that WeakSet, then that object is
inaccessible and is automatically removed from the WeakSet. WeakSet
implementations must detect and remove such objects and any associated
resources.
An implementation may impose an arbitrarily determined latency
between the time an object contained in a WeakSet becomes inaccessible
and the time when the object is removed from the WeakSet. If this
latency was observable to ECMAScript program, it would be a source of
indeterminacy that could impact program execution. For that reason, an
ECMAScript implementation must not provide any means to determine if a
WeakSet contains a particular object that does not require the observer
to present the observed object.
WeakSet objects must be implemented using either hash tables or
other mechanisms that, on average, provide access times that are
sublinear on the number of elements in the collection. The data
structure used in this WeakSet objects specification is only intended to
describe the required observable semantics of WeakSet objects. It is
not intended to be a viable implementation model.
Note
See the NOTE in
23.3
23.4.1
The WeakSet Constructor
The WeakSet
constructor
is the intrinsic object
%WeakSet%
is the initial value of the
WeakSet
property of the
global object
creates and initializes a new WeakSet object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
WeakSet
behaviour must include a
super
call to the
WeakSet
constructor
to create and initialize the subclass instance with the internal state necessary to support the
WeakSet.prototype
built-in methods.
23.4.1.1
WeakSet ( [
iterable
] )
When the
WeakSet
function is called with optional argument
iterable
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
set
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%WeakSetPrototype%"
, « [[WeakSetData]] »).
Set
set
.[[WeakSetData]] to a new empty
List
If
iterable
is not present, set
iterable
to
undefined
If
iterable
is either
undefined
or
null
, return
set
Let
adder
be ?
Get
set
"add"
).
If
IsCallable
adder
) is
false
, throw a
TypeError
exception.
Let
iteratorRecord
be ?
GetIterator
iterable
).
Repeat,
Let
next
be ?
IteratorStep
iteratorRecord
).
If
next
is
false
, return
set
Let
nextValue
be ?
IteratorValue
next
).
Let
status
be
Call
adder
set
, «
nextValue
»).
If
status
is an
abrupt completion
, return ?
IteratorClose
iteratorRecord
status
).
23.4.2
Properties of the WeakSet Constructor
The WeakSet
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
23.4.2.1
WeakSet.prototype
The initial value of
WeakSet.prototype
is the intrinsic
%WeakSetPrototype%
object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
23.4.3
Properties of the WeakSet Prototype Object
The WeakSet prototype object:
is the intrinsic object
%WeakSetPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[WeakSetData]] internal slot.
23.4.3.1
WeakSet.prototype.add (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakSetData]] internal slot, throw a
TypeError
exception.
If
Type
value
) is not Object, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakSetData]].
For each
that is an element of
entries
, do
If
is not
empty
and
SameValue
value
) is
true
, then
Return
Append
value
as the last element of
entries
Return
23.4.3.2
WeakSet.prototype.constructor
The initial value of
WeakSet.prototype.constructor
is the
%WeakSet%
intrinsic object.
23.4.3.3
WeakSet.prototype.delete (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakSetData]] internal slot, throw a
TypeError
exception.
If
Type
value
) is not Object, return
false
Let
entries
be the
List
that is
.[[WeakSetData]].
For each
that is an element of
entries
, do
If
is not
empty
and
SameValue
value
) is
true
, then
Replace the element of
entries
whose value is
with an element whose value is
empty
Return
true
Return
false
Note
The value
empty
is used as a
specification device to indicate that an entry has been deleted. Actual
implementations may take other actions such as physically removing the
entry from internal data structures.
23.4.3.4
WeakSet.prototype.has (
value
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[WeakSetData]] internal slot, throw a
TypeError
exception.
Let
entries
be the
List
that is
.[[WeakSetData]].
If
Type
value
) is not Object, return
false
For each
that is an element of
entries
, do
If
is not
empty
and
SameValue
value
) is
true
, return
true
Return
false
23.4.3.5
WeakSet.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"WeakSet"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
23.4.4
Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties
from the WeakSet prototype. WeakSet instances also have a
[[WeakSetData]] internal slot.
24
Structured Data
24.1
ArrayBuffer Objects
24.1.1
Abstract Operations For ArrayBuffer Objects
24.1.1.1
AllocateArrayBuffer (
constructor
byteLength
The abstract operation AllocateArrayBuffer with arguments
constructor
and
byteLength
is used to create an ArrayBuffer object. It performs the following steps:
Let
obj
be ?
OrdinaryCreateFromConstructor
constructor
"%ArrayBufferPrototype%"
, « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).
Assert
byteLength
is an integer value ≥ 0.
Let
block
be ?
CreateByteDataBlock
byteLength
).
Set
obj
.[[ArrayBufferData]] to
block
Set
obj
.[[ArrayBufferByteLength]] to
byteLength
Return
obj
24.1.1.2
IsDetachedBuffer (
arrayBuffer
The abstract operation IsDetachedBuffer with argument
arrayBuffer
performs the following steps:
Assert
Type
arrayBuffer
) is Object and it has an [[ArrayBufferData]] internal slot.
If
arrayBuffer
.[[ArrayBufferData]] is
null
, return
true
Return
false
24.1.1.3
DetachArrayBuffer (
arrayBuffer
[ ,
key
] )
The abstract operation DetachArrayBuffer with argument
arrayBuffer
and optional argument
key
performs the following steps:
Assert
Type
arrayBuffer
) is Object and it has [[ArrayBufferData]], [[ArrayBufferByteLength]], and [[ArrayBufferDetachKey]] internal slots.
Assert
IsSharedArrayBuffer
arrayBuffer
) is
false
If
key
is not present, set
key
to
undefined
If
SameValue
arrayBuffer
.[[ArrayBufferDetachKey]],
key
) is
false
, throw a
TypeError
exception.
Set
arrayBuffer
.[[ArrayBufferData]] to
null
Set
arrayBuffer
.[[ArrayBufferByteLength]] to 0.
Return
NormalCompletion
null
).
Note
Detaching an ArrayBuffer instance disassociates the
Data Block
used as its backing store from the instance and sets the byte length of
the buffer to 0. No operations defined by this specification use the
DetachArrayBuffer abstract operation. However, an ECMAScript
implementation or host environment may define such operations.
24.1.1.4
CloneArrayBuffer (
srcBuffer
srcByteOffset
srcLength
cloneConstructor
The abstract operation CloneArrayBuffer takes four parameters, an ArrayBuffer
srcBuffer
, an integer offset
srcByteOffset
, an integer length
srcLength
, and a
constructor
function
cloneConstructor
. It creates a new ArrayBuffer whose data is a copy of
srcBuffer
's data over the range starting at
srcByteOffset
and continuing for
srcLength
bytes. This operation performs the following steps:
Assert
Type
srcBuffer
) is Object and it has an [[ArrayBufferData]] internal slot.
Assert
IsConstructor
cloneConstructor
) is
true
Let
targetBuffer
be ?
AllocateArrayBuffer
cloneConstructor
srcLength
).
If
IsDetachedBuffer
srcBuffer
) is
true
, throw a
TypeError
exception.
Let
srcBlock
be
srcBuffer
.[[ArrayBufferData]].
Let
targetBlock
be
targetBuffer
.[[ArrayBufferData]].
Perform
CopyDataBlockBytes
targetBlock
, 0,
srcBlock
srcByteOffset
srcLength
).
Return
targetBuffer
24.1.1.5
RawBytesToNumber (
type
rawBytes
isLittleEndian
The abstract operation RawBytesToNumber takes three parameters, a String
type
, a
List
rawBytes
, and a Boolean
isLittleEndian
. This operation performs the following steps:
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
isLittleEndian
is
false
, reverse the order of the elements of
rawBytes
If
type
is
"Float32"
, then
Let
value
be the byte elements of
rawBytes
concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary32 value.
If
value
is an IEEE 754-2008 binary32 NaN value, return the
NaN
Number value.
Return the Number value that corresponds to
value
If
type
is
"Float64"
, then
Let
value
be the byte elements of
rawBytes
concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary64 value.
If
value
is an IEEE 754-2008 binary64 NaN value, return the
NaN
Number value.
Return the Number value that corresponds to
value
If the first code unit of
type
is the code unit 0x0055 (LATIN CAPITAL LETTER U), then
Let
intValue
be the byte elements of
rawBytes
concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let
intValue
be the byte elements of
rawBytes
concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number of bit length
elementSize
× 8.
Return the Number value that corresponds to
intValue
24.1.1.6
GetValueFromBuffer (
arrayBuffer
byteIndex
type
isTypedArray
order
[ ,
isLittleEndian
] )
The abstract operation GetValueFromBuffer takes six parameters, an ArrayBuffer or SharedArrayBuffer
arrayBuffer
, an integer
byteIndex
, a String
type
, a Boolean
isTypedArray
, a String
order
, and optionally a Boolean
isLittleEndian
. This operation performs the following steps:
Assert
IsDetachedBuffer
arrayBuffer
) is
false
Assert
: There are sufficient bytes in
arrayBuffer
starting at
byteIndex
to represent a value of
type
Assert
byteIndex
is an integer value ≥ 0.
Let
block
be
arrayBuffer
.[[ArrayBufferData]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
IsSharedArrayBuffer
arrayBuffer
) is
true
, then
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
If
isTypedArray
is
true
and
type
is
"Int8"
"Uint8"
"Int16"
"Uint16"
"Int32"
, or
"Uint32"
, let
noTear
be
true
; otherwise let
noTear
be
false
Let
rawValue
be a
List
of length
elementSize
of nondeterministically chosen byte values.
NOTE: In implementations,
rawValue
is the result of a non-atomic or atomic read instruction on the
underlying hardware. The nondeterminism is a semantic prescription of
the
memory model
to describe observable behaviour of hardware with weak consistency.
Let
readEvent
be
ReadSharedMemory
{ [[Order]]:
order
, [[NoTear]]:
noTear
, [[Block]]:
block
, [[ByteIndex]]:
byteIndex
, [[ElementSize]]:
elementSize
}.
Append
readEvent
to
eventList
Append
Chosen Value Record
{ [[Event]]:
readEvent
, [[ChosenValue]]:
rawValue
} to
execution
.[[ChosenValues]].
Else, let
rawValue
be a
List
of
elementSize
containing, in order, the
elementSize
sequence of bytes starting with
block
byteIndex
].
If
isLittleEndian
is not present, set
isLittleEndian
to the value of the [[LittleEndian]] field of the
surrounding agent
's
Agent Record
Return
RawBytesToNumber
type
rawValue
isLittleEndian
).
24.1.1.7
NumberToRawBytes (
type
value
isLittleEndian
The abstract operation NumberToRawBytes takes three parameters, a String
type
, a Number
value
, and a Boolean
isLittleEndian
. This operation performs the following steps:
If
type
is
"Float32"
, then
Let
rawBytes
be a
List
containing the 4 bytes that are the result of converting
value
to IEEE 754-2008 binary32 format using “Round to nearest, ties to even” rounding mode. If
isLittleEndian
is
false
, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If
value
is
NaN
rawBytes
may be set to any implementation chosen IEEE 754-2008 binary32 format
Not-a-Number encoding. An implementation must always choose the same
encoding for each implementation distinguishable
NaN
value.
Else if
type
is
"Float64"
, then
Let
rawBytes
be a
List
containing the 8 bytes that are the IEEE 754-2008 binary64 format encoding of
value
. If
isLittleEndian
is
false
, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If
value
is
NaN
rawBytes
may be set to any implementation chosen IEEE 754-2008 binary64 format
Not-a-Number encoding. An implementation must always choose the same
encoding for each implementation distinguishable
NaN
value.
Else,
Let
be the Number value of the Element Size specified in
Table 59
for Element Type
type
Let
convOp
be the abstract operation named in the Conversion Operation column in
Table 59
for Element Type
type
Let
intValue
be
convOp
value
).
If
intValue
≥ 0, then
Let
rawBytes
be a
List
containing the
-byte binary encoding of
intValue
. If
isLittleEndian
is
false
, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Else,
Let
rawBytes
be a
List
containing the
-byte binary 2's complement encoding of
intValue
. If
isLittleEndian
is
false
, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Return
rawBytes
24.1.1.8
SetValueInBuffer (
arrayBuffer
byteIndex
type
value
isTypedArray
order
[ ,
isLittleEndian
] )
The abstract operation SetValueInBuffer takes seven parameters, an ArrayBuffer or SharedArrayBuffer
arrayBuffer
, an integer
byteIndex
, a String
type
, a Number
value
, a Boolean
isTypedArray
, a String
order
, and optionally a Boolean
isLittleEndian
. This operation performs the following steps:
Assert
IsDetachedBuffer
arrayBuffer
) is
false
Assert
: There are sufficient bytes in
arrayBuffer
starting at
byteIndex
to represent a value of
type
Assert
byteIndex
is an integer value ≥ 0.
Assert
Type
value
) is Number.
Let
block
be
arrayBuffer
.[[ArrayBufferData]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
isLittleEndian
is not present, set
isLittleEndian
to the value of the [[LittleEndian]] field of the
surrounding agent
's
Agent Record
Let
rawBytes
be
NumberToRawBytes
type
value
isLittleEndian
).
If
IsSharedArrayBuffer
arrayBuffer
) is
true
, then
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
If
isTypedArray
is
true
and
type
is
"Int8"
"Uint8"
"Int16"
"Uint16"
"Int32"
, or
"Uint32"
, let
noTear
be
true
; otherwise let
noTear
be
false
Append
WriteSharedMemory
{ [[Order]]:
order
, [[NoTear]]:
noTear
, [[Block]]:
block
, [[ByteIndex]]:
byteIndex
, [[ElementSize]]:
elementSize
, [[Payload]]:
rawBytes
} to
eventList
Else, store the individual bytes of
rawBytes
into
block
, in order, starting at
block
byteIndex
].
Return
NormalCompletion
undefined
).
24.1.1.9
GetModifySetValueInBuffer (
arrayBuffer
byteIndex
type
value
op
[ ,
isLittleEndian
] )
The abstract operation GetModifySetValueInBuffer takes six parameters, a SharedArrayBuffer
arrayBuffer
, a nonnegative integer
byteIndex
, a String
type
, a Number
value
, a semantic function
op
, and optionally a Boolean
isLittleEndian
. This operation performs the following steps:
Assert
IsSharedArrayBuffer
arrayBuffer
) is
true
Assert
: There are sufficient bytes in
arrayBuffer
starting at
byteIndex
to represent a value of
type
Assert
byteIndex
is an integer value ≥ 0.
Assert
Type
value
) is Number.
Let
block
be
arrayBuffer
.[[ArrayBufferData]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
isLittleEndian
is not present, set
isLittleEndian
to the value of the [[LittleEndian]] field of the
surrounding agent
's
Agent Record
Let
rawBytes
be
NumberToRawBytes
type
value
isLittleEndian
).
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
Let
rawBytesRead
be a
List
of length
elementSize
of nondeterministically chosen byte values.
NOTE: In implementations,
rawBytesRead
is the result of a load-link, of a load-exclusive, or of an operand of a
read-modify-write instruction on the underlying hardware. The
nondeterminism is a semantic prescription of the
memory model
to describe observable behaviour of hardware with weak consistency.
Let
rmwEvent
be
ReadModifyWriteSharedMemory
{ [[Order]]:
"SeqCst"
, [[NoTear]]:
true
, [[Block]]:
block
, [[ByteIndex]]:
byteIndex
, [[ElementSize]]:
elementSize
, [[Payload]]:
rawBytes
, [[ModifyOp]]:
op
}.
Append
rmwEvent
to
eventList
Append
Chosen Value Record
{ [[Event]]:
rmwEvent
, [[ChosenValue]]:
rawBytesRead
} to
execution
.[[ChosenValues]].
Return
RawBytesToNumber
type
rawBytesRead
isLittleEndian
).
24.1.2
The ArrayBuffer Constructor
The ArrayBuffer
constructor
is the intrinsic object
%ArrayBuffer%
is the initial value of the
ArrayBuffer
property of the
global object
creates and initializes a new ArrayBuffer object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
ArrayBuffer
behaviour must include a
super
call to the
ArrayBuffer
constructor
to create and initialize subclass instances with the internal state necessary to support the
ArrayBuffer.prototype
built-in methods.
24.1.2.1
ArrayBuffer (
length
When the
ArrayBuffer
function is called with argument
length
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
byteLength
be ?
ToIndex
length
).
Return ?
AllocateArrayBuffer
(NewTarget,
byteLength
).
24.1.3
Properties of the ArrayBuffer Constructor
The ArrayBuffer
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
24.1.3.1
ArrayBuffer.isView (
arg
The
isView
function takes one argument
arg
, and performs the following steps:
If
Type
arg
) is not Object, return
false
If
arg
has a [[ViewedArrayBuffer]] internal slot, return
true
Return
false
24.1.3.2
ArrayBuffer.prototype
The initial value of
ArrayBuffer.prototype
is the intrinsic object
%ArrayBufferPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
24.1.3.3
get ArrayBuffer [ @@species ]
ArrayBuffer[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
ArrayBuffer prototype methods normally use their
this
object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its @@species property.
24.1.4
Properties of the ArrayBuffer Prototype Object
The ArrayBuffer prototype object:
is the intrinsic object
%ArrayBufferPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
24.1.4.1
get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
) is
true
, throw a
TypeError
exception.
If
IsDetachedBuffer
) is
true
, throw a
TypeError
exception.
Let
length
be
.[[ArrayBufferByteLength]].
Return
length
24.1.4.2
ArrayBuffer.prototype.constructor
The initial value of
ArrayBuffer.prototype.constructor
is the intrinsic object
%ArrayBuffer%
24.1.4.3
ArrayBuffer.prototype.slice (
start
end
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
) is
true
, throw a
TypeError
exception.
If
IsDetachedBuffer
) is
true
, throw a
TypeError
exception.
Let
len
be
.[[ArrayBufferByteLength]].
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
first
be
max
((
len
relativeStart
), 0); else let
first
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
newLen
be
max
final
first
, 0).
Let
ctor
be ?
SpeciesConstructor
%ArrayBuffer%
).
Let
new
be ?
Construct
ctor
, «
newLen
»).
If
new
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
new
) is
true
, throw a
TypeError
exception.
If
IsDetachedBuffer
new
) is
true
, throw a
TypeError
exception.
If
SameValue
new
) is
true
, throw a
TypeError
exception.
If
new
.[[ArrayBufferByteLength]] <
newLen
, throw a
TypeError
exception.
NOTE: Side-effects of the above steps may have detached
If
IsDetachedBuffer
) is
true
, throw a
TypeError
exception.
Let
fromBuf
be
.[[ArrayBufferData]].
Let
toBuf
be
new
.[[ArrayBufferData]].
Perform
CopyDataBlockBytes
toBuf
, 0,
fromBuf
first
newLen
).
Return
new
24.1.4.4
ArrayBuffer.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"ArrayBuffer"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
24.1.5
Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the ArrayBuffer
prototype object. ArrayBuffer instances each have an [[ArrayBufferData]]
internal slot, an [[ArrayBufferByteLength]] internal slot, and an
[[ArrayBufferDetachKey]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is
null
are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than
undefined
need to have all
DetachArrayBuffer
calls passing that same "detach key" as an argument, otherwise a
TypeError will result. This internal slot is only ever set by certain
embedding environments, not by algorithms in this specification.
24.2
SharedArrayBuffer Objects
24.2.1
Abstract Operations for SharedArrayBuffer Objects
24.2.1.1
AllocateSharedArrayBuffer (
constructor
byteLength
The abstract operation AllocateSharedArrayBuffer with arguments
constructor
and
byteLength
is used to create a SharedArrayBuffer object. It performs the following steps:
Let
obj
be ?
OrdinaryCreateFromConstructor
constructor
"%SharedArrayBufferPrototype%"
, « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
Assert
byteLength
is a nonnegative integer.
Let
block
be ?
CreateSharedByteDataBlock
byteLength
).
Set
obj
.[[ArrayBufferData]] to
block
Set
obj
.[[ArrayBufferByteLength]] to
byteLength
Return
obj
24.2.1.2
IsSharedArrayBuffer (
obj
IsSharedArrayBuffer tests whether an object is an
ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs
the following steps:
Assert
Type
obj
) is Object and it has an [[ArrayBufferData]] internal slot.
Let
bufferData
be
obj
.[[ArrayBufferData]].
If
bufferData
is
null
, return
false
If
bufferData
is a
Data Block
, return
false
Assert
bufferData
is a
Shared Data Block
Return
true
24.2.2
The SharedArrayBuffer Constructor
The SharedArrayBuffer
constructor
is the intrinsic object
%SharedArrayBuffer%
is the initial value of the
SharedArrayBuffer
property of the
global object
creates and initializes a new SharedArrayBuffer object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
SharedArrayBuffer
behaviour must include a
super
call to the
SharedArrayBuffer
constructor
to create and initialize subclass instances with the internal state necessary to support the
SharedArrayBuffer.prototype
built-in methods.
Note
Unlike an
ArrayBuffer
, a
SharedArrayBuffer
cannot become detached, and its internal [[ArrayBufferData]] slot is never
null
24.2.2.1
SharedArrayBuffer ( [
length
] )
When the
SharedArrayBuffer
function is called with optional argument
length
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
Let
byteLength
be ?
ToIndex
length
).
Return ?
AllocateSharedArrayBuffer
(NewTarget,
byteLength
).
24.2.3
Properties of the SharedArrayBuffer Constructor
The SharedArrayBuffer
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
24.2.3.1
SharedArrayBuffer.prototype
The initial value of
SharedArrayBuffer.prototype
is the intrinsic object
%SharedArrayBufferPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
24.2.3.2
get SharedArrayBuffer [ @@species ]
SharedArrayBuffer[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
24.2.4
Properties of the SharedArrayBuffer Prototype Object
The SharedArrayBuffer prototype object:
is the intrinsic object
%SharedArrayBufferPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
24.2.4.1
get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
) is
false
, throw a
TypeError
exception.
Let
length
be
.[[ArrayBufferByteLength]].
Return
length
24.2.4.2
SharedArrayBuffer.prototype.constructor
The initial value of
SharedArrayBuffer.prototype.constructor
is the intrinsic object
%SharedArrayBuffer%
24.2.4.3
SharedArrayBuffer.prototype.slice (
start
end
The following steps are taken:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
) is
false
, throw a
TypeError
exception.
Let
len
be
.[[ArrayBufferByteLength]].
Let
relativeStart
be ?
ToInteger
start
).
If
relativeStart
< 0, let
first
be
max
((
len
relativeStart
), 0); else let
first
be
min
relativeStart
len
).
If
end
is
undefined
, let
relativeEnd
be
len
; else let
relativeEnd
be ?
ToInteger
end
).
If
relativeEnd
< 0, let
final
be
max
((
len
relativeEnd
), 0); else let
final
be
min
relativeEnd
len
).
Let
newLen
be
max
final
first
, 0).
Let
ctor
be ?
SpeciesConstructor
%SharedArrayBuffer%
).
Let
new
be ?
Construct
ctor
, «
newLen
»).
If
new
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
If
IsSharedArrayBuffer
new
) is
false
, throw a
TypeError
exception.
If
new
.[[ArrayBufferData]] and
.[[ArrayBufferData]] are the same
Shared Data Block
values, throw a
TypeError
exception.
If
new
.[[ArrayBufferByteLength]] <
newLen
, throw a
TypeError
exception.
Let
fromBuf
be
.[[ArrayBufferData]].
Let
toBuf
be
new
.[[ArrayBufferData]].
Perform
CopyDataBlockBytes
toBuf
, 0,
fromBuf
first
newLen
).
Return
new
24.2.4.4
SharedArrayBuffer.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"SharedArrayBuffer"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
24.2.5
Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the
SharedArrayBuffer prototype object. SharedArrayBuffer instances each
have an [[ArrayBufferData]] internal slot and an
[[ArrayBufferByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
24.3
DataView Objects
24.3.1
Abstract Operations For DataView Objects
24.3.1.1
GetViewValue (
view
requestIndex
isLittleEndian
type
The abstract operation GetViewValue with arguments
view
requestIndex
isLittleEndian
, and
type
is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps:
If
Type
view
) is not Object, throw a
TypeError
exception.
If
view
does not have a [[DataView]] internal slot, throw a
TypeError
exception.
Assert
view
has a [[ViewedArrayBuffer]] internal slot.
Let
getIndex
be ?
ToIndex
requestIndex
).
Set
isLittleEndian
to
ToBoolean
isLittleEndian
).
Let
buffer
be
view
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
viewOffset
be
view
.[[ByteOffset]].
Let
viewSize
be
view
.[[ByteLength]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
getIndex
elementSize
viewSize
, throw a
RangeError
exception.
Let
bufferIndex
be
getIndex
viewOffset
Return
GetValueFromBuffer
buffer
bufferIndex
type
false
"Unordered"
isLittleEndian
).
24.3.1.2
SetViewValue (
view
requestIndex
isLittleEndian
type
value
The abstract operation SetViewValue with arguments
view
requestIndex
isLittleEndian
type
, and
value
is used by functions on DataView instances to store values into the view's buffer. It performs the following steps:
If
Type
view
) is not Object, throw a
TypeError
exception.
If
view
does not have a [[DataView]] internal slot, throw a
TypeError
exception.
Assert
view
has a [[ViewedArrayBuffer]] internal slot.
Let
getIndex
be ?
ToIndex
requestIndex
).
Let
numberValue
be ?
ToNumber
value
).
Set
isLittleEndian
to
ToBoolean
isLittleEndian
).
Let
buffer
be
view
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
viewOffset
be
view
.[[ByteOffset]].
Let
viewSize
be
view
.[[ByteLength]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for Element Type
type
If
getIndex
elementSize
viewSize
, throw a
RangeError
exception.
Let
bufferIndex
be
getIndex
viewOffset
Return
SetValueInBuffer
buffer
bufferIndex
type
numberValue
false
"Unordered"
isLittleEndian
).
24.3.2
The DataView Constructor
The DataView
constructor
is the intrinsic object
%DataView%
is the initial value of the
DataView
property of the
global object
creates and initializes a new DataView object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
DataView
behaviour must include a
super
call to the
DataView
constructor
to create and initialize subclass instances with the internal state necessary to support the
DataView.prototype
built-in methods.
24.3.2.1
DataView (
buffer
[ ,
byteOffset
[ ,
byteLength
] ] )
When the
DataView
function is called with at least one argument
buffer
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
If
Type
buffer
) is not Object, throw a
TypeError
exception.
If
buffer
does not have an [[ArrayBufferData]] internal slot, throw a
TypeError
exception.
Let
offset
be ?
ToIndex
byteOffset
).
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
bufferByteLength
be
buffer
.[[ArrayBufferByteLength]].
If
offset
bufferByteLength
, throw a
RangeError
exception.
If
byteLength
is either not present or
undefined
, then
Let
viewByteLength
be
bufferByteLength
offset
Else,
Let
viewByteLength
be ?
ToIndex
byteLength
).
If
offset
viewByteLength
bufferByteLength
, throw a
RangeError
exception.
Let
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%DataViewPrototype%"
, « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »).
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Set
.[[ViewedArrayBuffer]] to
buffer
Set
.[[ByteLength]] to
viewByteLength
Set
.[[ByteOffset]] to
offset
Return
24.3.3
Properties of the DataView Constructor
The DataView
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
24.3.3.1
DataView.prototype
The initial value of
DataView.prototype
is the intrinsic object
%DataViewPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
24.3.4
Properties of the DataView Prototype Object
The DataView prototype object:
is the intrinsic object
%DataViewPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.
24.3.4.1
get DataView.prototype.buffer
DataView.prototype.buffer
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[DataView]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
Return
buffer
24.3.4.2
get DataView.prototype.byteLength
DataView.prototype.byteLength
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[DataView]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
size
be
.[[ByteLength]].
Return
size
24.3.4.3
get DataView.prototype.byteOffset
DataView.prototype.byteOffset
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
If
does not have a [[DataView]] internal slot, throw a
TypeError
exception.
Assert
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
.[[ViewedArrayBuffer]].
If
IsDetachedBuffer
buffer
) is
true
, throw a
TypeError
exception.
Let
offset
be
.[[ByteOffset]].
Return
offset
24.3.4.4
DataView.prototype.constructor
The initial value of
DataView.prototype.constructor
is the intrinsic object
%DataView%
24.3.4.5
DataView.prototype.getFloat32 (
byteOffset
[ ,
littleEndian
] )
When the
getFloat32
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Float32"
).
24.3.4.6
DataView.prototype.getFloat64 (
byteOffset
[ ,
littleEndian
] )
When the
getFloat64
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Float64"
).
24.3.4.7
DataView.prototype.getInt8 (
byteOffset
When the
getInt8
method is called with argument
byteOffset
, the following steps are taken:
Let
be the
this
value.
Return ?
GetViewValue
byteOffset
true
"Int8"
).
24.3.4.8
DataView.prototype.getInt16 (
byteOffset
[ ,
littleEndian
] )
When the
getInt16
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Int16"
).
24.3.4.9
DataView.prototype.getInt32 (
byteOffset
[ ,
littleEndian
] )
When the
getInt32
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Int32"
).
24.3.4.10
DataView.prototype.getUint8 (
byteOffset
When the
getUint8
method is called with argument
byteOffset
, the following steps are taken:
Let
be the
this
value.
Return ?
GetViewValue
byteOffset
true
"Uint8"
).
24.3.4.11
DataView.prototype.getUint16 (
byteOffset
[ ,
littleEndian
] )
When the
getUint16
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Uint16"
).
24.3.4.12
DataView.prototype.getUint32 (
byteOffset
[ ,
littleEndian
] )
When the
getUint32
method is called with argument
byteOffset
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
GetViewValue
byteOffset
littleEndian
"Uint32"
).
24.3.4.13
DataView.prototype.setFloat32 (
byteOffset
value
[ ,
littleEndian
] )
When the
setFloat32
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Float32"
value
).
24.3.4.14
DataView.prototype.setFloat64 (
byteOffset
value
[ ,
littleEndian
] )
When the
setFloat64
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Float64"
value
).
24.3.4.15
DataView.prototype.setInt8 (
byteOffset
value
When the
setInt8
method is called with arguments
byteOffset
and
value
, the following steps are taken:
Let
be the
this
value.
Return ?
SetViewValue
byteOffset
true
"Int8"
value
).
24.3.4.16
DataView.prototype.setInt16 (
byteOffset
value
[ ,
littleEndian
] )
When the
setInt16
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Int16"
value
).
24.3.4.17
DataView.prototype.setInt32 (
byteOffset
value
[ ,
littleEndian
] )
When the
setInt32
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Int32"
value
).
24.3.4.18
DataView.prototype.setUint8 (
byteOffset
value
When the
setUint8
method is called with arguments
byteOffset
and
value
, the following steps are taken:
Let
be the
this
value.
Return ?
SetViewValue
byteOffset
true
"Uint8"
value
).
24.3.4.19
DataView.prototype.setUint16 (
byteOffset
value
[ ,
littleEndian
] )
When the
setUint16
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Uint16"
value
).
24.3.4.20
DataView.prototype.setUint32 (
byteOffset
value
[ ,
littleEndian
] )
When the
setUint32
method is called with arguments
byteOffset
and
value
and optional argument
littleEndian
, the following steps are taken:
Let
be the
this
value.
If
littleEndian
is not present, set
littleEndian
to
false
Return ?
SetViewValue
byteOffset
littleEndian
"Uint32"
value
).
24.3.4.21
DataView.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"DataView"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
24.3.5
Properties of DataView Instances
DataView instances are ordinary objects that inherit properties
from the DataView prototype object. DataView instances each have
[[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]]
internal slots.
Note
The value of the [[DataView]] internal slot is not used
within this specification. The simple presence of that internal slot is
used within the specification to identify objects created using the
DataView
constructor
24.4
The Atomics Object
The Atomics object:
is the intrinsic object
%Atomics%
is the initial value of the
Atomics
property of the
global object
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly
(atomically) on shared memory array cells as well as functions that let
agents wait for and dispatch primitive events. When used with
discipline, the Atomics functions allow multi-
agent
programs that communicate through shared memory to execute in a
well-understood order even on parallel CPUs. The rules that govern
shared-memory communication are provided by the
memory model
, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of the
memory model
section.
24.4.1
Abstract Operations for Atomics
24.4.1.1
ValidateSharedIntegerTypedArray (
typedArray
[ ,
onlyInt32
] )
The abstract operation ValidateSharedIntegerTypedArray takes one argument
typedArray
and an optional Boolean
onlyInt32
. It performs the following steps:
If
onlyInt32
is not present, set
onlyInt32
to
false
If
Type
typedArray
) is not Object, throw a
TypeError
exception.
If
typedArray
does not have a [[TypedArrayName]] internal slot, throw a
TypeError
exception.
Let
typeName
be
typedArray
.[[TypedArrayName]].
If
onlyInt32
is
true
, then
If
typeName
is not
"Int32Array"
, throw a
TypeError
exception.
Else,
If
typeName
is not
"Int8Array"
"Uint8Array"
"Int16Array"
"Uint16Array"
"Int32Array"
, or
"Uint32Array"
, throw a
TypeError
exception.
Assert
typedArray
has a [[ViewedArrayBuffer]] internal slot.
Let
buffer
be
typedArray
.[[ViewedArrayBuffer]].
If
IsSharedArrayBuffer
buffer
) is
false
, throw a
TypeError
exception.
Return
buffer
24.4.1.2
ValidateAtomicAccess (
typedArray
requestIndex
The abstract operation ValidateAtomicAccess takes two arguments,
typedArray
and
requestIndex
. It performs the following steps:
Assert
typedArray
is an Object that has a [[ViewedArrayBuffer]] internal slot.
Let
accessIndex
be ?
ToIndex
requestIndex
).
Let
length
be
typedArray
.[[ArrayLength]].
Assert
accessIndex
≥ 0.
If
accessIndex
length
, throw a
RangeError
exception.
Return
accessIndex
24.4.1.3
GetWaiterList (
block
WaiterList
is a semantic object that contains an ordered list of those agents that are waiting on a location (
block
) in shared memory;
block
is a
Shared Data Block
and
a byte offset into the memory of
block
The
agent cluster
has a store of WaiterList objects; the store is indexed by (
block
). WaiterLists are
agent
-independent: a lookup in the store of WaiterLists by (
block
) will result in the same WaiterList object in any
agent
in the
agent cluster
Operations on a WaiterList -- adding and removing waiting
agents, traversing the list of agents, suspending and notifying agents
on the list -- may only be performed by agents that have entered the
WaiterList's critical section.
The abstract operation GetWaiterList takes two arguments, a
Shared Data Block
block
and a nonnegative integer
. It performs the following steps:
Assert
block
is a
Shared Data Block
Assert
and
+ 3 are valid byte offsets within the memory of
block
Assert
is divisible by 4.
Return the
WaiterList
that is referenced by the pair (
block
).
24.4.1.4
EnterCriticalSection (
WL
The abstract operation EnterCriticalSection takes one argument, a
WaiterList
WL
. It performs the following steps:
Assert
: The calling
agent
is not in the critical section for any
WaiterList
Wait until no
agent
is in the critical section for
WL
, then enter the critical section for
WL
(without allowing any other
agent
to enter).
24.4.1.5
LeaveCriticalSection (
WL
The abstract operation LeaveCriticalSection takes one argument, a
WaiterList
WL
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Leave the critical section for
WL
24.4.1.6
AddWaiter (
WL
The abstract operation AddWaiter takes two arguments, a
WaiterList
WL
and an
agent
signifier
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Assert
is not on the list of waiters in any
WaiterList
Add
to the end of the list of waiters in
WL
24.4.1.7
RemoveWaiter (
WL
The abstract operation RemoveWaiter takes two arguments, a
WaiterList
WL
and an
agent
signifier
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Assert
is on the list of waiters in
WL
Remove
from the list of waiters in
WL
24.4.1.8
RemoveWaiters (
WL
The abstract operation RemoveWaiters takes two arguments, a
WaiterList
WL
and nonnegative integer
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Let
be a new empty
List
Let
be a reference to the list of waiters in
WL
Repeat, while
> 0 and
is not an empty
List
Let
be the first waiter in
Add
to the end of
Remove
from
Subtract 1 from
Return
24.4.1.9
Suspend (
WL
timeout
The abstract operation Suspend takes three arguments, a
WaiterList
WL
, an
agent
signifier
, and a nonnegative, non-
NaN
Number
timeout
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Assert
is equal to
AgentSignifier
().
Assert
is on the list of waiters in
WL
Assert
AgentCanSuspend
() is
true
Perform
LeaveCriticalSection
WL
) and suspend
for up to
timeout
milliseconds, performing the combined operation in such a way that a
notification that arrives after the critical section is exited but
before the suspension takes effect is not lost.
can notify either because the timeout expired or because it was notified explicitly by another
agent
calling
NotifyWaiter
WL
), and not for any other reasons at all.
Perform
EnterCriticalSection
WL
).
If
was notified explicitly by another
agent
calling
NotifyWaiter
WL
), return
true
Return
false
24.4.1.10
NotifyWaiter (
WL
The abstract operation NotifyWaiter takes two arguments, a
WaiterList
WL
and an
agent
signifier
. It performs the following steps:
Assert
: The calling
agent
is in the critical section for
WL
Assert
is on the list of waiters in
WL
Let
execution
be the [[CandidateExecution]] field of the
surrounding agent
's
Agent Record
Let
eventsRecord
be the
Agent Events Record
in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier
().
Let
agentSynchronizesWith
be
eventsRecord
.[[AgentSynchronizesWith]].
Let
notifierEventList
be
eventsRecord
.[[EventList]].
Let
waiterEventList
be the [[EventList]] field of the element in
execution
.[[EventsRecords]] whose [[AgentSignifier]] is
Let
notifyEvent
and
waitEvent
be new
Synchronize
events.
Append
notifyEvent
to
notifierEventList
Append
waitEvent
to
waiterEventList
Append (
notifyEvent
waitEvent
) to
agentSynchronizesWith
Notify the
agent
Note
The embedding may delay notifying
, e.g. for resource management reasons, but
must eventually be notified in order to guarantee forward progress.
24.4.1.11
AtomicReadModifyWrite (
typedArray
index
value
op
The abstract operation AtomicReadModifyWrite takes four arguments,
typedArray
index
value
, and a pure combining operation
op
. The pure combining operation
op
takes two
List
of byte values arguments and returns a
List
of byte values. The operation atomically loads a value, combines it
with another value, and stores the result of the combination. It returns
the loaded value. It performs the following steps:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
Let
be ?
ToInteger
value
).
Let
arrayTypeName
be
typedArray
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
elementSize
) +
offset
Return
GetModifySetValueInBuffer
buffer
indexedPosition
elementType
op
).
24.4.1.12
AtomicLoad (
typedArray
index
The abstract operation AtomicLoad takes two arguments,
typedArray
index
. The operation atomically loads a value and returns the loaded value. It performs the following steps:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
Let
arrayTypeName
be
typedArray
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
elementSize
) +
offset
Return
GetValueFromBuffer
buffer
indexedPosition
elementType
true
"SeqCst"
).
24.4.2
Atomics.add (
typedArray
index
value
Let
add
denote a semantic function of two
List
of byte values arguments that applies the addition operation to the Number values corresponding to the
List
of byte values arguments and returns a
List
of byte values corresponding to the result of that operation.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
add
).
24.4.3
Atomics.and (
typedArray
index
value
Let
and
denote a semantic function of two
List
of byte values arguments that applies the bitwise-and operation element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
and
).
24.4.4
Atomics.compareExchange (
typedArray
index
expectedValue
replacementValue
The following steps are taken:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
Let
expected
be ?
ToInteger
expectedValue
).
Let
replacement
be ?
ToInteger
replacementValue
).
Let
arrayTypeName
be
typedArray
.[[TypedArrayName]].
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Let
isLittleEndian
be the value of the [[LittleEndian]] field of the
surrounding agent
's
Agent Record
Let
expectedBytes
be
NumberToRawBytes
elementType
expected
isLittleEndian
).
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
elementSize
) +
offset
Let
compareExchange
denote a semantic function of two
List
of byte values arguments that returns the second argument if the first argument is element-wise equal to
expectedBytes
Return
GetModifySetValueInBuffer
buffer
indexedPosition
elementType
replacement
compareExchange
).
24.4.5
Atomics.exchange (
typedArray
index
value
Let
second
denote a semantic function of two
List
of byte values arguments that returns its second argument.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
second
).
24.4.6
Atomics.isLockFree (
size
The following steps are taken:
Let
be ?
ToInteger
size
).
Let
AR
be the
Agent Record
of the
surrounding agent
If
equals 1, return
AR
.[[IsLockFree1]].
If
equals 2, return
AR
.[[IsLockFree2]].
If
equals 4, return
true
Return
false
Note
Atomics.isLockFree
() is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (
compareExchange
load
store
add
sub
and
or
xor
, or
exchange
) on a datum of size
bytes will be performed without the calling
agent
acquiring a lock outside the
bytes comprising the datum, then
Atomics.isLockFree
) will return
true
High-performance algorithms will use Atomics.isLockFree to determine
whether to use locks or atomic operations in critical sections. If an
atomic primitive is not lock-free then it is often more efficient for an
algorithm to provide its own locking.
Atomics.isLockFree
(4) always returns
true
as that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.
24.4.7
Atomics.load (
typedArray
index
The following steps are taken:
Return ?
AtomicLoad
typedArray
index
).
24.4.8
Atomics.or (
typedArray
index
value
Let
or
denote a semantic function of two
List
of byte values arguments that applies the bitwise-or operation element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
or
).
24.4.9
Atomics.store (
typedArray
index
value
The following steps are taken:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
Let
be ?
ToInteger
value
).
Let
arrayTypeName
be
typedArray
.[[TypedArrayName]].
Let
elementSize
be the Number value of the Element Size value specified in
Table 59
for
arrayTypeName
Let
elementType
be the String value of the Element Type value in
Table 59
for
arrayTypeName
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
elementSize
) +
offset
Perform
SetValueInBuffer
buffer
indexedPosition
elementType
true
"SeqCst"
).
Return
24.4.10
Atomics.sub (
typedArray
index
value
Let
subtract
denote a semantic function of two
List
of byte values arguments that applies the subtraction operation to the Number values corresponding to the
List
of byte values arguments and returns a
List
of byte values corresponding to the result of that operation.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
subtract
).
24.4.11
Atomics.wait (
typedArray
index
value
timeout
Atomics.wait
puts the calling
agent
in a wait queue and puts it to sleep until it is notified or the sleep times out. The following steps are taken:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
true
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
Let
be ?
ToInt32
value
).
Let
be ?
ToNumber
timeout
).
If
is
NaN
, let
be
+∞
, else let
be
max
, 0).
Let
be
AgentCanSuspend
().
If
is
false
, throw a
TypeError
exception.
Let
block
be
buffer
.[[ArrayBufferData]].
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
× 4) +
offset
Let
WL
be
GetWaiterList
block
indexedPosition
).
Perform
EnterCriticalSection
WL
).
Let
be !
AtomicLoad
typedArray
).
If
is not equal to
, then
Perform
LeaveCriticalSection
WL
).
Return the String
"not-equal"
Let
be
AgentSignifier
().
Perform
AddWaiter
WL
).
Let
notified
be
Suspend
WL
).
If
notified
is
true
, then
Assert
is not on the list of waiters in
WL
Else,
Perform
RemoveWaiter
WL
).
Perform
LeaveCriticalSection
WL
).
If
notified
is
true
, return the String
"ok"
Return the String
"timed-out"
24.4.12
Atomics.notify (
typedArray
index
count
Atomics.notify
notifies some agents that are sleeping in the wait queue. The following steps are taken:
Let
buffer
be ?
ValidateSharedIntegerTypedArray
typedArray
true
).
Let
be ?
ValidateAtomicAccess
typedArray
index
).
If
count
is
undefined
, let
be
+∞
Else,
Let
intCount
be ?
ToInteger
count
).
Let
be
max
intCount
, 0).
Let
block
be
buffer
.[[ArrayBufferData]].
Let
offset
be
typedArray
.[[ByteOffset]].
Let
indexedPosition
be (
× 4) +
offset
Let
WL
be
GetWaiterList
block
indexedPosition
).
Let
be 0.
Perform
EnterCriticalSection
WL
).
Let
be
RemoveWaiters
WL
).
Repeat, while
is not an empty
List
Let
be the first
agent
in
Remove
from the front of
Perform
NotifyWaiter
WL
).
Add 1 to
Perform
LeaveCriticalSection
WL
).
Return
24.4.13
Atomics.xor (
typedArray
index
value
Let
xor
denote a semantic function of two
List
of byte values arguments that applies the bitwise-xor operation element-wise to the two arguments and returns a
List
of byte values corresponding to the result of that operation.
The following steps are taken:
Return ?
AtomicReadModifyWrite
typedArray
index
value
xor
).
24.4.14
Atomics [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Atomics"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
24.5
The JSON Object
The JSON object:
is the intrinsic object
%JSON%
is the initial value of the
JSON
property of the
global object
is an ordinary object.
contains two functions,
parse
and
stringify
, that are used to parse and construct JSON texts.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON
interchange format used in this specification is exactly that described
by ECMA-404. Conforming implementations of
JSON.parse
and
JSON.stringify
must support the exact interchange format described in the ECMA-404
specification without any deletions or extensions to the format.
24.5.1
JSON.parse (
text
[ ,
reviver
] )
The
parse
function parses a JSON text (a
JSON-formatted String) and produces an ECMAScript value. The JSON format
represents literals, arrays, and objects with a syntax similar to the
syntax for ECMAScript literals, Array Initializers, and Object
Initializers. After parsing, JSON objects are realized as ECMAScript
objects. JSON arrays are realized as ECMAScript Array instances. JSON
strings, numbers, booleans, and null are realized as ECMAScript Strings,
Numbers, Booleans, and
null
The optional
reviver
parameter is a function that takes two parameters,
key
and
value
. It can filter and transform the results. It is called with each of the
key
value
pairs produced by the parse, and its return value is used instead of
the original value. If it returns what it received, the structure is not
modified. If it returns
undefined
then the property is deleted from the result.
Let
JText
be ?
ToString
text
).
Parse
JText
interpreted as UTF-16 encoded Unicode points (
6.1.4
) as a JSON text as specified in ECMA-404. Throw a
SyntaxError
exception if
JText
is not a valid JSON text as defined in that specification.
Let
scriptText
be the
string-concatenation
of
"("
JText
, and
");"
Let
completion
be the result of parsing and evaluating
scriptText
as if it was the source text of an ECMAScript
Script
. The extended PropertyDefinitionEvaluation semantics defined in
B.3.1
must not be used during the evaluation.
Let
unfiltered
be
completion
.[[Value]].
Assert
unfiltered
is either a String, Number, Boolean, Null, or an Object that is defined by either an
ArrayLiteral
or an
ObjectLiteral
If
IsCallable
reviver
) is
true
, then
Let
root
be
ObjectCreate
%ObjectPrototype%
).
Let
rootName
be the empty String.
Let
status
be
CreateDataProperty
root
rootName
unfiltered
).
Assert
status
is
true
Return ?
InternalizeJSONProperty
root
rootName
).
Else,
Return
unfiltered
This function is the
%JSONParse%
intrinsic object.
The
"length"
property of the
parse
function is 2.
Note
Valid JSON text is a subset of the ECMAScript
PrimaryExpression
syntax as modified by Step 4 above. Step 2 verifies that
JText
conforms to that subset, and step 6 verifies that that parsing and evaluation returns a value of an appropriate type.
24.5.1.1
Runtime Semantics: InternalizeJSONProperty (
holder
name
The abstract operation InternalizeJSONProperty is a recursive abstract operation that takes two parameters: a
holder
object and the String
name
of a property in that object. InternalizeJSONProperty uses the value of
reviver
that was originally passed to the above parse function.
Let
val
be ?
Get
holder
name
).
If
Type
val
) is Object, then
Let
isArray
be ?
IsArray
val
).
If
isArray
is
true
, then
Let
be 0.
Let
len
be ?
ToLength
(?
Get
val
"length"
)).
Repeat, while
len
Let
newElement
be ?
InternalizeJSONProperty
val
, !
ToString
)).
If
newElement
is
undefined
, then
Perform ?
val
.[[Delete]](!
ToString
)).
Else,
Perform ?
CreateDataProperty
val
, !
ToString
),
newElement
).
NOTE: This algorithm intentionally does not throw an exception if
CreateDataProperty
returns
false
Add 1 to
Else,
Let
keys
be ?
EnumerableOwnPropertyNames
val
"key"
).
For each String
in
keys
, do
Let
newElement
be ?
InternalizeJSONProperty
val
).
If
newElement
is
undefined
, then
Perform ?
val
.[[Delete]](
).
Else,
Perform ?
CreateDataProperty
val
newElement
).
NOTE: This algorithm intentionally does not throw an exception if
CreateDataProperty
returns
false
Return ?
Call
reviver
holder
, «
name
val
»).
It is not permitted for a conforming implementation of
JSON.parse
to extend the JSON grammars. If an implementation wishes to support a
modified or extended JSON interchange format it must do so by defining a
different parse function.
Note
In the case where there are duplicate name Strings within
an object, lexically preceding values for the same key shall be
overwritten.
24.5.2
JSON.stringify (
value
[ ,
replacer
[ ,
space
] ] )
The
stringify
function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, or
undefined
. It can take three parameters. The
value
parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or
null
. The optional
replacer
parameter is either a function that alters the way objects and arrays
are stringified, or an array of Strings and Numbers that acts as an
inclusion list for selecting the object properties that will be
stringified. The optional
space
parameter is a String or Number that allows the result to have white space injected into it to improve human readability.
These are the steps in stringifying an object:
Let
stack
be a new empty
List
Let
indent
be the empty String.
Let
PropertyList
and
ReplacerFunction
be
undefined
If
Type
replacer
) is Object, then
If
IsCallable
replacer
) is
true
, then
Set
ReplacerFunction
to
replacer
Else,
Let
isArray
be ?
IsArray
replacer
).
If
isArray
is
true
, then
Set
PropertyList
to a new empty
List
Let
len
be ?
ToLength
(?
Get
replacer
"length"
)).
Let
be 0.
Repeat, while
len
Let
be ?
Get
replacer
, !
ToString
)).
Let
item
be
undefined
If
Type
) is String, set
item
to
Else if
Type
) is Number, set
item
to !
ToString
).
Else if
Type
) is Object, then
If
has a [[StringData]] or [[NumberData]] internal slot, set
item
to ?
ToString
).
If
item
is not
undefined
and
item
is not currently an element of
PropertyList
, then
Append
item
to the end of
PropertyList
Increase
by 1.
If
Type
space
) is Object, then
If
space
has a [[NumberData]] internal slot, then
Set
space
to ?
ToNumber
space
).
Else if
space
has a [[StringData]] internal slot, then
Set
space
to ?
ToString
space
).
If
Type
space
) is Number, then
Set
space
to
min
(10, !
ToInteger
space
)).
Let
gap
be the String value containing
space
occurrences of the code unit 0x0020 (SPACE). This will be the empty String if
space
is less than 1.
Else if
Type
space
) is String, then
If the length of
space
is 10 or less, let
gap
be
space
; otherwise let
gap
be the String value consisting of the first 10 code units of
space
Else,
Let
gap
be the empty String.
Let
wrapper
be
ObjectCreate
%ObjectPrototype%
).
Let
status
be
CreateDataProperty
wrapper
, the empty String,
value
).
Assert
status
is
true
Return ?
SerializeJSONProperty
(the empty String,
wrapper
).
The
"length"
property of the
stringify
function is 3.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If
value
is or contains a cyclic structure, then the stringify function must throw a
TypeError
exception. This is an example of a value that cannot be stringified:
a = [];
a[
] = a;
my_text =
JSON
.stringify(a);
// This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The
null
value is rendered in JSON text as the String
null
The
undefined
value is not rendered.
The
true
value is rendered in JSON text as the String
true
The
false
value is rendered in JSON text as the String
false
Note 3
String values are wrapped in QUOTATION MARK (
) code units. The code units
and
are escaped with
prefixes. Control characters code units are replaced with escape sequences
\u
HHHH, or with the shorter forms,
\b
(BACKSPACE),
\f
(FORM FEED),
\n
(LINE FEED),
\r
(CARRIAGE RETURN),
\t
(CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling
ToString
number
).
NaN
and Infinity regardless of sign are represented as the String
null
Note 5
Values that do not have a JSON representation (such as
undefined
and functions) do not produce a String. Instead they produce the
undefined
value. In arrays these values are represented as the String
null
. In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed
by zero or more properties, separated with a U+002C (COMMA), closed
with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String
representing the key or
property name
a U+003A (COLON), and then the stringified property value. An array is
rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or
more values, separated with a U+002C (COMMA), closed with a U+005D
(RIGHT SQUARE BRACKET).
24.5.2.1
Runtime Semantics: SerializeJSONProperty (
key
holder
The abstract operation SerializeJSONProperty with arguments
key
, and
holder
has access to
ReplacerFunction
from the invocation of the
stringify
method. Its algorithm is as follows:
Let
value
be ?
Get
holder
key
).
If
Type
value
) is Object, then
Let
toJSON
be ?
Get
value
"toJSON"
).
If
IsCallable
toJSON
) is
true
, then
Set
value
to ?
Call
toJSON
value
, «
key
»).
If
ReplacerFunction
is not
undefined
, then
Set
value
to ?
Call
ReplacerFunction
holder
, «
key
value
»).
If
Type
value
) is Object, then
If
value
has a [[NumberData]] internal slot, then
Set
value
to ?
ToNumber
value
).
Else if
value
has a [[StringData]] internal slot, then
Set
value
to ?
ToString
value
).
Else if
value
has a [[BooleanData]] internal slot, then
Set
value
to
value
.[[BooleanData]].
If
value
is
null
, return
"null"
If
value
is
true
, return
"true"
If
value
is
false
, return
"false"
If
Type
value
) is String, return
QuoteJSONString
value
).
If
Type
value
) is Number, then
If
value
is finite, return !
ToString
value
).
Return
"null"
If
Type
value
) is Object and
IsCallable
value
) is
false
, then
Let
isArray
be ?
IsArray
value
).
If
isArray
is
true
, return ?
SerializeJSONArray
value
).
Return ?
SerializeJSONObject
value
).
Return
undefined
24.5.2.2
Runtime Semantics: QuoteJSONString (
value
The abstract operation QuoteJSONString with argument
value
wraps a String value in QUOTATION MARK code units and escapes certain other code units within it.
This operation interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4
Let
product
be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
Let
cpList
be a
List
containing in order the code points of
value
when interpreted as a sequence of UTF-16 encoded code points as described in
6.1.4
For each code point
in
cpList
, do
If
is listed in the Code Point column of
Table 62
, then
Set
product
to the
string-concatenation
of
product
and the Escape Sequence for
as specified in
Table 62
Else if
has a numeric value less than 0x0020 (SPACE), or if
has the same numeric value as a
leading surrogate
or
trailing surrogate
, then
Let
unit
be the code unit whose numeric value is that of
Set
product
to the
string-concatenation
of
product
and
UnicodeEscape
unit
).
Else,
Set
product
to the
string-concatenation
of
product
and the
UTF16Encoding
of
Set
product
to the
string-concatenation
of
product
and the code unit 0x0022 (QUOTATION MARK).
Return
product
Table 62: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
24.5.2.3
Runtime Semantics: UnicodeEscape (
The abstract operation UnicodeEscape takes a code unit argument
and represents it as a Unicode escape sequence.
Let
be the numeric value of
Assert
≤ 0xFFFF.
Return the
string-concatenation
of:
the code unit 0x005C (REVERSE SOLIDUS)
"u"
the String representation of
, formatted as a four-digit lowercase hexadecimal number, padded to the left with zeroes if necessary
24.5.2.4
Runtime Semantics: SerializeJSONObject (
value
The abstract operation SerializeJSONObject with argument
value
serializes an object. It has access to the
stack
indent
gap
, and
PropertyList
values of the current invocation of the
stringify
method.
If
stack
contains
value
, throw a
TypeError
exception because the structure is cyclical.
Append
value
to
stack
Let
stepback
be
indent
Set
indent
to the
string-concatenation
of
indent
and
gap
If
PropertyList
is not
undefined
, then
Let
be
PropertyList
Else,
Let
be ?
EnumerableOwnPropertyNames
value
"key"
).
Let
partial
be a new empty
List
For each element
of
, do
Let
strP
be ?
SerializeJSONProperty
value
).
If
strP
is not
undefined
, then
Let
member
be
QuoteJSONString
).
Set
member
to the
string-concatenation
of
member
and
":"
If
gap
is not the empty String, then
Set
member
to the
string-concatenation
of
member
and the code unit 0x0020 (SPACE).
Set
member
to the
string-concatenation
of
member
and
strP
Append
member
to
partial
If
partial
is empty, then
Let
final
be
"{}"
Else,
If
gap
is the empty String, then
Let
properties
be the String value formed by concatenating all the element Strings of
partial
with each adjacent pair of Strings separated with the code unit 0x002C
(COMMA). A comma is not inserted either before the first String or after
the last String.
Let
final
be the
string-concatenation
of
"{"
properties
, and
"}"
Else
gap
is not the empty String,
Let
separator
be the
string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and
indent
Let
properties
be the String value formed by concatenating all the element Strings of
partial
with each adjacent pair of Strings separated with
separator
. The
separator
String is not inserted either before the first String or after the last String.
Let
final
be the
string-concatenation
of
"{"
, the code unit 0x000A (LINE FEED),
indent
properties
, the code unit 0x000A (LINE FEED),
stepback
, and
"}"
Remove the last element of
stack
Set
indent
to
stepback
Return
final
24.5.2.5
Runtime Semantics: SerializeJSONArray (
value
The abstract operation SerializeJSONArray with argument
value
serializes an array. It has access to the
stack
indent
, and
gap
values of the current invocation of the
stringify
method.
If
stack
contains
value
, throw a
TypeError
exception because the structure is cyclical.
Append
value
to
stack
Let
stepback
be
indent
Set
indent
to the
string-concatenation
of
indent
and
gap
Let
partial
be a new empty
List
Let
len
be ?
ToLength
(?
Get
value
"length"
)).
Let
index
be 0.
Repeat, while
index
len
Let
strP
be ?
SerializeJSONProperty
(!
ToString
index
),
value
).
If
strP
is
undefined
, then
Append
"null"
to
partial
Else,
Append
strP
to
partial
Increment
index
by 1.
If
partial
is empty, then
Let
final
be
"[]"
Else,
If
gap
is the empty String, then
Let
properties
be the String value formed by concatenating all the element Strings of
partial
with each adjacent pair of Strings separated with the code unit 0x002C
(COMMA). A comma is not inserted either before the first String or after
the last String.
Let
final
be the
string-concatenation
of
"["
properties
, and
"]"
Else,
Let
separator
be the
string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and
indent
Let
properties
be the String value formed by concatenating all the element Strings of
partial
with each adjacent pair of Strings separated with
separator
. The
separator
String is not inserted either before the first String or after the last String.
Let
final
be the
string-concatenation
of
"["
, the code unit 0x000A (LINE FEED),
indent
properties
, the code unit 0x000A (LINE FEED),
stepback
, and
"]"
Remove the last element of
stack
Set
indent
to
stepback
Return
final
Note
The representation of arrays includes only the elements between zero and
array.length
- 1
inclusive. Properties whose keys are not
array indexes
are excluded from the stringification. An array is stringified as an
opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing
RIGHT SQUARE BRACKET.
24.5.3
JSON [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"JSON"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25
Control Abstraction Objects
25.1
Iteration
25.1.1
Common Iteration Interfaces
An interface is a set of property keys whose associated values
match a specific specification. Any object that provides all the
properties as described by an interface's specification
conforms
to that interface. An interface is not represented by a distinct
object. There may be many separately implemented objects that conform to
any interface. An individual object may conform to multiple interfaces.
25.1.1.1
The
Iterable
Interface
The
Iterable
interface includes the property described in
Table 63
Table 63:
Iterable
Interface Required Properties
Property
Value
Requirements
@@iterator
A function that returns an
Iterator
object.
The returned object must conform to the
Iterator
interface.
25.1.1.2
The
Iterator
Interface
An object that implements the
Iterator
interface must include the property in
Table 64
. Such objects may also implement the properties in
Table 65
Table 64:
Iterator
Interface Required Properties
Property
Value
Requirements
next
A function that returns an
IteratorResult
object.
The returned object must conform to the
IteratorResult
interface. If a previous call to the
next
method of an
Iterator
has returned an
IteratorResult
object whose
done
property is
true
, then all subsequent calls to the
next
method of that object should also return an
IteratorResult
object whose
done
property is
true
. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target
Iterator
. The for-of statement and other common users of
Iterators
do not pass any arguments, so
Iterator
objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 65:
Iterator
Interface Optional Properties
Property
Value
Requirements
return
A function that returns an
IteratorResult
object.
The returned object must conform to the
IteratorResult
interface. Invoking this method notifies the
Iterator
object that the caller does not intend to make any more
next
method calls to the
Iterator
. The returned
IteratorResult
object will typically have a
done
property whose value is
true
, and a
value
property with the value passed as the argument of the
return
method. However, this requirement is not enforced.
throw
A function that returns an
IteratorResult
object.
The returned object must conform to the
IteratorResult
interface. Invoking this method notifies the
Iterator
object that the caller has detected an error condition. The argument
may be used to identify the error condition and typically will be an
exception object. A typical response is to
throw
the value passed as the argument. If the method does not
throw
, the returned
IteratorResult
object will typically have a
done
property whose value is
true
Note 2
Typically callers of these methods should check for their
existence before invoking them. Certain ECMAScript language features
including
for
of
yield*
, and
array destructuring call these methods after performing an existence
check. Most ECMAScript library functions that accept
Iterable
objects as arguments also conditionally call them.
25.1.1.3
The
AsyncIterable
Interface
The
AsyncIterable
interface includes the properties described in
Table 66
Table 66:
AsyncIterable
Interface Required Properties
Property
Value
Requirements
@@asyncIterator
A function that returns an
AsyncIterator
object.
The returned object must conform to the
AsyncIterator
interface.
25.1.1.4
The
AsyncIterator
Interface
An object that implements the
AsyncIterator
interface must include the properties in
Table 67
. Such objects may also implement the properties in
Table 68
Table 67:
AsyncIterator
Interface Required Properties
Property
Value
Requirements
next
A function that returns a promise for an
IteratorResult
object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the
IteratorResult
interface. If a previous call to the
next
method of an
AsyncIterator
has returned a promise for an
IteratorResult
object whose
done
property is
true
, then all subsequent calls to the
next
method of that object should also return a promise for an
IteratorResult
object whose
done
property is
true
. However, this requirement is not enforced.
Additionally, the
IteratorResult
object that serves as a fulfillment value should have a
value
property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target
AsyncIterator
. The
for
await
of
statement and other common users of
AsyncIterators
do not pass any arguments, so
AsyncIterator
objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 68:
AsyncIterator
Interface Optional Properties
Property
Value
Requirements
return
A function that returns a promise for an
IteratorResult
object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the
IteratorResult
interface. Invoking this method notifies the
AsyncIterator
object that the caller does not intend to make any more
next
method calls to the
AsyncIterator
. The returned promise will fulfill with an
IteratorResult
object which will typically have a
done
property whose value is
true
, and a
value
property with the value passed as the argument of the
return
method. However, this requirement is not enforced.
Additionally, the
IteratorResult
object that serves as a fulfillment value should have a
value
property whose value is not a promise (or "thenable"). If the argument
value is used in the typical manner, then if it is a rejected promise, a
promise rejected with the same reason should be returned; if it is a
fulfilled promise, then its fulfillment value should be used as the
value
property of the returned promise's
IteratorResult
object fulfillment value. However, these requirements are also not enforced.
throw
A function that returns a promise for an
IteratorResult
object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the
IteratorResult
interface. Invoking this method notifies the
AsyncIterator
object that the caller has detected an error condition. The argument
may be used to identify the error condition and typically will be an
exception object. A typical response is to return a rejected promise
which rejects with the value passed as the argument.
If the returned promise is fulfilled, the
IteratorResult
fulfillment value will typically have a
done
property whose value is
true
. Additionally, it should have a
value
property whose value is not a promise (or "thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their
existence before invoking them. Certain ECMAScript language features
including
for
await
of
and
yield*
call these methods after performing an existence check.
25.1.1.5
The IteratorResult Interface
The
IteratorResult
interface includes the properties listed in
Table 69
Table 69:
IteratorResult
Interface Properties
Property
Value
Requirements
done
Either
true
or
false
This is the result status of an
iterator
next
method call. If the end of the iterator was reached
done
is
true
. If the end was not reached
done
is
false
and a value is available. If a
done
property (either own or inherited) does not exist, it is consider to have the value
false
value
Any
ECMAScript language value
If done is
false
, this is the current iteration element value. If done is
true
, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value,
value
is
undefined
. In that case, the
value
property may be absent from the conforming object if it does not inherit an explicit
value
property.
25.1.2
The %IteratorPrototype% Object
The
%IteratorPrototype%
object:
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
Note
All objects defined in this specification that implement the
Iterator interface also inherit from %IteratorPrototype%. ECMAScript
code may also define objects that inherit from %IteratorPrototype%.The
%IteratorPrototype% object provides a place where additional methods
that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
Object
.getPrototypeOf(
Object
.getPrototypeOf([][
Symbol
.iterator]()))
25.1.2.1
%IteratorPrototype% [ @@iterator ] ( )
The following steps are taken:
Return the
this
value.
The value of the
name
property of this function is
"[Symbol.iterator]"
25.1.3
The %AsyncIteratorPrototype% Object
The
%AsyncIteratorPrototype%
object:
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
Note
All objects defined in this specification that implement the
AsyncIterator interface also inherit from %AsyncIteratorPrototype%.
ECMAScript code may also define objects that inherit from
%AsyncIteratorPrototype%.The %AsyncIteratorPrototype% object provides a
place where additional methods that are applicable to all async iterator
objects may be added.
25.1.3.1
%AsyncIteratorPrototype% [ @@asyncIterator ] ( )
The following steps are taken:
Return the
this
value.
The value of the
name
property of this function is
"[Symbol.asyncIterator]"
25.1.4
Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a named
constructor
for Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by the
CreateAsyncFromSyncIterator
abstract operation as needed.
25.1.4.1
CreateAsyncFromSyncIterator (
syncIteratorRecord
The abstract operation CreateAsyncFromSyncIterator is used to create an async iterator
Record
from a synchronous iterator
Record
. It performs the following steps:
Let
asyncIterator
be !
ObjectCreate
%AsyncFromSyncIteratorPrototype%
, « [[SyncIteratorRecord]] »).
Set
asyncIterator
.[[SyncIteratorRecord]] to
syncIteratorRecord
Return ?
GetIterator
asyncIterator
async
).
25.1.4.2
The %AsyncFromSyncIteratorPrototype% Object
The
%AsyncFromSyncIteratorPrototype%
object:
has properties that are inherited by all Async-from-Sync Iterator Objects.
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%AsyncIteratorPrototype%
has the following properties:
25.1.4.2.1
%AsyncFromSyncIteratorPrototype%.next (
value
Let
be the
this
value.
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
If
Type
) is not Object, or if
does not have a [[SyncIteratorRecord]] internal slot, then
Let
invalidIteratorError
be a newly created
TypeError
object.
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
invalidIteratorError
»).
Return
promiseCapability
.[[Promise]].
Let
syncIteratorRecord
be
.[[SyncIteratorRecord]].
Let
result
be
IteratorNext
syncIteratorRecord
value
).
IfAbruptRejectPromise
result
promiseCapability
).
Return !
AsyncFromSyncIteratorContinuation
result
promiseCapability
).
25.1.4.2.2
%AsyncFromSyncIteratorPrototype%.return (
value
Let
be the
this
value.
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
If
Type
) is not Object, or if
does not have a [[SyncIteratorRecord]] internal slot, then
Let
invalidIteratorError
be a newly created
TypeError
object.
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
invalidIteratorError
»).
Return
promiseCapability
.[[Promise]].
Let
syncIterator
be
.[[SyncIteratorRecord]].[[Iterator]].
Let
return
be
GetMethod
syncIterator
"return"
).
IfAbruptRejectPromise
return
promiseCapability
).
If
return
is
undefined
, then
Let
iterResult
be !
CreateIterResultObject
value
true
).
Perform !
Call
promiseCapability
.[[Resolve]],
undefined
, «
iterResult
»).
Return
promiseCapability
.[[Promise]].
Let
result
be
Call
return
syncIterator
, «
value
»).
IfAbruptRejectPromise
result
promiseCapability
).
If
Type
result
) is not Object, then
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, « a newly created
TypeError
object »).
Return
promiseCapability
.[[Promise]].
Return !
AsyncFromSyncIteratorContinuation
result
promiseCapability
).
25.1.4.2.3
%AsyncFromSyncIteratorPrototype%.throw (
value
Let
be the
this
value.
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
If
Type
) is not Object, or if
does not have a [[SyncIteratorRecord]] internal slot, then
Let
invalidIteratorError
be a newly created
TypeError
object.
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
invalidIteratorError
»).
Return
promiseCapability
.[[Promise]].
Let
syncIterator
be
.[[SyncIteratorRecord]].[[Iterator]].
Let
throw
be
GetMethod
syncIterator
"throw"
).
IfAbruptRejectPromise
throw
promiseCapability
).
If
throw
is
undefined
, then
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
value
»).
Return
promiseCapability
.[[Promise]].
Let
result
be
Call
throw
syncIterator
, «
value
»).
IfAbruptRejectPromise
result
promiseCapability
).
If
Type
result
) is not Object, then
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, « a newly created
TypeError
object »).
Return
promiseCapability
.[[Promise]].
Return !
AsyncFromSyncIteratorContinuation
result
promiseCapability
).
25.1.4.2.4
%AsyncFromSyncIteratorPrototype% [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Async-from-Sync Iterator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.1.4.2.5
Async-from-Sync Iterator Value Unwrap Functions
An async-from-sync iterator value unwrap function is an anonymous built-in function that is used by methods of
%AsyncFromSyncIteratorPrototype%
when processing the
value
field of an
IteratorResult
object, in order to wait for its value if it is a promise and re-package the result in a new "unwrapped"
IteratorResult
object. Each async iterator value unwrap function has a [[Done]] internal slot.
When an async-from-sync iterator value unwrap function is called with argument
value
, the following steps are taken:
Let
be the
active function object
Return !
CreateIterResultObject
value
.[[Done]]).
25.1.4.3
Properties of Async-from-Sync Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that inherit properties from the
%AsyncFromSyncIteratorPrototype%
intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed in
Table 70
Table 70: Internal Slots of Async-from-Sync Iterator Instances
Internal Slot
Description
[[SyncIteratorRecord]]
Record
, of the type returned by
GetIterator
, representing the original synchronous iterator which is being adapted.
25.1.4.4
AsyncFromSyncIteratorContinuation (
result
promiseCapability
Let
done
be
IteratorComplete
result
).
IfAbruptRejectPromise
done
promiseCapability
).
Let
value
be
IteratorValue
result
).
IfAbruptRejectPromise
value
promiseCapability
).
Let
valueWrapper
be ?
PromiseResolve
%Promise%
, «
value
»).
Let
steps
be the algorithm steps defined in
Async-from-Sync Iterator Value Unwrap Functions
Let
onFulfilled
be
CreateBuiltinFunction
steps
, « [[Done]] »).
Set
onFulfilled
.[[Done]] to
done
Perform !
PerformPromiseThen
valueWrapper
onFulfilled
undefined
promiseCapability
).
Return
promiseCapability
.[[Promise]].
25.2
GeneratorFunction Objects
GeneratorFunction objects are functions that are usually created by evaluating
GeneratorDeclaration
s,
GeneratorExpression
s, and
GeneratorMethod
s. They may also be created by calling the
%GeneratorFunction%
intrinsic.
Figure 5 (Informative): Generator Objects Relationships
25.2.1
The GeneratorFunction Constructor
The GeneratorFunction
constructor
is the intrinsic object
%GeneratorFunction%
creates and initializes a new GeneratorFunction object when called as a function rather than as a
constructor
. Thus the function call
GeneratorFunction (…)
is equivalent to the object creation expression
new GeneratorFunction (…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
GeneratorFunction
behaviour must include a
super
call to the
GeneratorFunction
constructor
to create and initialize subclass instances with the internal slots
necessary for built-in GeneratorFunction behaviour. All ECMAScript
syntactic forms for defining generator function objects create direct
instances of
GeneratorFunction
. There is no syntactic means to create instances of
GeneratorFunction
subclasses.
25.2.1.1
GeneratorFunction (
p1
p2
, … ,
pn
body
The last argument specifies the body (executable code) of a
generator function; any preceding arguments specify formal parameters.
When the
GeneratorFunction
function is called with some arguments
p1
p2
, … ,
pn
body
(where
might be 0, that is, there are no “
” arguments, and where
body
might also not be provided), the following steps are taken:
Let
be the
active function object
Let
args
be the
argumentsList
that was passed to this function by [[Call]] or [[Construct]].
Return ?
CreateDynamicFunction
, NewTarget,
"generator"
args
).
Note
See NOTE for
19.2.1.1
25.2.2
Properties of the GeneratorFunction Constructor
The GeneratorFunction
constructor
is a standard built-in
function object
that inherits from the
Function
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%Function%
has a
name
property whose value is
"GeneratorFunction"
has the following properties:
25.2.2.1
GeneratorFunction.length
This is a
data property
with a value of 1. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.2.2.2
GeneratorFunction.prototype
The initial value of
GeneratorFunction.prototype
is the intrinsic object
%Generator%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
25.2.3
Properties of the GeneratorFunction Prototype Object
The GeneratorFunction prototype object:
is an ordinary object.
is not a
function object
and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in
Table 27
or
Table 71
is the value of the
prototype
property of the intrinsic object
%GeneratorFunction%
is the intrinsic object
%Generator%
(see Figure 2).
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
25.2.3.1
GeneratorFunction.prototype.constructor
The initial value of
GeneratorFunction.prototype.constructor
is the intrinsic object
%GeneratorFunction%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.2.3.2
GeneratorFunction.prototype.prototype
The value of
GeneratorFunction.prototype.prototype
is the
%GeneratorPrototype%
intrinsic object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.2.3.3
GeneratorFunction.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"GeneratorFunction"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.2.4
GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 27
. The value of the [[FunctionKind]] internal slot for all such instances is
"generator"
Each GeneratorFunction instance has the following own properties:
25.2.4.1
length
The specification for the
"length"
property of Function instances given in
19.2.4.1
also applies to GeneratorFunction instances.
25.2.4.2
name
The specification for the
name
property of Function instances given in
19.2.4.2
also applies to GeneratorFunction instances.
25.2.4.3
prototype
Whenever a GeneratorFunction instance is created another
ordinary object is also created and is the initial value of the
generator function's
prototype
property. The value of the
prototype property is used to initialize the [[Prototype]] internal slot
of a newly created Generator object when the generator
function object
is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
Unlike Function instances, the object that is the value of the a GeneratorFunction's
prototype
property does not have a
constructor
property whose value is the GeneratorFunction instance.
25.3
AsyncGeneratorFunction Objects
AsyncGeneratorFunction objects are functions that are usually created by evaluating
AsyncGeneratorDeclaration
AsyncGeneratorExpression
, and
AsyncGeneratorMethod
syntactic productions. They may also be created by calling the
%AsyncGeneratorFunction%
intrinsic.
25.3.1
The AsyncGeneratorFunction Constructor
The AsyncGeneratorFunction
constructor
is the intrinsic object
%AsyncGeneratorFunction%
creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as a
constructor
. Thus the function call
AsyncGeneratorFunction (...)
is equivalent to the object creation expression
new AsyncGeneratorFunction (...)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
AsyncGeneratorFunction
behaviour must include a
super
call to the
AsyncGeneratorFunction
constructor
to create and initialize subclass instances with the internal slots
necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript
syntactic forms for defining async generator function objects create
direct instances of
AsyncGeneratorFunction
. There is no syntactic means to create instances of
AsyncGeneratorFunction
subclasses.
25.3.1.1
AsyncGeneratorFunction (
p1
p2
, ...,
pn
body
The last argument specifies the body (executable code) of an
async generator function; any preceding arguments specify formal
parameters.
When the
AsyncGeneratorFunction
function is called with some arguments
p1
p2
, … ,
pn
body
(where
might be 0, that is, there are no "
" arguments, and where
body
might also not be provided), the following steps are taken:
Let
be the
active function object
Let
args
be the
argumentsList
that was passed to this function by [[Call]] or [[Construct]].
Return ?
CreateDynamicFunction
, NewTarget,
"async generator"
args
).
Note
See NOTE for
19.2.1.1
25.3.2
Properties of the AsyncGeneratorFunction Constructor
The AsyncGeneratorFunction
constructor
is a standard built-in
function object
that inherits from the
Function
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%Function%
has a
name
property whose value is
"AsyncGeneratorFunction"
has the following properties:
25.3.2.1
AsyncGeneratorFunction.length
This is a
data property
with a value of 1. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.3.2.2
AsyncGeneratorFunction.prototype
The initial value of
AsyncGeneratorFunction.prototype
is the intrinsic object
%AsyncGenerator%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
25.3.3
Properties of the AsyncGeneratorFunction Prototype Object
The AsyncGeneratorFunction prototype object:
is an ordinary object.
is not a
function object
and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in
Table 27
or
Table 72
is the value of the
prototype
property of the intrinsic object
%AsyncGeneratorFunction%
is the intrinsic object
%AsyncGenerator%
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
25.3.3.1
AsyncGeneratorFunction.prototype.constructor
The initial value of
AsyncGeneratorFunction.prototype.constructor
is the intrinsic object
%AsyncGeneratorFunction%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.3.3.2
AsyncGeneratorFunction.prototype.prototype
The value of
AsyncGeneratorFunction.prototype.prototype
is the
%AsyncGeneratorPrototype%
intrinsic object.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.3.3.3
AsyncGeneratorFunction.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"AsyncGeneratorFunction"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.3.4
AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 27
. The value of the [[FunctionKind]] internal slot for all such instances is
"generator"
Each AsyncGeneratorFunction instance has the following own properties:
25.3.4.1
length
The value of the
"length"
property is an integer
that indicates the typical number of arguments expected by the
AsyncGeneratorFunction. However, the language permits the function to be
invoked with some other number of arguments. The behaviour of an
AsyncGeneratorFunction when invoked on a number of arguments other than
the number specified by its
"length"
property depends on the function.
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.3.4.2
name
The specification for the
name
property of Function instances given in
19.2.4.2
also applies to AsyncGeneratorFunction instances.
25.3.4.3
prototype
Whenever an AsyncGeneratorFunction instance is created
another ordinary object is also created and is the initial value of the
async generator function's
prototype
property. The value of
the prototype property is used to initialize the [[Prototype]] internal
slot of a newly created AsyncGenerator object when the generator
function object
is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
Note
Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's
prototype
property does not have a
constructor
property whose value is the AsyncGeneratorFunction instance.
25.4
Generator Objects
A Generator object is an instance of a generator function and conforms to both the
Iterator
and
Iterable
interfaces.
Generator instances directly inherit properties from the object that is the value of the
prototype
property of the Generator function that created the instance. Generator
instances indirectly inherit properties from the Generator Prototype
intrinsic,
%GeneratorPrototype%
25.4.1
Properties of the Generator Prototype Object
The Generator prototype object:
is the intrinsic object
%GeneratorPrototype%
is the initial value of the
prototype
property of the intrinsic object
%Generator%
(the GeneratorFunction.prototype).
is an ordinary object.
is not a Generator instance and does not have a [[GeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%IteratorPrototype%
has properties that are indirectly inherited by all Generator instances.
25.4.1.1
Generator.prototype.constructor
The initial value of
Generator.prototype.constructor
is the intrinsic object
%Generator%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.4.1.2
Generator.prototype.next (
value
The
next
method performs the following steps:
Let
be the
this
value.
Return ?
GeneratorResume
value
).
25.4.1.3
Generator.prototype.return (
value
The
return
method performs the following steps:
Let
be the
this
value.
Let
be
Completion
{ [[Type]]:
return
, [[Value]]:
value
, [[Target]]:
empty
}.
Return ?
GeneratorResumeAbrupt
).
25.4.1.4
Generator.prototype.throw (
exception
The
throw
method performs the following steps:
Let
be the
this
value.
Let
be
ThrowCompletion
exception
).
Return ?
GeneratorResumeAbrupt
).
25.4.1.5
Generator.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Generator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.4.2
Properties of Generator Instances
Generator instances are initially created with the internal slots described in
Table 71
Table 71: Internal Slots of Generator Instances
Internal Slot
Description
[[GeneratorState]]
The current execution state of the generator. The possible values are:
undefined
"suspendedStart"
"suspendedYield"
"executing"
, and
"completed"
[[GeneratorContext]]
The
execution context
that is used when executing the code of this generator.
25.4.3
Generator Abstract Operations
25.4.3.1
GeneratorStart (
generator
generatorBody
The abstract operation GeneratorStart with arguments
generator
and
generatorBody
performs the following steps:
Assert
: The value of
generator
.[[GeneratorState]] is
undefined
Let
genContext
be the
running execution context
Set the Generator component of
genContext
to
generator
Set the code evaluation state of
genContext
such that when evaluation is resumed for that
execution context
the following steps will be performed:
Let
result
be the result of evaluating
generatorBody
Assert
: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
Remove
genContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
Set
generator
.[[GeneratorState]] to
"completed"
Once a generator enters the
"completed"
state it never leaves it and its associated
execution context
is never resumed. Any execution state associated with
generator
can be discarded at this point.
If
result
.[[Type]] is
normal
, let
resultValue
be
undefined
Else if
result
.[[Type]] is
return
, let
resultValue
be
result
.[[Value]].
Else,
Assert
result
.[[Type]] is
throw
Return
Completion
result
).
Return
CreateIterResultObject
resultValue
true
).
Set
generator
.[[GeneratorContext]] to
genContext
Set
generator
.[[GeneratorState]] to
"suspendedStart"
Return
NormalCompletion
undefined
).
25.4.3.2
GeneratorValidate (
generator
The abstract operation GeneratorValidate with argument
generator
performs the following steps:
If
Type
generator
) is not Object, throw a
TypeError
exception.
If
generator
does not have a [[GeneratorState]] internal slot, throw a
TypeError
exception.
Assert
generator
also has a [[GeneratorContext]] internal slot.
Let
state
be
generator
.[[GeneratorState]].
If
state
is
"executing"
, throw a
TypeError
exception.
Return
state
25.4.3.3
GeneratorResume (
generator
value
The abstract operation GeneratorResume with arguments
generator
and
value
performs the following steps:
Let
state
be ?
GeneratorValidate
generator
).
If
state
is
"completed"
, return
CreateIterResultObject
undefined
true
).
Assert
state
is either
"suspendedStart"
or
"suspendedYield"
Let
genContext
be
generator
.[[GeneratorContext]].
Let
methodContext
be the
running execution context
Suspend
methodContext
Set
generator
.[[GeneratorState]] to
"executing"
Push
genContext
onto the
execution context stack
genContext
is now the
running execution context
Resume the suspended evaluation of
genContext
using
NormalCompletion
value
) as the result of the operation that suspended it. Let
result
be the value returned by the resumed computation.
Assert
: When we return here,
genContext
has already been removed from the
execution context stack
and
methodContext
is the currently
running execution context
Return
Completion
result
).
25.4.3.4
GeneratorResumeAbrupt (
generator
abruptCompletion
The abstract operation GeneratorResumeAbrupt with arguments
generator
and
abruptCompletion
performs the following steps:
Let
state
be ?
GeneratorValidate
generator
).
If
state
is
"suspendedStart"
, then
Set
generator
.[[GeneratorState]] to
"completed"
Once a generator enters the
"completed"
state it never leaves it and its associated
execution context
is never resumed. Any execution state associated with
generator
can be discarded at this point.
Set
state
to
"completed"
If
state
is
"completed"
, then
If
abruptCompletion
.[[Type]] is
return
, then
Return
CreateIterResultObject
abruptCompletion
.[[Value]],
true
).
Return
Completion
abruptCompletion
).
Assert
state
is
"suspendedYield"
Let
genContext
be
generator
.[[GeneratorContext]].
Let
methodContext
be the
running execution context
Suspend
methodContext
Set
generator
.[[GeneratorState]] to
"executing"
Push
genContext
onto the
execution context stack
genContext
is now the
running execution context
Resume the suspended evaluation of
genContext
using
abruptCompletion
as the result of the operation that suspended it. Let
result
be the completion record returned by the resumed computation.
Assert
: When we return here,
genContext
has already been removed from the
execution context stack
and
methodContext
is the currently
running execution context
Return
Completion
result
).
25.4.3.5
GetGeneratorKind ( )
Let
genContext
be the
running execution context
If
genContext
does not have a Generator component, return
non-generator
Let
generator
be the Generator component of
genContext
If
generator
has an [[AsyncGeneratorState]] internal slot, return
async
Else, return
sync
25.4.3.6
GeneratorYield (
iterNextObj
The abstract operation GeneratorYield with argument
iterNextObj
performs the following steps:
Assert
iterNextObj
is an Object that implements the
IteratorResult
interface.
Let
genContext
be the
running execution context
Assert
genContext
is the
execution context
of a generator.
Let
generator
be the value of the Generator component of
genContext
Assert
GetGeneratorKind
() is
sync
Set
generator
.[[GeneratorState]] to
"suspendedYield"
Remove
genContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
Set the code evaluation state of
genContext
such that when evaluation is resumed with a
Completion
resumptionValue
the following steps will be performed:
Return
resumptionValue
NOTE: This returns to the evaluation of the
YieldExpression
that originally called this abstract operation.
Return
NormalCompletion
iterNextObj
).
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of
genContext
25.5
AsyncGenerator Objects
An AsyncGenerator object is an instance of an async generator
function and conforms to both the AsyncIterator and AsyncIterable
interfaces.
AsyncGenerator instances directly inherit properties from the object that is the value of the
prototype
property of the AsyncGenerator function that created the instance.
AsyncGenerator instances indirectly inherit properties from the
AsyncGenerator Prototype intrinsic,
%AsyncGeneratorPrototype%
25.5.1
Properties of the AsyncGenerator Prototype Object
The AsyncGenerator prototype object:
is the intrinsic object
%AsyncGeneratorPrototype%
is the initial value of the
prototype
property of the intrinsic object
%AsyncGenerator%
(the AsyncGeneratorFunction.prototype).
is an ordinary object.
is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
has a [[Prototype]] internal slot whose value is the intrinsic object
%AsyncIteratorPrototype%
has properties that are indirectly inherited by all AsyncGenerator instances.
25.5.1.1
AsyncGenerator.prototype.constructor
The initial value of
AsyncGenerator.prototype.constructor
is the intrinsic object
%AsyncGenerator%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.5.1.2
AsyncGenerator.prototype.next (
value
Let
generator
be the
this
value.
Let
completion
be
NormalCompletion
value
).
Return !
AsyncGeneratorEnqueue
generator
completion
).
25.5.1.3
AsyncGenerator.prototype.return (
value
Let
generator
be the
this
value.
Let
completion
be
Completion
{ [[Type]]:
return
, [[Value]]:
value
, [[Target]]:
empty
}.
Return !
AsyncGeneratorEnqueue
generator
completion
).
25.5.1.4
AsyncGenerator.prototype.throw (
exception
Let
generator
be the
this
value.
Let
completion
be
ThrowCompletion
exception
).
Return !
AsyncGeneratorEnqueue
generator
completion
).
25.5.1.5
AsyncGenerator.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"AsyncGenerator"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.5.2
Properties of AsyncGenerator Instances
AsyncGenerator instances are initially created with the internal slots described below:
Table 72: Internal Slots of AsyncGenerator Instances
Internal Slot
Description
[[AsyncGeneratorState]]
The current execution state of the async generator. The possible values are:
undefined
"suspendedStart"
"suspendedYield"
"executing"
"awaiting-return"
, and
"completed"
[[AsyncGeneratorContext]]
The
execution context
that is used when executing the code of this async generator.
[[AsyncGeneratorQueue]]
List
of AsyncGeneratorRequest records which represent requests to resume the async generator.
25.5.3
AsyncGenerator Abstract Operations
25.5.3.1
AsyncGeneratorRequest Records
The AsyncGeneratorRequest is a
Record
value used to store information about how an async generator should be
resumed and contains capabilities for fulfilling or rejecting the
corresponding promise.
They have the following fields:
Table 73: AsyncGeneratorRequest
Record
Fields
Field Name
Value
Meaning
[[Completion]]
Completion
record
The completion which should be used to resume the async generator.
[[Capability]]
A PromiseCapability record
The promise capabilities associated with this request.
25.5.3.2
AsyncGeneratorStart (
generator
generatorBody
Assert
generator
is an AsyncGenerator instance.
Assert
generator
.[[AsyncGeneratorState]] is
undefined
Let
genContext
be the
running execution context
Set the Generator component of
genContext
to
generator
Set the code evaluation state of
genContext
such that when evaluation is resumed for that
execution context
the following steps will be performed:
Let
result
be the result of evaluating
generatorBody
Assert
: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
Remove
genContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
Set
generator
.[[AsyncGeneratorState]] to
"completed"
If
result
is a normal completion, let
resultValue
be
undefined
Else,
Let
resultValue
be
result
.[[Value]].
If
result
.[[Type]] is not
return
, then
Return !
AsyncGeneratorReject
generator
resultValue
).
Return !
AsyncGeneratorResolve
generator
resultValue
true
).
Set
generator
.[[AsyncGeneratorContext]] to
genContext
Set
generator
.[[AsyncGeneratorState]] to
"suspendedStart"
Set
generator
.[[AsyncGeneratorQueue]] to a new empty
List
Return
undefined
25.5.3.3
AsyncGeneratorResolve (
generator
value
done
Assert
generator
is an AsyncGenerator instance.
Let
queue
be
generator
.[[AsyncGeneratorQueue]].
Assert
queue
is not an empty
List
Remove the first element from
queue
and let
next
be the value of that element.
Let
promiseCapability
be
next
.[[Capability]].
Let
iteratorResult
be !
CreateIterResultObject
value
done
).
Perform !
Call
promiseCapability
.[[Resolve]],
undefined
, «
iteratorResult
»).
Perform !
AsyncGeneratorResumeNext
generator
).
Return
undefined
25.5.3.4
AsyncGeneratorReject (
generator
exception
Assert
generator
is an AsyncGenerator instance.
Let
queue
be
generator
.[[AsyncGeneratorQueue]].
Assert
queue
is not an empty
List
Remove the first element from
queue
and let
next
be the value of that element.
Let
promiseCapability
be
next
.[[Capability]].
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
exception
»).
Perform !
AsyncGeneratorResumeNext
generator
).
Return
undefined
25.5.3.5
AsyncGeneratorResumeNext (
generator
Assert
generator
is an AsyncGenerator instance.
Let
state
be
generator
.[[AsyncGeneratorState]].
Assert
state
is not
"executing"
If
state
is
"awaiting-return"
, return
undefined
Let
queue
be
generator
.[[AsyncGeneratorQueue]].
If
queue
is an empty
List
, return
undefined
Let
next
be the value of the first element of
queue
Assert
next
is an AsyncGeneratorRequest record.
Let
completion
be
next
.[[Completion]].
If
completion
is an
abrupt completion
, then
If
state
is
"suspendedStart"
, then
Set
generator
.[[AsyncGeneratorState]] to
"completed"
Set
state
to
"completed"
If
state
is
"completed"
, then
If
completion
.[[Type]] is
return
, then
Set
generator
.[[AsyncGeneratorState]] to
"awaiting-return"
Let
promise
be ?
PromiseResolve
%Promise%
, «
completion
.[[Value]] »).
Let
stepsFulfilled
be the algorithm steps defined in
AsyncGeneratorResumeNext Return Processor Fulfilled Functions
Let
onFulfilled
be
CreateBuiltinFunction
stepsFulfilled
, « [[Generator]] »).
Set
onFulfilled
.[[Generator]] to
generator
Let
stepsRejected
be the algorithm steps defined in
AsyncGeneratorResumeNext Return Processor Rejected Functions
Let
onRejected
be
CreateBuiltinFunction
stepsRejected
, « [[Generator]] »).
Set
onRejected
.[[Generator]] to
generator
Perform !
PerformPromiseThen
promise
onFulfilled
onRejected
).
Return
undefined
Else,
Assert
completion
.[[Type]] is
throw
Perform !
AsyncGeneratorReject
generator
completion
.[[Value]]).
Return
undefined
Else if
state
is
"completed"
, return !
AsyncGeneratorResolve
generator
undefined
true
).
Assert
state
is either
"suspendedStart"
or
"suspendedYield"
Let
genContext
be
generator
.[[AsyncGeneratorContext]].
Let
callerContext
be the
running execution context
Suspend
callerContext
Set
generator
.[[AsyncGeneratorState]] to
"executing"
Push
genContext
onto the
execution context stack
genContext
is now the
running execution context
Resume the suspended evaluation of
genContext
using
completion
as the result of the operation that suspended it. Let
result
be the completion record returned by the resumed computation.
Assert
result
is never an
abrupt completion
Assert
: When we return here,
genContext
has already been removed from the
execution context stack
and
callerContext
is the currently
running execution context
Return
undefined
25.5.3.5.1
AsyncGeneratorResumeNext Return Processor Fulfilled Functions
An
AsyncGeneratorResumeNext
return processor fulfilled function is an anonymous built-in function that is used as part of the
AsyncGeneratorResumeNext
specification device to unwrap promises passed in to the
AsyncGenerator.prototype.return (
value
method. Each
AsyncGeneratorResumeNext
return processor fulfilled function has a [[Generator]] internal slot.
When an
AsyncGeneratorResumeNext
return processor fulfilled function is called with argument
value
, the following steps are taken:
Let
be the
active function object
Set
.[[Generator]].[[AsyncGeneratorState]] to
"completed"
Return !
AsyncGeneratorResolve
.[[Generator]],
value
true
).
The
"length"
property of an
AsyncGeneratorResumeNext
return processor fulfilled function is 1.
25.5.3.5.2
AsyncGeneratorResumeNext Return Processor Rejected Functions
An
AsyncGeneratorResumeNext
return processor rejected function is an anonymous built-in function that is used as part of the
AsyncGeneratorResumeNext
specification device to unwrap promises passed in to the
AsyncGenerator.prototype.return (
value
method. Each
AsyncGeneratorResumeNext
return processor rejected function has a [[Generator]] internal slot.
When an
AsyncGeneratorResumeNext
return processor rejected function is called with argument
reason
, the following steps are taken:
Let
be the
active function object
Set
.[[Generator]].[[AsyncGeneratorState]] to
"completed"
Return !
AsyncGeneratorReject
.[[Generator]],
reason
).
The
"length"
property of an
AsyncGeneratorResumeNext
return processor rejected function is 1.
25.5.3.6
AsyncGeneratorEnqueue (
generator
completion
Assert
completion
is a
Completion Record
Let
promiseCapability
be !
NewPromiseCapability
%Promise%
).
If
Type
generator
) is not Object, or if
generator
does not have an [[AsyncGeneratorState]] internal slot, then
Let
badGeneratorError
be a newly created
TypeError
object.
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
badGeneratorError
»).
Return
promiseCapability
.[[Promise]].
Let
queue
be
generator
.[[AsyncGeneratorQueue]].
Let
request
be AsyncGeneratorRequest { [[Completion]]:
completion
, [[Capability]]:
promiseCapability
}.
Append
request
to the end of
queue
Let
state
be
generator
.[[AsyncGeneratorState]].
If
state
is not
"executing"
, then
Perform !
AsyncGeneratorResumeNext
generator
).
Return
promiseCapability
.[[Promise]].
25.5.3.7
AsyncGeneratorYield (
value
The abstract operation AsyncGeneratorYield with argument
value
performs the following steps:
Let
genContext
be the
running execution context
Assert
genContext
is the
execution context
of a generator.
Let
generator
be the value of the Generator component of
genContext
Assert
GetGeneratorKind
() is
async
Set
value
to ?
Await
value
).
Set
generator
.[[AsyncGeneratorState]] to
"suspendedYield"
Remove
genContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
Set the code evaluation state of
genContext
such that when evaluation is resumed with a
Completion
resumptionValue
the following steps will be performed:
If
resumptionValue
.[[Type]] is not
return
, return
Completion
resumptionValue
).
Let
awaited
be
Await
resumptionValue
.[[Value]]).
If
awaited
.[[Type]] is
throw
, return
Completion
awaited
).
Assert
awaited
.[[Type]] is
normal
Return
Completion
{ [[Type]]:
return
, [[Value]]:
awaited
.[[Value]], [[Target]]:
empty
}.
NOTE: When one of the above steps returns, it returns to the evaluation of the
YieldExpression
production that originally called this abstract operation.
Return !
AsyncGeneratorResolve
generator
value
false
).
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of
genContext
25.6
Promise Objects
A Promise is an object that is used as a placeholder for the
eventual results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states:
fulfilled
rejected
, and
pending
A promise
is fulfilled if
p.then(f, r)
will immediately enqueue a Job to call the function
A promise
is rejected if
p.then(f, r)
will immediately enqueue a Job to call the function
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be
settled
if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is
resolved
if it is settled or if it has
been “locked in” to match the state of another promise. Attempting to
resolve or reject a resolved promise has no effect. A promise is
unresolved
if it is not resolved. An unresolved promise is always in the pending
state. A resolved promise may be pending, fulfilled or rejected.
25.6.1
Promise Abstract Operations
25.6.1.1
PromiseCapability Records
A PromiseCapability is a
Record
value used to encapsulate a promise object along with the functions
that are capable of resolving or rejecting that promise object.
PromiseCapability Records are produced by the
NewPromiseCapability
abstract operation.
PromiseCapability Records have the fields listed in
Table 74
Table 74: PromiseCapability
Record
Fields
Field Name
Value
Meaning
[[Promise]]
An object
An object that is usable as a promise.
[[Resolve]]
function object
The function that is used to resolve the given promise object.
[[Reject]]
function object
The function that is used to reject the given promise object.
25.6.1.1.1
IfAbruptRejectPromise (
value
capability
IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use a PromiseCapability
Record
. An algorithm step of the form:
IfAbruptRejectPromise
value
capability
).
means the same thing as:
If
value
is an
abrupt completion
, then
Perform ?
Call
capability
.[[Reject]],
undefined
, «
value
.[[Value]] »).
Return
capability
.[[Promise]].
Else if
value
is a
Completion Record
, set
value
to
value
.[[Value]].
25.6.1.2
PromiseReaction Records
The PromiseReaction is a
Record
value used to store information about how a promise should react when
it becomes resolved or rejected with a given value. PromiseReaction
records are created by the
PerformPromiseThen
abstract operation, and are used by a
PromiseReactionJob
PromiseReaction records have the fields listed in
Table 75
Table 75: PromiseReaction
Record
Fields
Field Name
Value
Meaning
[[Capability]]
A PromiseCapability
Record
, or
undefined
The capabilities of the promise for which this record provides a reaction handler.
[[Type]]
Either
"Fulfill"
or
"Reject"
The [[Type]] is used when [[Handler]] is
undefined
to allow for behaviour specific to the settlement type.
[[Handler]]
function object
or
undefined
The function that should be applied to the incoming
value, and whose return value will govern what happens to the derived
promise. If [[Handler]] is
undefined
, a function that depends on the value of [[Type]] will be used instead.
25.6.1.3
CreateResolvingFunctions (
promise
When CreateResolvingFunctions is performed with argument
promise
, the following steps are taken:
Let
alreadyResolved
be a new
Record
{ [[Value]]:
false
}.
Let
stepsResolve
be the algorithm steps defined in Promise Resolve Functions (
25.6.1.3.2
).
Let
resolve
be
CreateBuiltinFunction
stepsResolve
, « [[Promise]], [[AlreadyResolved]] »).
Set
resolve
.[[Promise]] to
promise
Set
resolve
.[[AlreadyResolved]] to
alreadyResolved
Let
stepsReject
be the algorithm steps defined in Promise Reject Functions (
25.6.1.3.1
).
Let
reject
be
CreateBuiltinFunction
stepsReject
, « [[Promise]], [[AlreadyResolved]] »).
Set
reject
.[[Promise]] to
promise
Set
reject
.[[AlreadyResolved]] to
alreadyResolved
Return a new
Record
{ [[Resolve]]:
resolve
, [[Reject]]:
reject
}.
25.6.1.3.1
Promise Reject Functions
A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.
When a promise reject function is called with argument
reason
, the following steps are taken:
Let
be the
active function object
Assert
has a [[Promise]] internal slot whose value is an Object.
Let
promise
be
.[[Promise]].
Let
alreadyResolved
be
.[[AlreadyResolved]].
If
alreadyResolved
.[[Value]] is
true
, return
undefined
Set
alreadyResolved
.[[Value]] to
true
Return
RejectPromise
promise
reason
).
The
"length"
property of a promise reject function is 1.
25.6.1.3.2
Promise Resolve Functions
A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.
When a promise resolve function is called with argument
resolution
, the following steps are taken:
Let
be the
active function object
Assert
has a [[Promise]] internal slot whose value is an Object.
Let
promise
be
.[[Promise]].
Let
alreadyResolved
be
.[[AlreadyResolved]].
If
alreadyResolved
.[[Value]] is
true
, return
undefined
Set
alreadyResolved
.[[Value]] to
true
If
SameValue
resolution
promise
) is
true
, then
Let
selfResolutionError
be a newly created
TypeError
object.
Return
RejectPromise
promise
selfResolutionError
).
If
Type
resolution
) is not Object, then
Return
FulfillPromise
promise
resolution
).
Let
then
be
Get
resolution
"then"
).
If
then
is an
abrupt completion
, then
Return
RejectPromise
promise
then
.[[Value]]).
Let
thenAction
be
then
.[[Value]].
If
IsCallable
thenAction
) is
false
, then
Return
FulfillPromise
promise
resolution
).
Perform
EnqueueJob
"PromiseJobs"
PromiseResolveThenableJob
, «
promise
resolution
thenAction
»).
Return
undefined
The
"length"
property of a promise resolve function is 1.
25.6.1.4
FulfillPromise (
promise
value
When the FulfillPromise abstract operation is called with arguments
promise
and
value
, the following steps are taken:
Assert
: The value of
promise
.[[PromiseState]] is
"pending"
Let
reactions
be
promise
.[[PromiseFulfillReactions]].
Set
promise
.[[PromiseResult]] to
value
Set
promise
.[[PromiseFulfillReactions]] to
undefined
Set
promise
.[[PromiseRejectReactions]] to
undefined
Set
promise
.[[PromiseState]] to
"fulfilled"
Return
TriggerPromiseReactions
reactions
value
).
25.6.1.5
NewPromiseCapability (
The abstract operation NewPromiseCapability takes a
constructor
function, and attempts to use that
constructor
function in the fashion of the built-in
Promise
constructor
to create a Promise object and extract its resolve and reject
functions. The promise plus the resolve and reject functions are used to
initialize a new PromiseCapability
Record
which is returned as the value of this abstract operation.
If
IsConstructor
) is
false
, throw a
TypeError
exception.
NOTE:
is assumed to be a
constructor
function that supports the parameter conventions of the
Promise
constructor
(see
25.6.3.1
).
Let
promiseCapability
be a new PromiseCapability { [[Promise]]:
undefined
, [[Resolve]]:
undefined
, [[Reject]]:
undefined
}.
Let
steps
be the algorithm steps defined in
GetCapabilitiesExecutor Functions
Let
executor
be
CreateBuiltinFunction
steps
, « [[Capability]] »).
Set
executor
.[[Capability]] to
promiseCapability
Let
promise
be ?
Construct
, «
executor
»).
If
IsCallable
promiseCapability
.[[Resolve]]) is
false
, throw a
TypeError
exception.
If
IsCallable
promiseCapability
.[[Reject]]) is
false
, throw a
TypeError
exception.
Set
promiseCapability
.[[Promise]] to
promise
Return
promiseCapability
Note
This abstract operation supports Promise subclassing, as it is generic on any
constructor
that calls a passed executor function argument in the same way as the Promise
constructor
. It is used to generalize static methods of the Promise
constructor
to any subclass.
25.6.1.5.1
GetCapabilitiesExecutor Functions
A GetCapabilitiesExecutor function is an anonymous built-in function that has a [[Capability]] internal slot.
When a GetCapabilitiesExecutor function is called with arguments
resolve
and
reject
, the following steps are taken:
Let
be the
active function object
Assert
has a [[Capability]] internal slot whose value is a PromiseCapability
Record
Let
promiseCapability
be
.[[Capability]].
If
promiseCapability
.[[Resolve]] is not
undefined
, throw a
TypeError
exception.
If
promiseCapability
.[[Reject]] is not
undefined
, throw a
TypeError
exception.
Set
promiseCapability
.[[Resolve]] to
resolve
Set
promiseCapability
.[[Reject]] to
reject
Return
undefined
The
"length"
property of a GetCapabilitiesExecutor function is 2.
25.6.1.6
IsPromise (
The abstract operation IsPromise checks for the promise brand on an object.
If
Type
) is not Object, return
false
If
does not have a [[PromiseState]] internal slot, return
false
Return
true
25.6.1.7
RejectPromise (
promise
reason
When the RejectPromise abstract operation is called with arguments
promise
and
reason
, the following steps are taken:
Assert
: The value of
promise
.[[PromiseState]] is
"pending"
Let
reactions
be
promise
.[[PromiseRejectReactions]].
Set
promise
.[[PromiseResult]] to
reason
Set
promise
.[[PromiseFulfillReactions]] to
undefined
Set
promise
.[[PromiseRejectReactions]] to
undefined
Set
promise
.[[PromiseState]] to
"rejected"
If
promise
.[[PromiseIsHandled]] is
false
, perform
HostPromiseRejectionTracker
promise
"reject"
).
Return
TriggerPromiseReactions
reactions
reason
).
25.6.1.8
TriggerPromiseReactions (
reactions
argument
The abstract operation TriggerPromiseReactions takes a
collection of PromiseReactionRecords and enqueues a new Job for each
record. Each such Job processes the [[Type]] and [[Handler]] of the
PromiseReactionRecord, and if the [[Handler]] is a function, calls it
passing the given argument. If the [[Handler]] is
undefined
, the behaviour is determined by the [[Type]].
For each
reaction
in
reactions
, in original insertion order, do
Perform
EnqueueJob
"PromiseJobs"
PromiseReactionJob
, «
reaction
argument
»).
Return
undefined
25.6.1.9
HostPromiseRejectionTracker (
promise
operation
HostPromiseRejectionTracker is an implementation-defined
abstract operation that allows host environments to track promise
rejections.
An implementation of HostPromiseRejectionTracker must
complete normally in all cases. The default implementation of
HostPromiseRejectionTracker is to unconditionally return an empty normal
completion.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its
operation
argument set to
"reject"
When a handler is added to a rejected promise for the first time, it is called with its
operation
argument set to
"handle"
A typical implementation of HostPromiseRejectionTracker
might try to notify developers of unhandled rejections, while also being
careful to notify them if such previous notifications are later
invalidated by new handlers being attached.
Note 2
If
operation
is
"handle"
, an implementation should not hold a reference to
promise
in a way that would interfere with garbage collection. An implementation may hold a reference to
promise
if
operation
is
"reject"
, since it is expected that rejections will be rare and not on hot code paths.
25.6.2
Promise Jobs
25.6.2.1
PromiseReactionJob (
reaction
argument
The job PromiseReactionJob with parameters
reaction
and
argument
applies the appropriate handler to the incoming value, and uses the
handler's return value to resolve or reject the derived promise
associated with that handler.
Assert
reaction
is a PromiseReaction
Record
Let
promiseCapability
be
reaction
.[[Capability]].
Let
type
be
reaction
.[[Type]].
Let
handler
be
reaction
.[[Handler]].
If
handler
is
undefined
, then
If
type
is
"Fulfill"
, let
handlerResult
be
NormalCompletion
argument
).
Else,
Assert
type
is
"Reject"
Let
handlerResult
be
ThrowCompletion
argument
).
Else, let
handlerResult
be
Call
handler
undefined
, «
argument
»).
If
promiseCapability
is
undefined
, then
Assert
handlerResult
is not an
abrupt completion
Return
NormalCompletion
empty
).
If
handlerResult
is an
abrupt completion
, then
Let
status
be
Call
promiseCapability
.[[Reject]],
undefined
, «
handlerResult
.[[Value]] »).
Else,
Let
status
be
Call
promiseCapability
.[[Resolve]],
undefined
, «
handlerResult
.[[Value]] »).
Return
Completion
status
).
25.6.2.2
PromiseResolveThenableJob (
promiseToResolve
thenable
then
The job PromiseResolveThenableJob with parameters
promiseToResolve
thenable
, and
then
performs the following steps:
Let
resolvingFunctions
be
CreateResolvingFunctions
promiseToResolve
).
Let
thenCallResult
be
Call
then
thenable
, «
resolvingFunctions
.[[Resolve]],
resolvingFunctions
.[[Reject]] »).
If
thenCallResult
is an
abrupt completion
, then
Let
status
be
Call
resolvingFunctions
.[[Reject]],
undefined
, «
thenCallResult
.[[Value]] »).
Return
Completion
status
).
Return
Completion
thenCallResult
).
Note
This Job uses the supplied thenable and its
then
method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the
then
method occurs after evaluation of any surrounding code has completed.
25.6.3
The Promise Constructor
The Promise
constructor
is the intrinsic object
%Promise%
is the initial value of the
Promise
property of the
global object
creates and initializes a new Promise object when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an
extends
clause of a class definition. Subclass constructors that intend to inherit the specified
Promise
behaviour must include a
super
call to the
Promise
constructor
to create and initialize the subclass instance with the internal state necessary to support the
Promise
and
Promise.prototype
built-in methods.
25.6.3.1
Promise (
executor
When the
Promise
function is called with argument
executor
, the following steps are taken:
If NewTarget is
undefined
, throw a
TypeError
exception.
If
IsCallable
executor
) is
false
, throw a
TypeError
exception.
Let
promise
be ?
OrdinaryCreateFromConstructor
(NewTarget,
"%PromisePrototype%"
, « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
Set
promise
.[[PromiseState]] to
"pending"
Set
promise
.[[PromiseFulfillReactions]] to a new empty
List
Set
promise
.[[PromiseRejectReactions]] to a new empty
List
Set
promise
.[[PromiseIsHandled]] to
false
Let
resolvingFunctions
be
CreateResolvingFunctions
promise
).
Let
completion
be
Call
executor
undefined
, «
resolvingFunctions
.[[Resolve]],
resolvingFunctions
.[[Reject]] »).
If
completion
is an
abrupt completion
, then
Perform ?
Call
resolvingFunctions
.[[Reject]],
undefined
, «
completion
.[[Value]] »).
Return
promise
Note
The
executor
argument must be a
function object
It is called for initiating and reporting completion of the possibly
deferred action represented by this Promise object. The executor is
called with two arguments:
resolve
and
reject
. These are functions that may be used by the
executor
function to report eventual completion or failure of the deferred
computation. Returning from the executor function does not mean that the
deferred action has been completed but only that the request to
eventually perform the deferred action has been accepted.
The
resolve
function that is passed to an
executor
function accepts a single argument. The
executor
code may eventually call the
resolve
function to indicate that it wishes to resolve the associated Promise object. The argument passed to the
resolve
function represents the eventual value of the deferred action and can
be either the actual fulfillment value or another Promise object which
will provide the value if it is fulfilled.
The
reject
function that is passed to an
executor
function accepts a single argument. The
executor
code may eventually call the
reject
function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the
reject
function is used as the rejection value of the promise. Typically it will be an
Error
object.
The resolve and reject functions passed to an
executor
function by the Promise
constructor
have the capability to actually resolve and reject the associated promise. Subclasses may have different
constructor
behaviour that passes in customized values for resolve and reject.
25.6.4
Properties of the Promise Constructor
The Promise
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
has the following properties:
25.6.4.1
Promise.all (
iterable
The
all
function returns a new promise which is
fulfilled with an array of fulfillment values for the passed promises,
or rejects with the reason of the first passed promise that rejects. It
resolves all elements of the passed iterable to promises as it runs this
algorithm.
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
promiseCapability
be ?
NewPromiseCapability
).
Let
iteratorRecord
be
GetIterator
iterable
).
IfAbruptRejectPromise
iteratorRecord
promiseCapability
).
Let
result
be
PerformPromiseAll
iteratorRecord
promiseCapability
).
If
result
is an
abrupt completion
, then
If
iteratorRecord
.[[Done]] is
false
, set
result
to
IteratorClose
iteratorRecord
result
).
IfAbruptRejectPromise
result
promiseCapability
).
Return
Completion
result
).
This function is the
%Promise_all%
intrinsic object.
Note
The
all
function requires its
this
value to be a
constructor
function that supports the parameter conventions of the
Promise
constructor
25.6.4.1.1
Runtime Semantics: PerformPromiseAll (
iteratorRecord
constructor
resultCapability
When the PerformPromiseAll abstract operation is called with arguments
iteratorRecord
constructor
, and
resultCapability
, the following steps are taken:
Assert
IsConstructor
constructor
) is
true
Assert
resultCapability
is a PromiseCapability
Record
Let
values
be a new empty
List
Let
remainingElementsCount
be a new
Record
{ [[Value]]: 1 }.
Let
index
be 0.
Repeat,
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, then
Set
iteratorRecord
.[[Done]] to
true
Set
remainingElementsCount
.[[Value]] to
remainingElementsCount
.[[Value]] - 1.
If
remainingElementsCount
.[[Value]] is 0, then
Let
valuesArray
be
CreateArrayFromList
values
).
Perform ?
Call
resultCapability
.[[Resolve]],
undefined
, «
valuesArray
»).
Return
resultCapability
.[[Promise]].
Let
nextValue
be
IteratorValue
next
).
If
nextValue
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
nextValue
).
Append
undefined
to
values
Let
nextPromise
be ?
Invoke
constructor
"resolve"
, «
nextValue
»).
Let
steps
be the algorithm steps defined in
Promise.all
Resolve Element Functions
Let
resolveElement
be
CreateBuiltinFunction
steps
, « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
Set
resolveElement
.[[AlreadyCalled]] to a new
Record
{ [[Value]]:
false
}.
Set
resolveElement
.[[Index]] to
index
Set
resolveElement
.[[Values]] to
values
Set
resolveElement
.[[Capability]] to
resultCapability
Set
resolveElement
.[[RemainingElements]] to
remainingElementsCount
Set
remainingElementsCount
.[[Value]] to
remainingElementsCount
.[[Value]] + 1.
Perform ?
Invoke
nextPromise
"then"
, «
resolveElement
resultCapability
.[[Reject]] »).
Increase
index
by 1.
25.6.4.1.2
Promise.all
Resolve Element Functions
Promise.all
resolve element function is an anonymous built-in function that is used to resolve a specific
Promise.all
element. Each
Promise.all
resolve element function has [[Index]], [[Values]], [[Capability]],
[[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a
Promise.all
resolve element function is called with argument
, the following steps are taken:
Let
be the
active function object
Let
alreadyCalled
be
.[[AlreadyCalled]].
If
alreadyCalled
.[[Value]] is
true
, return
undefined
Set
alreadyCalled
.[[Value]] to
true
Let
index
be
.[[Index]].
Let
values
be
.[[Values]].
Let
promiseCapability
be
.[[Capability]].
Let
remainingElementsCount
be
.[[RemainingElements]].
Set
values
index
] to
Set
remainingElementsCount
.[[Value]] to
remainingElementsCount
.[[Value]] - 1.
If
remainingElementsCount
.[[Value]] is 0, then
Let
valuesArray
be
CreateArrayFromList
values
).
Return ?
Call
promiseCapability
.[[Resolve]],
undefined
, «
valuesArray
»).
Return
undefined
The
"length"
property of a
Promise.all
resolve element function is 1.
25.6.4.2
Promise.prototype
The initial value of
Promise.prototype
is the intrinsic object
%PromisePrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
25.6.4.3
Promise.race (
iterable
The
race
function returns a new promise which is
settled in the same way as the first passed promise to settle. It
resolves all elements of the passed
iterable
to promises as it runs this algorithm.
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
promiseCapability
be ?
NewPromiseCapability
).
Let
iteratorRecord
be
GetIterator
iterable
).
IfAbruptRejectPromise
iteratorRecord
promiseCapability
).
Let
result
be
PerformPromiseRace
iteratorRecord
promiseCapability
).
If
result
is an
abrupt completion
, then
If
iteratorRecord
.[[Done]] is
false
, set
result
to
IteratorClose
iteratorRecord
result
).
IfAbruptRejectPromise
result
promiseCapability
).
Return
Completion
result
).
Note 1
If the
iterable
argument is empty or if none of the promises in
iterable
ever settle then the pending promise returned by this method will never be settled.
Note 2
The
race
function expects its
this
value to be a
constructor
function that supports the parameter conventions of the
Promise
constructor
. It also expects that its
this
value provides a
resolve
method.
25.6.4.3.1
Runtime Semantics: PerformPromiseRace (
iteratorRecord
constructor
resultCapability
When the PerformPromiseRace abstract operation is called with arguments
iteratorRecord
constructor
, and
resultCapability
, the following steps are taken:
Assert
IsConstructor
constructor
) is
true
Assert
resultCapability
is a PromiseCapability
Record
Repeat,
Let
next
be
IteratorStep
iteratorRecord
).
If
next
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
next
).
If
next
is
false
, then
Set
iteratorRecord
.[[Done]] to
true
Return
resultCapability
.[[Promise]].
Let
nextValue
be
IteratorValue
next
).
If
nextValue
is an
abrupt completion
, set
iteratorRecord
.[[Done]] to
true
ReturnIfAbrupt
nextValue
).
Let
nextPromise
be ?
Invoke
constructor
"resolve"
, «
nextValue
»).
Perform ?
Invoke
nextPromise
"then"
, «
resultCapability
.[[Resolve]],
resultCapability
.[[Reject]] »).
25.6.4.4
Promise.reject (
The
reject
function returns a new promise rejected with the passed argument.
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Let
promiseCapability
be ?
NewPromiseCapability
).
Perform ?
Call
promiseCapability
.[[Reject]],
undefined
, «
»).
Return
promiseCapability
.[[Promise]].
This function is the
%Promise_reject%
intrinsic object.
Note
The
reject
function expects its
this
value to be a
constructor
function that supports the parameter conventions of the
Promise
constructor
25.6.4.5
Promise.resolve (
The
resolve
function returns either a new
promise resolved with the passed argument, or the argument itself if the
argument is a promise produced by this
constructor
Let
be the
this
value.
If
Type
) is not Object, throw a
TypeError
exception.
Return ?
PromiseResolve
).
This function is the
%Promise_resolve%
intrinsic object.
Note
The
resolve
function expects its
this
value to be a
constructor
function that supports the parameter conventions of the
Promise
constructor
25.6.4.5.1
PromiseResolve (
The abstract operation PromiseResolve, given a
constructor
and a value, returns a new promise resolved with that value.
Assert
Type
) is Object.
If
IsPromise
) is
true
, then
Let
xConstructor
be ?
Get
"constructor"
).
If
SameValue
xConstructor
) is
true
, return
Let
promiseCapability
be ?
NewPromiseCapability
).
Perform ?
Call
promiseCapability
.[[Resolve]],
undefined
, «
»).
Return
promiseCapability
.[[Promise]].
25.6.4.6
get Promise [ @@species ]
Promise[@@species]
is an
accessor property
whose set accessor function is
undefined
. Its get accessor function performs the following steps:
Return the
this
value.
The value of the
name
property of this function is
"get [Symbol.species]"
Note
Promise prototype methods normally use their
this
object's
constructor
to create a derived object. However, a subclass
constructor
may over-ride that default behaviour by redefining its @@species property.
25.6.5
Properties of the Promise Prototype Object
The Promise prototype object:
is the intrinsic object
%PromisePrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is an ordinary object.
does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.
25.6.5.1
Promise.prototype.catch (
onRejected
When the
catch
method is called with argument
onRejected
, the following steps are taken:
Let
promise
be the
this
value.
Return ?
Invoke
promise
"then"
, «
undefined
onRejected
»).
25.6.5.2
Promise.prototype.constructor
The initial value of
Promise.prototype.constructor
is the intrinsic object
%Promise%
25.6.5.3
Promise.prototype.finally (
onFinally
When the
finally
method is called with argument
onFinally
, the following steps are taken:
Let
promise
be the
this
value.
If
Type
promise
) is not Object, throw a
TypeError
exception.
Let
be ?
SpeciesConstructor
promise
%Promise%
).
Assert
IsConstructor
) is
true
If
IsCallable
onFinally
) is
false
, then
Let
thenFinally
be
onFinally
Let
catchFinally
be
onFinally
Else,
Let
stepsThenFinally
be the algorithm steps defined in
Then Finally Functions
Let
thenFinally
be
CreateBuiltinFunction
stepsThenFinally
, « [[Constructor]], [[OnFinally]] »).
Set
thenFinally
.[[Constructor]] to
Set
thenFinally
.[[OnFinally]] to
onFinally
Let
stepsCatchFinally
be the algorithm steps defined in
Catch Finally Functions
Let
catchFinally
be
CreateBuiltinFunction
stepsCatchFinally
, « [[Constructor]], [[OnFinally]] »).
Set
catchFinally
.[[Constructor]] to
Set
catchFinally
.[[OnFinally]] to
onFinally
Return ?
Invoke
promise
"then"
, «
thenFinally
catchFinally
»).
25.6.5.3.1
Then Finally Functions
A Then Finally function is an anonymous built-in function
that has a [[Constructor]] and an [[OnFinally]] internal slot. The value
of the [[Constructor]] internal slot is a
Promise
-like
constructor
function object
, and the value of the [[OnFinally]] internal slot is a
function object
When a Then Finally function is called with argument
value
, the following steps are taken:
Let
be the
active function object
Let
onFinally
be
.[[OnFinally]].
Assert
IsCallable
onFinally
) is
true
Let
result
be ?
Call
onFinally
undefined
).
Let
be
.[[Constructor]].
Assert
IsConstructor
) is
true
Let
promise
be ?
PromiseResolve
result
).
Let
valueThunk
be equivalent to a function that returns
value
Return ?
Invoke
promise
"then"
, «
valueThunk
»).
The
"length"
property of a Then Finally function is
25.6.5.3.2
Catch Finally Functions
A Catch Finally function is an anonymous built-in function
that has a [[Constructor]] and an [[OnFinally]] internal slot. The value
of the [[Constructor]] internal slot is a
Promise
-like
constructor
function object
, and the value of the [[OnFinally]] internal slot is a
function object
When a Catch Finally function is called with argument
reason
, the following steps are taken:
Let
be the
active function object
Let
onFinally
be
.[[OnFinally]].
Assert
IsCallable
onFinally
) is
true
Let
result
be ?
Call
onFinally
undefined
).
Let
be
.[[Constructor]].
Assert
IsConstructor
) is
true
Let
promise
be ?
PromiseResolve
result
).
Let
thrower
be equivalent to a function that throws
reason
Return ?
Invoke
promise
"then"
, «
thrower
»).
The
"length"
property of a Catch Finally function is
25.6.5.4
Promise.prototype.then (
onFulfilled
onRejected
When the
then
method is called with arguments
onFulfilled
and
onRejected
, the following steps are taken:
Let
promise
be the
this
value.
If
IsPromise
promise
) is
false
, throw a
TypeError
exception.
Let
be ?
SpeciesConstructor
promise
%Promise%
).
Let
resultCapability
be ?
NewPromiseCapability
).
Return
PerformPromiseThen
promise
onFulfilled
onRejected
resultCapability
).
This function is the
%PromiseProto_then%
intrinsic object.
25.6.5.4.1
PerformPromiseThen (
promise
onFulfilled
onRejected
[ ,
resultCapability
] )
The abstract operation PerformPromiseThen performs the “then” operation on
promise
using
onFulfilled
and
onRejected
as its settlement actions. If
resultCapability
is passed, the result is stored by updating
resultCapability
's
promise. (If it is not passed, then PerformPromiseThen is being called
by a specification-internal operation where the result does not matter.)
Assert
IsPromise
promise
) is
true
If
resultCapability
is present, then
Assert
resultCapability
is a PromiseCapability
Record
Else,
Set
resultCapability
to
undefined
If
IsCallable
onFulfilled
) is
false
, then
Set
onFulfilled
to
undefined
If
IsCallable
onRejected
) is
false
, then
Set
onRejected
to
undefined
Let
fulfillReaction
be the PromiseReaction { [[Capability]]:
resultCapability
, [[Type]]:
"Fulfill"
, [[Handler]]:
onFulfilled
}.
Let
rejectReaction
be the PromiseReaction { [[Capability]]:
resultCapability
, [[Type]]:
"Reject"
, [[Handler]]:
onRejected
}.
If
promise
.[[PromiseState]] is
"pending"
, then
Append
fulfillReaction
as the last element of the
List
that is
promise
.[[PromiseFulfillReactions]].
Append
rejectReaction
as the last element of the
List
that is
promise
.[[PromiseRejectReactions]].
Else if
promise
.[[PromiseState]] is
"fulfilled"
, then
Let
value
be
promise
.[[PromiseResult]].
Perform
EnqueueJob
"PromiseJobs"
PromiseReactionJob
, «
fulfillReaction
value
»).
Else,
Assert
: The value of
promise
.[[PromiseState]] is
"rejected"
Let
reason
be
promise
.[[PromiseResult]].
If
promise
.[[PromiseIsHandled]] is
false
, perform
HostPromiseRejectionTracker
promise
"handle"
).
Perform
EnqueueJob
"PromiseJobs"
PromiseReactionJob
, «
rejectReaction
reason
»).
Set
promise
.[[PromiseIsHandled]] to
true
If
resultCapability
is
undefined
, then
Return
undefined
Else,
Return
resultCapability
.[[Promise]].
25.6.5.5
Promise.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the String value
"Promise"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.6.6
Properties of Promise Instances
Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic,
%PromisePrototype%
). Promise instances are initially created with the internal slots described in
Table 76
Table 76: Internal Slots of Promise Instances
Internal Slot
Description
[[PromiseState]]
A String value that governs how a promise will react to incoming calls to its
then
method. The possible values are:
"pending"
"fulfilled"
, and
"rejected"
[[PromiseResult]]
The value with which the promise has been fulfilled or
rejected, if any. Only meaningful if [[PromiseState]] is not
"pending"
[[PromiseFulfillReactions]]
List
of PromiseReaction records to be processed when/if the promise transitions from the
"pending"
state to the
"fulfilled"
state.
[[PromiseRejectReactions]]
List
of PromiseReaction records to be processed when/if the promise transitions from the
"pending"
state to the
"rejected"
state.
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a
fulfillment or rejection handler; used in unhandled rejection tracking.
25.7
AsyncFunction Objects
AsyncFunction objects are functions that are usually created by evaluating
AsyncFunctionDeclaration
s,
AsyncFunctionExpression
s,
AsyncMethod
s, and
AsyncArrowFunction
s. They may also be created by calling the
%AsyncFunction%
intrinsic.
25.7.1
The AsyncFunction Constructor
The AsyncFunction
constructor
is the intrinsic object
%AsyncFunction%
is a subclass of
Function
creates and initializes a new AsyncFunction object when called as a function rather than as a
constructor
. Thus the function call
AsyncFunction(…)
is equivalent to the object creation expression
new AsyncFunction(…)
with the same arguments.
is designed to be subclassable. It may be used as the value of an
extends
clause of a class definition. Subclass constructors that intend to
inherit the specified AsyncFunction behaviour must include a
super
call to the
AsyncFunction
constructor
to create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour.
25.7.1.1
AsyncFunction (
p1
p2
, … ,
pn
body
The last argument specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.
When the
AsyncFunction
function is called with some arguments
p1
p2
, …,
pn
body
(where
might be 0, that is, there are no
arguments, and where
body
might also not be provided), the following steps are taken:
Let
be the
active function object
Let
args
be the
argumentsList
that was passed to this function by [[Call]] or [[Construct]].
Return
CreateDynamicFunction
, NewTarget,
"async"
args
).
Note
See NOTE for
19.2.1.1
25.7.2
Properties of the AsyncFunction Constructor
The AsyncFunction
constructor
is a standard built-in
function object
that inherits from the
Function
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%Function%
has a
name
property whose value is
"AsyncFunction"
has the following properties:
25.7.2.1
AsyncFunction.length
This is a
data property
with a value of 1. This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.7.2.2
AsyncFunction.prototype
The initial value of
AsyncFunction.prototype
is the intrinsic object
%AsyncFunctionPrototype%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
25.7.3
Properties of the AsyncFunction Prototype Object
The AsyncFunction prototype object:
is an ordinary object.
is not a
function object
and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in
Table 27
is the value of the
prototype
property of the intrinsic object
%AsyncFunction%
is the intrinsic object
%AsyncFunctionPrototype%
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
25.7.3.1
AsyncFunction.prototype.constructor
The initial value of
AsyncFunction.prototype.constructor
is the intrinsic object
%AsyncFunction%
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.7.3.2
AsyncFunction.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is the string value
"AsyncFunction"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
true
}.
25.7.4
AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript
function object
and has the internal slots listed in
Table 27
. The value of the [[FunctionKind]] internal slot for all such instances is
"async"
AsyncFunction instances are not constructors and do not have a
[[Construct]] internal method. AsyncFunction instances do not have a
prototype property as they are not constructable.
Each AsyncFunction instance has the following own properties:
25.7.4.1
length
The specification for the
"length"
property of Function instances given in
19.2.4.1
also applies to AsyncFunction instances.
25.7.4.2
name
The specification for the
name
property of Function instances given in
19.2.4.2
also applies to AsyncFunction instances.
25.7.5
Async Functions Abstract Operations
25.7.5.1
AsyncFunctionStart (
promiseCapability
asyncFunctionBody
Let
runningContext
be the
running execution context
Let
asyncContext
be a copy of
runningContext
Set the code evaluation state of
asyncContext
such that when evaluation is resumed for that
execution context
the following steps will be performed:
Let
result
be the result of evaluating
asyncFunctionBody
Assert
If we return here, the async function either threw an exception or
performed an implicit or explicit return; all awaiting is done.
Remove
asyncContext
from the
execution context stack
and restore the
execution context
that is at the top of the
execution context stack
as the
running execution context
If
result
.[[Type]] is
normal
, then
Perform !
Call
promiseCapability
.[[Resolve]],
undefined
, «
undefined
»).
Else if
result
.[[Type]] is
return
, then
Perform !
Call
promiseCapability
.[[Resolve]],
undefined
, «
result
.[[Value]] »).
Else,
Assert
result
.[[Type]] is
throw
Perform !
Call
promiseCapability
.[[Reject]],
undefined
, «
result
.[[Value]] »).
Return.
Push
asyncContext
onto the
execution context stack
asyncContext
is now the
running execution context
Resume the suspended evaluation of
asyncContext
. Let
result
be the value returned by the resumed computation.
Assert
: When we return here,
asyncContext
has already been removed from the
execution context stack
and
runningContext
is the currently
running execution context
Assert
result
is a normal completion with a value of
undefined
. The possible sources of completion values are
Await
or, if the async function doesn't await anything, the step 3.g above.
Return.
26
Reflection
26.1
The Reflect Object
The Reflect object:
is the intrinsic object
%Reflect%
is the initial value of the
Reflect
property of the
global object
is an ordinary object.
has a [[Prototype]] internal slot whose value is the intrinsic object
%ObjectPrototype%
is not a
function object
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the
new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
26.1.1
Reflect.apply (
target
thisArgument
argumentsList
When the
apply
function is called with arguments
target
thisArgument
, and
argumentsList
, the following steps are taken:
If
IsCallable
target
) is
false
, throw a
TypeError
exception.
Let
args
be ?
CreateListFromArrayLike
argumentsList
).
Perform
PrepareForTailCall
().
Return ?
Call
target
thisArgument
args
).
26.1.2
Reflect.construct (
target
argumentsList
[ ,
newTarget
] )
When the
construct
function is called with arguments
target
argumentsList
, and
newTarget
, the following steps are taken:
If
IsConstructor
target
) is
false
, throw a
TypeError
exception.
If
newTarget
is not present, set
newTarget
to
target
Else if
IsConstructor
newTarget
) is
false
, throw a
TypeError
exception.
Let
args
be ?
CreateListFromArrayLike
argumentsList
).
Return ?
Construct
target
args
newTarget
).
26.1.3
Reflect.defineProperty (
target
propertyKey
attributes
When the
defineProperty
function is called with arguments
target
propertyKey
, and
attributes
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
Let
desc
be ?
ToPropertyDescriptor
attributes
).
Return ?
target
.[[DefineOwnProperty]](
key
desc
).
26.1.4
Reflect.deleteProperty (
target
propertyKey
When the
deleteProperty
function is called with arguments
target
and
propertyKey
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
Return ?
target
.[[Delete]](
key
).
26.1.5
Reflect.get (
target
propertyKey
[ ,
receiver
] )
When the
get
function is called with arguments
target
propertyKey
, and
receiver
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
If
receiver
is not present, then
Set
receiver
to
target
Return ?
target
.[[Get]](
key
receiver
).
26.1.6
Reflect.getOwnPropertyDescriptor (
target
propertyKey
When the
getOwnPropertyDescriptor
function is called with arguments
target
and
propertyKey
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
Let
desc
be ?
target
.[[GetOwnProperty]](
key
).
Return
FromPropertyDescriptor
desc
).
26.1.7
Reflect.getPrototypeOf (
target
When the
getPrototypeOf
function is called with argument
target
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Return ?
target
.[[GetPrototypeOf]]().
26.1.8
Reflect.has (
target
propertyKey
When the
has
function is called with arguments
target
and
propertyKey
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
Return ?
target
.[[HasProperty]](
key
).
26.1.9
Reflect.isExtensible (
target
When the
isExtensible
function is called with argument
target
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Return ?
target
.[[IsExtensible]]().
26.1.10
Reflect.ownKeys (
target
When the
ownKeys
function is called with argument
target
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
keys
be ?
target
.[[OwnPropertyKeys]]().
Return
CreateArrayFromList
keys
).
26.1.11
Reflect.preventExtensions (
target
When the
preventExtensions
function is called with argument
target
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Return ?
target
.[[PreventExtensions]]().
26.1.12
Reflect.set (
target
propertyKey
[ ,
receiver
] )
When the
set
function is called with arguments
target
propertyKey
, and
receiver
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
Let
key
be ?
ToPropertyKey
propertyKey
).
If
receiver
is not present, then
Set
receiver
to
target
Return ?
target
.[[Set]](
key
receiver
).
26.1.13
Reflect.setPrototypeOf (
target
proto
When the
setPrototypeOf
function is called with arguments
target
and
proto
, the following steps are taken:
If
Type
target
) is not Object, throw a
TypeError
exception.
If
Type
proto
) is not Object and
proto
is not
null
, throw a
TypeError
exception.
Return ?
target
.[[SetPrototypeOf]](
proto
).
26.2
Proxy Objects
26.2.1
The Proxy Constructor
The Proxy
constructor
is the intrinsic object
%Proxy%
is the initial value of the
Proxy
property of the
global object
creates and initializes a new proxy
exotic object
when called as a
constructor
is not intended to be called as a function and will throw an exception when called in that manner.
26.2.1.1
Proxy (
target
handler
When
Proxy
is called with arguments
target
and
handler
, it performs the following steps:
If NewTarget is
undefined
, throw a
TypeError
exception.
Return ?
ProxyCreate
target
handler
).
26.2.2
Properties of the Proxy Constructor
The Proxy
constructor
has a [[Prototype]] internal slot whose value is the intrinsic object
%FunctionPrototype%
does not have a
prototype
property because proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
has the following properties:
26.2.2.1
Proxy.revocable (
target
handler
The
Proxy.revocable
function is used to create a revocable Proxy object. When
Proxy.revocable
is called with arguments
target
and
handler
, the following steps are taken:
Let
be ?
ProxyCreate
target
handler
).
Let
steps
be the algorithm steps defined in
Proxy Revocation Functions
Let
revoker
be
CreateBuiltinFunction
steps
, « [[RevocableProxy]] »).
Set
revoker
.[[RevocableProxy]] to
Let
result
be
ObjectCreate
%ObjectPrototype%
).
Perform
CreateDataProperty
result
"proxy"
).
Perform
CreateDataProperty
result
"revoke"
revoker
).
Return
result
26.2.2.1.1
Proxy Revocation Functions
A Proxy revocation function is an anonymous function that has the ability to invalidate a specific Proxy object.
Each Proxy revocation function has a [[RevocableProxy]] internal slot.
When a Proxy revocation function is called, the following steps are taken:
Let
be the
active function object
Let
be
.[[RevocableProxy]].
If
is
null
, return
undefined
Set
.[[RevocableProxy]] to
null
Assert
is a Proxy object.
Set
.[[ProxyTarget]] to
null
Set
.[[ProxyHandler]] to
null
Return
undefined
The
"length"
property of a Proxy revocation function is 0.
26.3
Module Namespace Objects
A Module Namespace Object is a module namespace
exotic object
that provides runtime property-based access to a module's exported bindings. There is no
constructor
function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an
ImportDeclaration
that includes a
NameSpaceImport
In addition to the properties specified in
9.4.6
each Module Namespace Object has the following own property:
26.3.1
@@toStringTag
The initial value of the @@toStringTag property is the String value
"Module"
This property has the attributes { [[Writable]]:
false
, [[Enumerable]]:
false
, [[Configurable]]:
false
}.
27
Memory Model
The memory consistency model, or
memory model
, specifies the possible orderings of
Shared Data Block
events, arising via accessing TypedArray instances backed by a
SharedArrayBuffer and via methods on the Atomics object. When the
program has no data races (defined below), the ordering of events
appears as sequentially consistent, i.e., as an interleaving of actions
from each
agent
When the program has data races, shared memory operations may appear
sequentially inconsistent. For example, programs may exhibit
causality-violating behaviour and other astonishments. These
astonishments arise from compiler transforms and the design of CPUs
(e.g., out-of-order execution and speculation). The memory model defines
both the precise conditions under which a program exhibits sequentially
consistent behaviour as well as the possible values read from data
races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by
abstract operations
on SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by the
abstract operations
on SharedArrayBuffers. It bears stressing that the model is not
expressible algorithmically, unlike the rest of this specification. The
nondeterministic introduction of events by
abstract operations
is the interface between the operational semantics of ECMAScript
evaluation and the axiomatic semantics of the memory model. The
semantics of these events is defined by considering graphs of all events
in an evaluation. These are neither Static Semantics nor Runtime
Semantics. There is no demonstrated algorithmic implementation, but
instead a set of constraints that determine if a particular event graph
is allowed or disallowed.
27.1
Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two
groups, atomic accesses and data accesses, defined below. Atomic
accesses are sequentially consistent, i.e., there is a strict total
ordering of events agreed upon by all agents in an
agent cluster
. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.
Shared Data Block event
is either a
ReadSharedMemory
WriteSharedMemory
, or
ReadModifyWriteSharedMemory
Record
Table 77:
ReadSharedMemory
Event Fields
Field Name
Value
Meaning
[[Order]]
"SeqCst"
or
"Unordered"
The weakest ordering guaranteed by the
memory model
for the event.
[[NoTear]]
A Boolean
Whether this event is allowed to read from multiple write events on equal range as this event.
[[Block]]
Shared Data Block
The block the event operates on.
[[ByteIndex]]
A nonnegative integer
The byte address of the read in [[Block]].
[[ElementSize]]
A nonnegative integer
The size of the read.
Table 78:
WriteSharedMemory
Event Fields
Field Name
Value
Meaning
[[Order]]
"SeqCst"
"Unordered"
, or
"Init"
The weakest ordering guaranteed by the
memory model
for the event.
[[NoTear]]
A Boolean
Whether this event is allowed to be read from multiple read events with equal range as this event.
[[Block]]
Shared Data Block
The block the event operates on.
[[ByteIndex]]
A nonnegative integer
The byte address of the write in [[Block]].
[[ElementSize]]
A nonnegative integer
The size of the write.
[[Payload]]
List
The
List
of byte values to be read by other events.
Table 79:
ReadModifyWriteSharedMemory
Event Fields
Field Name
Value
Meaning
[[Order]]
"SeqCst"
Read-modify-write events are always sequentially consistent.
[[NoTear]]
true
Read-modify-write events cannot tear.
[[Block]]
Shared Data Block
The block the event operates on.
[[ByteIndex]]
A nonnegative integer
The byte address of the read-modify-write in [[Block]].
[[ElementSize]]
A nonnegative integer
The size of the read-modify-write.
[[Payload]]
List
The
List
of byte values to be passed to [[ModifyOp]].
[[ModifyOp]]
A semantic function
A pure semantic function that returns a modified
List
of byte values from a read
List
of byte values and [[Payload]].
These events are introduced by
abstract operations
or by methods on the Atomics object.
Some operations may also introduce
Synchronize
events. A
Synchronize event
has no fields, and exists purely to directly constrain the permitted orderings of other events.
In addition to
Shared Data Block
and Synchronize events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory event be the Set of contiguous integers from
its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events'
ranges are equal when the events have the same [[Block]], and the ranges
are element-wise equal. Two events' ranges are overlapping when the
events have the same [[Block]], the ranges are not equal and their
intersection is non-empty. Two events' ranges are disjoint when the
events do not have the same [[Block]] or their ranges are neither equal
nor overlapping.
Note 2
Examples of host-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one
agent
to another (e.g., by
postMessage
in a browser), starting and stopping agents, and communicating within the
agent cluster
via channels other than shared memory. It is assumed those events are appended to
agent-order
during evaluation like the other SharedArrayBuffer events.
Events are ordered within candidate executions by the relations defined below.
27.2
Agent Events Records
An
Agent Events Record
is a
Record
with the following fields.
Table 80:
Agent Events Record
Fields
Field Name
Value
Meaning
[[AgentSignifier]]
A value that admits equality testing
The
agent
whose evaluation resulted in this ordering.
[[EventList]]
List
of events
Events are appended to the list during evaluation.
[[AgentSynchronizesWith]]
List
of pairs of
Synchronize
events
Synchronize
relationships introduced by the operational semantics.
27.3
Chosen Value Records
Chosen Value Record
is a
Record
with the following fields.
Table 81:
Chosen Value Record
Fields
Field Name
Value
Meaning
[[Event]]
Shared Data Block event
The
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event that was introduced for this chosen value.
[[ChosenValue]]
List
of byte values
The bytes that were nondeterministically chosen during evaluation.
27.4
Candidate Executions
candidate execution
of the evaluation of an
agent cluster
is a
Record
with the following fields.
Table 82: Candidate Execution
Record
Fields
Field Name
Value
Meaning
[[EventsRecords]]
List
of
Agent
Events Records.
Maps an
agent
to Lists of events appended during the evaluation.
[[ChosenValues]]
List
of Chosen Value Records.
Maps
ReadSharedMemory
or
ReadModifyWriteSharedMemory
events to the
List
of byte values chosen during the evaluation.
[[AgentOrder]]
An
agent-order
Relation
Defined below.
[[ReadsBytesFrom]]
reads-bytes-from
semantic function.
Defined below.
[[ReadsFrom]]
reads-from
Relation
Defined below.
[[HostSynchronizesWith]]
host-synchronizes-with
Relation
Defined below.
[[SynchronizesWith]]
synchronizes-with
Relation
Defined below.
[[HappensBefore]]
happens-before
Relation
Defined below.
An
empty candidate execution
is a candidate execution
Record
whose fields are empty Lists and Relations.
27.5
Abstract Operations for the Memory Model
27.5.1
EventSet (
execution
The abstract operation EventSet takes one argument, a
candidate execution
execution
. It performs the following steps:
Let
events
be an empty Set.
For each
Agent Events Record
aer
in
execution
.[[EventsRecords]], do
For each event
in
aer
.[[EventList]], do
Add
to
events
Return
events
27.5.2
SharedDataBlockEventSet (
execution
The abstract operation SharedDataBlockEventSet takes one argument, a
candidate execution
execution
. It performs the following steps:
Let
events
be an empty Set.
For each event
in
EventSet
execution
), do
If
is a
ReadSharedMemory
WriteSharedMemory
, or
ReadModifyWriteSharedMemory
event, add
to
events
Return
events
27.5.3
SynchronizeEventSet (
execution
The abstract operation SynchronizeEventSet takes one argument, a
candidate execution
execution
. It performs the following steps:
Let
events
be an empty Set.
For each event
in
EventSet
execution
), do
If
is a
Synchronize event
, add
to
events
Return
events
27.5.4
HostEventSet (
execution
The abstract operation HostEventSet takes one argument, a
candidate execution
execution
. It performs the following steps:
Let
events
be an empty Set.
For each event
in
EventSet
execution
), do
If
is not in
SharedDataBlockEventSet
execution
), add
to
events
Return
events
27.5.5
ComposeWriteEventBytes (
execution
byteIndex
Ws
The abstract operation ComposeWriteEventBytes takes four arguments, a
candidate execution
execution
, a nonnegative integer
byteIndex
, and a
List
Ws
of
WriteSharedMemory
or
ReadModifyWriteSharedMemory
events. It performs the following steps:
Let
byteLocation
be
byteIndex
Let
bytesRead
be a new empty
List
For each element
of
Ws
in
List
order, do
Assert
has
byteLocation
in its range.
Let
payloadIndex
be
byteLocation
.[[ByteIndex]].
If
is a
WriteSharedMemory
event, then
Let
byte
be
.[[Payload]][
payloadIndex
].
Else,
Assert
is a
ReadModifyWriteSharedMemory
event.
Let
bytes
be
ValueOfReadEvent
execution
).
Let
bytesModified
be
.[[ModifyOp]](
bytes
.[[Payload]]).
Let
byte
be
bytesModified
payloadIndex
].
Append
byte
to
bytesRead
Increment
byteLocation
by 1.
Return
bytesRead
Note 1
The semantic function [[ModifyOp]] is given by the function properties on the Atomics object that introduce
ReadModifyWriteSharedMemory
events.
Note 2
This abstract operation composes a
List
of write events into a
List
of byte values. It is used in the event semantics of
ReadSharedMemory
and
ReadModifyWriteSharedMemory
events.
27.5.6
ValueOfReadEvent (
execution
The abstract operation ValueOfReadEvent takes two arguments, a
candidate execution
execution
and a
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event
. It performs the following steps:
Assert
is a
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event.
Let
Ws
be
execution
.[[ReadsBytesFrom]](
).
Assert
Ws
is a
List
of
WriteSharedMemory
or
ReadModifyWriteSharedMemory
events with length equal to
.[[ElementSize]].
Return
ComposeWriteEventBytes
execution
.[[ByteIndex]],
Ws
).
27.6
Relations of Candidate Executions
27.6.1
agent-order
For a
candidate execution
execution
execution
.[[AgentOrder]] is a
Relation
on events that satisfies the following.
For each pair (
) in
EventSet
execution
), (
) is in
execution
.[[AgentOrder]] if there is some
Agent Events Record
aer
in
execution
.[[EventsRecords]] such that
and
are in
aer
.[[EventList]] and
is before
in
List
order of
aer
.[[EventList]].
Note
Each
agent
introduces events in a per-
agent
strict total order
during the evaluation. This is the union of those strict total orders.
27.6.2
reads-bytes-from
For a
candidate execution
execution
execution
.[[ReadsBytesFrom]] is a semantic function from events in
SharedDataBlockEventSet
execution
) to Lists of events in
SharedDataBlockEventSet
execution
) that satisfies the following conditions.
For each
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
),
execution
.[[ReadsBytesFrom]](
) is a
List
of length equal to
.[[ElementSize]] of
WriteSharedMemory
or
ReadModifyWriteSharedMemory
events
Ws
such that all of the following are true.
Each event
with index
in
Ws
has
.[[ByteIndex]] +
in its range.
is not in
Ws
27.6.3
reads-from
For a
candidate execution
execution
execution
.[[ReadsFrom]] is the least
Relation
on events that satisfies the following.
For each pair (
) in
SharedDataBlockEventSet
execution
), (
) is in
execution
.[[ReadsFrom]] if
is in
execution
.[[ReadsBytesFrom]](
).
27.6.4
host-synchronizes-with
For a
candidate execution
execution
execution
.[[HostSynchronizesWith]] is a host-provided
strict partial order
on host-specific events that satisfies at least the following.
If (
) is in
execution
.[[HostSynchronizesWith]],
and
are in
HostEventSet
execution
).
There is no cycle in the union of
execution
.[[HostSynchronizesWith]] and
execution
.[[AgentOrder]].
Note 1
For two host-specific events
and
host-synchronizes-with
implies
happens-before
Note 2
The host-synchronizes-with relation allows the host to provide additional synchronization mechanisms, such as
postMessage
between HTML workers.
27.6.5
synchronizes-with
For a
candidate execution
execution
execution
.[[SynchronizesWith]] is the least
Relation
on events that satisfies the following.
For each pair (
) in
execution
.[[ReadsFrom]], (
) is in
execution
.[[SynchronizesWith]] if all the following are true.
.[[Order]] is
"SeqCst"
.[[Order]] is
"SeqCst"
or
"Init"
If
.[[Order]] is
"SeqCst"
, then
and
have equal ranges.
If
.[[Order]] is
"Init"
, then for each event
such that (
) is in
execution
.[[ReadsFrom]],
.[[Order]] is
"Init"
For each element
eventsRecord
of
execution
.[[EventsRecords]], the following is true.
For each pair (
Sw
) in
eventsRecord
.[[AgentSynchronizesWith]], (
Sw
) is in
execution
.[[SynchronizesWith]].
For each pair (
) in
execution
.[[HostSynchronizesWith]], (
) is in
execution
.[[SynchronizesWith]].
Note 1
Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.
Note 2
Not all
"SeqCst"
events related by
reads-from
are related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.
Note 3
For
Shared Data Block
events
and
such that
synchronizes-with
may
reads-from
other writes than
27.6.6
happens-before
For a
candidate execution
execution
execution
.[[HappensBefore]] is the least
Relation
on events that satisfies the following.
For each pair (
) in
execution
.[[AgentOrder]], (
) is in
execution
.[[HappensBefore]].
For each pair (
) in
execution
.[[SynchronizesWith]], (
) is in
execution
.[[HappensBefore]].
For each pair (
) in
SharedDataBlockEventSet
execution
), (
) is in
execution
.[[HappensBefore]] if
.[[Order]] is
"Init"
and
and
have overlapping ranges.
For each pair (
) in
EventSet
execution
), (
) is in
execution
.[[HappensBefore]] if there is an event
such that the pairs (
) and (
) are in
execution
.[[HappensBefore]].
Note
Because happens-before is a superset of
agent-order
, candidate executions are consistent with the single-thread evaluation semantics of ECMAScript.
27.7
Properties of Valid Executions
27.7.1
Valid Chosen Reads
candidate execution
execution
has valid chosen reads if the following abstract operation returns
true
For each
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
), do
Let
chosenValueRecord
be the element of
execution
.[[ChosenValues]] whose [[Event]] field is
Let
chosenValue
be
chosenValueRecord
.[[ChosenValue]].
Let
readValue
be
ValueOfReadEvent
execution
).
Let
chosenLen
be the number of elements of
chosenValue
Let
readLen
be the number of elements of
readValue
If
chosenLen
is not equal to
readLen
, then
Return
false
If
chosenValue
] is not equal to
readValue
] for any integer value
in the range 0 through
chosenLen
, exclusive, then
Return
false
Return
true
27.7.2
Coherent Reads
candidate execution
execution
has coherent reads if the following abstract operation returns
true
For each
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
), do
Let
Ws
be
execution
.[[ReadsBytesFrom]](
).
Let
byteLocation
be
.[[ByteIndex]].
For each element
of
Ws
in
List
order, do
If (
) is in
execution
.[[HappensBefore]], then
Return
false
If there is a
WriteSharedMemory
or
ReadModifyWriteSharedMemory
event
that has
byteLocation
in its range such that the pairs (
) and (
) are in
execution
.[[HappensBefore]], then
Return
false
Increment
byteLocation
by 1.
Return
true
27.7.3
Tear Free Reads
candidate execution
execution
has tear free reads if the following abstract operation returns
true
For each
ReadSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
), do
If
.[[NoTear]] is
true
, then
Assert
: The remainder of dividing
.[[ByteIndex]] by
.[[ElementSize]] is 0.
For each event
such that (
) is in
execution
.[[ReadsFrom]] and
.[[NoTear]] is
true
, do
If
and
have equal ranges, and there is an event
such that
and
have equal ranges,
.[[NoTear]] is
true
is not
, and (
) is in
execution
.[[ReadsFrom]], then
Return
false
Return
true
Note
An event's [[NoTear]] field is
true
when that event was introduced via accessing an integer TypedArray, and
false
when introduced via accessing a floating point TypedArray or DataView.
Intuitively, this requirement says when a memory range is
accessed in an aligned fashion via an integer TypedArray, a single write
event on that range must "win" when in a data race with other write
events with equal ranges. More precisely, this requirement says an
aligned read event cannot read a value composed of bytes from multiple,
different write events all with equal ranges. It is possible, however,
for an aligned read event to read from multiple write events with
overlapping ranges.
27.7.4
Sequentially Consistent Atomics
For a
candidate execution
execution
, memory-order is a
strict total order
of all events in
EventSet
execution
) that satisfies the following.
For each pair (
) in
execution
.[[HappensBefore]], (
) is in memory-order.
For each pair (
) in
execution
.[[SynchronizesWith]], (
) is in memory-order if there is no
WriteSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
) with equal range as
such that
is not
, and the pairs (
) and (
) are in memory-order.
Note 1
This clause additionally constrains
"SeqCst"
events on equal ranges.
For each
WriteSharedMemory
or
ReadModifyWriteSharedMemory
event
in
SharedDataBlockEventSet
execution
), if
.[[Order]] is
"SeqCst"
, then it is not the case that there is an infinite number of
ReadSharedMemory
or
ReadModifyWriteSharedMemory
events in
SharedDataBlockEventSet
execution
) with equal range that is memory-order before
Note 2
This clause together with the forward progress guarantee on agents ensure the liveness condition that
"SeqCst"
writes become visible to
"SeqCst"
reads with equal range in finite time.
candidate execution
has sequentially consistent atomics if a memory-order exists.
Note 3
While memory-order includes all events in
EventSet
execution
), those that are not constrained by
happens-before
or
synchronizes-with
are allowed to occur anywhere in the order.
27.7.5
Valid Executions
candidate execution
execution
is a valid execution (or simply an execution) if all of the following are true.
The host provides a
host-synchronizes-with
Relation
for
execution
.[[HostSynchronizesWith]].
execution
.[[HappensBefore]] is a
strict partial order
execution
has valid chosen reads.
execution
has coherent reads.
execution
has tear free reads.
execution
has sequentially consistent atomics.
All programs have at least one valid execution.
27.8
Races
For an execution
execution
, two events
and
in
SharedDataBlockEventSet
execution
) are in a race if the following abstract operation returns
true
If
is not
, then
If the pairs (
) and (
) are not in
execution
.[[HappensBefore]], then
If
and
are both
WriteSharedMemory
or
ReadModifyWriteSharedMemory
events and
and
do not have disjoint ranges, then
Return
true
If either (
) or (
) is in
execution
.[[ReadsFrom]], then
Return
true
Return
false
27.9
Data Races
For an execution
execution
, two events
and
in
SharedDataBlockEventSet
execution
) are in a data race if the following abstract operation returns
true
If
and
are in a race in
execution
, then
If
.[[Order]] is not
"SeqCst"
or
.[[Order]] is not
"SeqCst"
, then
Return
true
If
and
have overlapping ranges, then
Return
true
Return
false
27.10
Data Race Freedom
An execution
execution
is data race free if there are no two events in
SharedDataBlockEventSet
execution
) that are in a data race.
A program is data race free if all its executions are data race free.
The
memory model
guarantees sequential consistency of all events for data race free programs.
27.11
Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so
that it is impossible for there to be concurrent non-atomic operations
on the same memory location. Data race free programs have interleaving
semantics where each step in the evaluation semantics of each
agent
are interleaved with each other. For data race free programs, it is not necessary to understand the details of the
memory model
. The details are unlikely to build intuition that will help one to better write ECMAScript.
More generally, even if a program is not data race free it may
have predictable behaviour, so long as atomic operations are not
involved in any data races and the operations that race all have the
same access size. The simplest way to arrange for atomics not to be
involved in races is to ensure that different memory cells are used by
atomic and non-atomic operations and that atomic accesses of different
sizes are not used to access the same cells at the same time.
Effectively, the program should treat shared memory as strongly typed as
much as possible. One still cannot depend on the ordering and timing of
non-atomic accesses that race, but if memory is treated as strongly
typed the racing accesses will not "tear" (bits of their values will not
be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.
It is desirable to allow most program transformations that are valid in a single-
agent
setting in a multi-
agent
setting, to ensure that the performance of each
agent
in a multi-
agent
program is as good as it would be in a single-
agent
setting. Frequently these transformations are hard to judge. We outline
some rules about program transformations that are intended to be taken
as normative (in that they are implied by the
memory model
or stronger than what the
memory model
implies) but which are likely not exhaustive. These rules are intended
to apply to program transformations that precede the introductions of
the events that make up the
agent-order
Let an
agent-order slice
be the subset of the
agent-order
pertaining to a single
agent
Let
possible read values
of a read event be the set of all values of
ValueOfReadEvent
for that event across all valid executions.
Any transformation of an agent-order slice that is valid in the
absence of shared memory is valid in the presence of shared memory,
with the following exceptions.
Atomics are carved in stone
: Program transformations must not cause the
"SeqCst"
events in an agent-order slice to be reordered with its
"Unordered"
operations, nor its
"SeqCst"
operations to be reordered with each other, nor may a program transformation remove a
"SeqCst"
operation from the
agent-order
(In practice, the prohibition on reorderings forces a compiler to assume that every
"SeqCst"
operation is a synchronization and included in the final
memory-order
, which it would usually have to assume anyway in the absence of inter-
agent
program analysis. It also forces the compiler to assume that every call where the callee's effects on the
memory-order
are unknown may contain
"SeqCst"
operations.)
Reads must be stable
: Any given shared memory read must only observe a single value in an execution.
(For example, if what is semantically a single read in the
program is executed multiple times then the program is subsequently
allowed to observe only one of the values read. A transformation known
as rematerialization can violate this rule.)
Writes must be stable
: All observable writes to shared memory must follow from program semantics in an execution.
(For example, a transformation may not introduce certain
observable writes, such as by using read-modify-write operations on a
larger location to write a smaller datum, writing a value to memory that
the program could not have written, or writing a just-read value back
to the location it was read from, if that location could have been
overwritten by another
agent
after the read.)
Possible read values must be nonempty
: Program transformations cannot cause the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have force in
memory model
insofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between two
"SeqCst"
operations, but the transformation may not remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging
multiple non-atomic reads from the same location, reordering non-atomic
reads, introducing speculative non-atomic reads, merging multiple
non-atomic writes to the same location, reordering non-atomic writes to
different locations, and hoisting non-atomic reads out of loops even if
that affects termination. Note in general that aliased TypedArrays make
it hard to prove that locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.
For architectures with memory models no weaker than those of
ARM or Power, non-atomic stores and loads may be compiled to bare stores
and loads on the target architecture. Atomic stores and loads may be
compiled down to instructions that guarantee sequential consistency. If
no such instructions exist, memory barriers are to be employed, such as
placing barriers on both sides of a bare store or load.
Read-modify-write operations may be compiled to read-modify-write
instructions on the target architectrue, such as
LOCK
-prefixed
instructions on x86, load-exclusive/store-exclusive instructions on
ARM, and load-link/store-conditional instructions on Power.
Specifically, the
memory model
is intended to allow code generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write
operations on larger data, but as non-lock-free atomics if the platform
does not have atomic operations of the appropriate size. (We already
assume that every platform has normal memory access operations of every
interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a
full fence, an atomic read-modify-write instruction sequence, and a full
fence.
Non-lock-free atomics compile to a spinlock acquire, a full
fence, a series of non-atomic load and store instructions, a full fence,
and a spinlock release.
That mapping is correct so long as an atomic operation on an
address range does not race with a non-atomic write or with an atomic
operation of different size. However, that is all we need: the
memory model
effectively demotes the atomic operations involved in a race to
non-atomic status. On the other hand, the naive mapping is quite strong:
it allows atomic operations to be used as sequentially consistent
fences, which the
memory model
does not actually guarantee.
A number of local improvements to those basic patterns are also intended to be legal:
There are obvious platform-dependent improvements that
remove redundant fences. For example, on x86 the fences around
lock-free atomic loads and stores can always be omitted except for the
fence following a store, and no fence is needed for lock-free
read-modify-write instructions, as these all use LOCK-prefixed
instructions. On many platforms there are fences of several strengths,
and weaker fences can be used in certain contexts without destroying
sequential consistency.
Most modern platforms support lock-free atomics for all the
data sizes required by ECMAScript atomics. Should non-lock-free atomics
be needed, the fences surrounding the body of the atomic operation can
usually be folded into the lock and unlock steps. The simplest solution
for non-lock-free atomics is to have a single lock word per
SharedArrayBuffer.
There are also more complicated platform-dependent local
improvements, requiring some code analysis. For example, two
back-to-back fences often have the same effect as a single fence, so if
code is generated for two atomic operations in sequence, only a single
fence need separate them. On x86, even a single fence separating atomic
stores can be omitted, as the fence following a store is only needed to
separate the store from a subsequent load.
Grammar Summary
A.1
Lexical Grammar
SourceCharacter
::
any Unicode code point
InputElementDiv
::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
RightBracePunctuator
InputElementRegExp
::
WhiteSpace
LineTerminator
Comment
CommonToken
RightBracePunctuator
RegularExpressionLiteral
InputElementRegExpOrTemplateTail
::
WhiteSpace
LineTerminator
Comment
CommonToken
RegularExpressionLiteral
TemplateSubstitutionTail
InputElementTemplateTail
::
WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
TemplateSubstitutionTail
WhiteSpace
::







LineTerminator
::




LineTerminatorSequence
::


[lookahead ≠





Comment
::
MultiLineComment
SingleLineComment
MultiLineComment
::
/*
MultiLineCommentChars
opt
*/
MultiLineCommentChars
::
MultiLineNotAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
PostAsteriskCommentChars
::
MultiLineNotForwardSlashOrAsteriskChar
MultiLineCommentChars
opt
PostAsteriskCommentChars
opt
MultiLineNotAsteriskChar
::
SourceCharacter
but not
MultiLineNotForwardSlashOrAsteriskChar
::
SourceCharacter
but not one of
or
SingleLineComment
::
//
SingleLineCommentChars
opt
SingleLineCommentChars
::
SingleLineCommentChar
SingleLineCommentChars
opt
SingleLineCommentChar
::
SourceCharacter
but not
LineTerminator
CommonToken
::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
Template
IdentifierName
::
IdentifierStart
IdentifierName
IdentifierPart
IdentifierStart
::
UnicodeIDStart
UnicodeEscapeSequence
IdentifierPart
::
UnicodeIDContinue
UnicodeEscapeSequence


UnicodeIDStart
::
any Unicode code point with the Unicode property “ID_Start”
UnicodeIDContinue
::
any Unicode code point with the Unicode property “ID_Continue”
ReservedWord
::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
Keyword
::
one of
await
break
case
catch
class
const
continue
debugger
default
delete
do
else
export
extends
finally
for
function
if
import
in
instanceof
new
return
super
switch
this
throw
try
typeof
var
void
while
with
yield
FutureReservedWord
::
enum
The following tokens are also considered to be
FutureReservedWord
s when parsing
strict mode code
implements
package
protected
interface
private
public
Punctuator
::
one of
...
<=
>=
==
!=
===
!==
**
++
--
<<
>>
>>>
&&
||
+=
-=
*=
%=
**=
<<=
>>=
>>>=
&=
|=
^=
=>
DivPunctuator
::
/=
RightBracePunctuator
::
NullLiteral
::
null
BooleanLiteral
::
true
false
NumericLiteral
::
DecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
DecimalLiteral
::
DecimalIntegerLiteral
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalIntegerLiteral
ExponentPart
opt
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
opt
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit
::
one of
NonZeroDigit
::
one of
ExponentPart
::
ExponentIndicator
SignedInteger
ExponentIndicator
::
one of
SignedInteger
::
DecimalDigits
DecimalDigits
DecimalDigits
BinaryIntegerLiteral
::
0b
BinaryDigits
0B
BinaryDigits
BinaryDigits
::
BinaryDigit
BinaryDigits
BinaryDigit
BinaryDigit
::
one of
OctalIntegerLiteral
::
0o
OctalDigits
0O
OctalDigits
OctalDigits
::
OctalDigit
OctalDigits
OctalDigit
OctalDigit
::
one of
HexIntegerLiteral
::
0x
HexDigits
0X
HexDigits
HexDigits
::
HexDigit
HexDigits
HexDigit
HexDigit
::
one of
StringLiteral
::
DoubleStringCharacters
opt
SingleStringCharacters
opt
DoubleStringCharacters
::
DoubleStringCharacter
DoubleStringCharacters
opt
SingleStringCharacters
::
SingleStringCharacter
SingleStringCharacters
opt
DoubleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator


EscapeSequence
LineContinuation
SingleStringCharacter
::
SourceCharacter
but not one of
or
or
LineTerminator


EscapeSequence
LineContinuation
LineContinuation
::
LineTerminatorSequence
EscapeSequence
::
CharacterEscapeSequence
[lookahead ∉
DecimalDigit
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence
::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter
::
one of
NonEscapeCharacter
::
SourceCharacter
but not one of
EscapeCharacter
or
LineTerminator
EscapeCharacter
::
SingleEscapeCharacter
DecimalDigit
HexEscapeSequence
::
HexDigit
HexDigit
UnicodeEscapeSequence
::
Hex4Digits
u{
CodePoint
Hex4Digits
::
HexDigit
HexDigit
HexDigit
HexDigit
RegularExpressionLiteral
::
RegularExpressionBody
RegularExpressionFlags
RegularExpressionBody
::
RegularExpressionFirstChar
RegularExpressionChars
RegularExpressionChars
::
[empty]
RegularExpressionChars
RegularExpressionChar
RegularExpressionFirstChar
::
RegularExpressionNonTerminator
but not one of
or
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar
::
RegularExpressionNonTerminator
but not one of
or
or
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence
::
RegularExpressionNonTerminator
RegularExpressionNonTerminator
::
SourceCharacter
but not
LineTerminator
RegularExpressionClass
::
RegularExpressionClassChars
RegularExpressionClassChars
::
[empty]
RegularExpressionClassChars
RegularExpressionClassChar
RegularExpressionClassChar
::
RegularExpressionNonTerminator
but not one of
or
RegularExpressionBackslashSequence
RegularExpressionFlags
::
[empty]
RegularExpressionFlags
IdentifierPart
Template
::
NoSubstitutionTemplate
TemplateHead
NoSubstitutionTemplate
::
TemplateCharacters
opt
TemplateHead
::
TemplateCharacters
opt
${
TemplateSubstitutionTail
::
TemplateMiddle
TemplateTail
TemplateMiddle
::
TemplateCharacters
opt
${
TemplateTail
::
TemplateCharacters
opt
TemplateCharacters
::
TemplateCharacter
TemplateCharacters
opt
TemplateCharacter
::
[lookahead ≠
EscapeSequence
NotEscapeSequence
LineContinuation
LineTerminatorSequence
SourceCharacter
but not one of
or
or
or
LineTerminator
NotEscapeSequence
::
DecimalDigit
DecimalDigit
but not
[lookahead ∉
HexDigit
HexDigit
[lookahead ∉
HexDigit
[lookahead ∉
HexDigit
[lookahead ≠
HexDigit
[lookahead ∉
HexDigit
HexDigit
HexDigit
[lookahead ∉
HexDigit
HexDigit
HexDigit
HexDigit
[lookahead ∉
HexDigit
[lookahead ∉
HexDigit
NotCodePoint
[lookahead ∉
HexDigit
CodePoint
[lookahead ∉
HexDigit
[lookahead ≠
NotCodePoint
::
HexDigits
but only if MV of
HexDigits
> 0x10FFFF
CodePoint
::
HexDigits
but only if MV of
HexDigits
≤ 0x10FFFF
A.2
Expressions
IdentifierReference
[Yield, Await]
Identifier
[~Yield]
yield
[~Await]
await
BindingIdentifier
[Yield, Await]
Identifier
yield
await
Identifier
IdentifierName
but not
ReservedWord
AsyncArrowBindingIdentifier
[Yield]
BindingIdentifier
[?Yield, +Await]
LabelIdentifier
[Yield, Await]
Identifier
[~Yield]
yield
[~Await]
await
PrimaryExpression
[Yield, Await]
this
IdentifierReference
[?Yield, ?Await]
Literal
ArrayLiteral
[?Yield, ?Await]
ObjectLiteral
[?Yield, ?Await]
FunctionExpression
ClassExpression
[?Yield, ?Await]
GeneratorExpression
AsyncFunctionExpression
AsyncGeneratorExpression
RegularExpressionLiteral
TemplateLiteral
[?Yield, ?Await, ~Tagged]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList
[Yield, Await]
Expression
[+In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingIdentifier
[?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingIdentifier
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
When processing an instance of the production
PrimaryExpression
[Yield, Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
the interpretation of
CoverParenthesizedExpressionAndArrowParameterList
is refined using the following grammar:
ParenthesizedExpression
[Yield, Await]
Expression
[+In, ?Yield, ?Await]
Literal
NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
ArrayLiteral
[Yield, Await]
Elision
opt
ElementList
[?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
ElementList
[Yield, Await]
Elision
opt
AssignmentExpression
[+In, ?Yield, ?Await]
Elision
opt
SpreadElement
[?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
AssignmentExpression
[+In, ?Yield, ?Await]
ElementList
[?Yield, ?Await]
Elision
opt
SpreadElement
[?Yield, ?Await]
Elision
Elision
SpreadElement
[Yield, Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
ObjectLiteral
[Yield, Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinitionList
[Yield, Await]
PropertyDefinition
[?Yield, ?Await]
PropertyDefinitionList
[?Yield, ?Await]
PropertyDefinition
[?Yield, ?Await]
PropertyDefinition
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
CoverInitializedName
[?Yield, ?Await]
PropertyName
[?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
MethodDefinition
[?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
PropertyName
[Yield, Await]
LiteralPropertyName
ComputedPropertyName
[?Yield, ?Await]
LiteralPropertyName
IdentifierName
StringLiteral
NumericLiteral
ComputedPropertyName
[Yield, Await]
AssignmentExpression
[+In, ?Yield, ?Await]
CoverInitializedName
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
Initializer
[In, Yield, Await]
AssignmentExpression
[?In, ?Yield, ?Await]
TemplateLiteral
[Yield, Await, Tagged]
NoSubstitutionTemplate
SubstitutionTemplate
[?Yield, ?Await, ?Tagged]
SubstitutionTemplate
[Yield, Await, Tagged]
TemplateHead
Expression
[+In, ?Yield, ?Await]
TemplateSpans
[?Yield, ?Await, ?Tagged]
TemplateSpans
[Yield, Await, Tagged]
TemplateTail
TemplateMiddleList
[?Yield, ?Await, ?Tagged]
TemplateTail
TemplateMiddleList
[Yield, Await, Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
TemplateMiddleList
[?Yield, ?Await, ?Tagged]
TemplateMiddle
Expression
[+In, ?Yield, ?Await]
MemberExpression
[Yield, Await]
PrimaryExpression
[?Yield, ?Await]
MemberExpression
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
MemberExpression
[?Yield, ?Await]
IdentifierName
MemberExpression
[?Yield, ?Await]
TemplateLiteral
[?Yield, ?Await, +Tagged]
SuperProperty
[?Yield, ?Await]
MetaProperty
new
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
SuperProperty
[Yield, Await]
super
Expression
[+In, ?Yield, ?Await]
super
IdentifierName
MetaProperty
NewTarget
NewTarget
new
target
NewExpression
[Yield, Await]
MemberExpression
[?Yield, ?Await]
new
NewExpression
[?Yield, ?Await]
CallExpression
[Yield, Await]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
SuperCall
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
CallExpression
[?Yield, ?Await]
IdentifierName
CallExpression
[?Yield, ?Await]
TemplateLiteral
[?Yield, ?Await, +Tagged]
CoverCallExpressionAndAsyncArrowHead
[Yield, Await]
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
When processing an instance of the production
CallExpression
[Yield, Await]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
the interpretation of
CoverCallExpressionAndAsyncArrowHead
is refined using the following grammar:
CallMemberExpression
[Yield, Await]
MemberExpression
[?Yield, ?Await]
Arguments
[?Yield, ?Await]
SuperCall
[Yield, Await]
super
Arguments
[?Yield, ?Await]
Arguments
[Yield, Await]
ArgumentList
[?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
ArgumentList
[Yield, Await]
AssignmentExpression
[+In, ?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
ArgumentList
[?Yield, ?Await]
...
AssignmentExpression
[+In, ?Yield, ?Await]
LeftHandSideExpression
[Yield, Await]
NewExpression
[?Yield, ?Await]
CallExpression
[?Yield, ?Await]
UpdateExpression
[Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
++
LeftHandSideExpression
[?Yield, ?Await]
[no
LineTerminator
here]
--
++
UnaryExpression
[?Yield, ?Await]
--
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[Yield, Await]
UpdateExpression
[?Yield, ?Await]
delete
UnaryExpression
[?Yield, ?Await]
void
UnaryExpression
[?Yield, ?Await]
typeof
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
UnaryExpression
[?Yield, ?Await]
[+Await]
AwaitExpression
[?Yield]
ExponentiationExpression
[Yield, Await]
UnaryExpression
[?Yield, ?Await]
UpdateExpression
[?Yield, ?Await]
**
ExponentiationExpression
[?Yield, ?Await]
MultiplicativeExpression
[Yield, Await]
ExponentiationExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
MultiplicativeOperator
ExponentiationExpression
[?Yield, ?Await]
MultiplicativeOperator
one of
AdditiveExpression
[Yield, Await]
MultiplicativeExpression
[?Yield, ?Await]
AdditiveExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
AdditiveExpression
[?Yield, ?Await]
MultiplicativeExpression
[?Yield, ?Await]
ShiftExpression
[Yield, Await]
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
<<
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
>>
AdditiveExpression
[?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
>>>
AdditiveExpression
[?Yield, ?Await]
RelationalExpression
[In, Yield, Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
<=
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
>=
ShiftExpression
[?Yield, ?Await]
RelationalExpression
[?In, ?Yield, ?Await]
instanceof
ShiftExpression
[?Yield, ?Await]
[+In]
RelationalExpression
[+In, ?Yield, ?Await]
in
ShiftExpression
[?Yield, ?Await]
EqualityExpression
[In, Yield, Await]
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
==
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
!=
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
===
RelationalExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
!==
RelationalExpression
[?In, ?Yield, ?Await]
BitwiseANDExpression
[In, Yield, Await]
EqualityExpression
[?In, ?Yield, ?Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
EqualityExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[In, Yield, Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
BitwiseANDExpression
[?In, ?Yield, ?Await]
BitwiseORExpression
[In, Yield, Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
BitwiseORExpression
[?In, ?Yield, ?Await]
BitwiseXORExpression
[?In, ?Yield, ?Await]
LogicalANDExpression
[In, Yield, Await]
BitwiseORExpression
[?In, ?Yield, ?Await]
LogicalANDExpression
[?In, ?Yield, ?Await]
&&
BitwiseORExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[In, Yield, Await]
LogicalANDExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[?In, ?Yield, ?Await]
||
LogicalANDExpression
[?In, ?Yield, ?Await]
ConditionalExpression
[In, Yield, Await]
LogicalORExpression
[?In, ?Yield, ?Await]
LogicalORExpression
[?In, ?Yield, ?Await]
AssignmentExpression
[+In, ?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
AssignmentExpression
[In, Yield, Await]
ConditionalExpression
[?In, ?Yield, ?Await]
[+Yield]
YieldExpression
[?In, ?Await]
ArrowFunction
[?In, ?Yield, ?Await]
AsyncArrowFunction
[?In, ?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentOperator
AssignmentExpression
[?In, ?Yield, ?Await]
In certain circumstances when processing an instance of the production
AssignmentExpression
[In, Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
the following grammar is used to refine the interpretation of
LeftHandSideExpression
AssignmentPattern
[Yield, Await]
ObjectAssignmentPattern
[?Yield, ?Await]
ArrayAssignmentPattern
[?Yield, ?Await]
ObjectAssignmentPattern
[Yield, Await]
AssignmentRestProperty
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentRestProperty
[?Yield, ?Await]
opt
ArrayAssignmentPattern
[Yield, Await]
Elision
opt
AssignmentRestElement
[?Yield, ?Await]
opt
AssignmentElementList
[?Yield, ?Await]
AssignmentElementList
[?Yield, ?Await]
Elision
opt
AssignmentRestElement
[?Yield, ?Await]
opt
AssignmentRestProperty
[Yield, Await]
...
DestructuringAssignmentTarget
[?Yield, ?Await]
AssignmentPropertyList
[Yield, Await]
AssignmentProperty
[?Yield, ?Await]
AssignmentPropertyList
[?Yield, ?Await]
AssignmentProperty
[?Yield, ?Await]
AssignmentElementList
[Yield, Await]
AssignmentElisionElement
[?Yield, ?Await]
AssignmentElementList
[?Yield, ?Await]
AssignmentElisionElement
[?Yield, ?Await]
AssignmentElisionElement
[Yield, Await]
Elision
opt
AssignmentElement
[?Yield, ?Await]
AssignmentProperty
[Yield, Await]
IdentifierReference
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
PropertyName
[?Yield, ?Await]
AssignmentElement
[?Yield, ?Await]
AssignmentElement
[Yield, Await]
DestructuringAssignmentTarget
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
AssignmentRestElement
[Yield, Await]
...
DestructuringAssignmentTarget
[?Yield, ?Await]
DestructuringAssignmentTarget
[Yield, Await]
LeftHandSideExpression
[?Yield, ?Await]
AssignmentOperator
one of
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
**=
Expression
[In, Yield, Await]
AssignmentExpression
[?In, ?Yield, ?Await]
Expression
[?In, ?Yield, ?Await]
AssignmentExpression
[?In, ?Yield, ?Await]
A.3
Statements
Statement
[Yield, Await, Return]
BlockStatement
[?Yield, ?Await, ?Return]
VariableStatement
[?Yield, ?Await]
EmptyStatement
ExpressionStatement
[?Yield, ?Await]
IfStatement
[?Yield, ?Await, ?Return]
BreakableStatement
[?Yield, ?Await, ?Return]
ContinueStatement
[?Yield, ?Await]
BreakStatement
[?Yield, ?Await]
[+Return]
ReturnStatement
[?Yield, ?Await]
WithStatement
[?Yield, ?Await, ?Return]
LabelledStatement
[?Yield, ?Await, ?Return]
ThrowStatement
[?Yield, ?Await]
TryStatement
[?Yield, ?Await, ?Return]
DebuggerStatement
Declaration
[Yield, Await]
HoistableDeclaration
[?Yield, ?Await, ~Default]
ClassDeclaration
[?Yield, ?Await, ~Default]
LexicalDeclaration
[+In, ?Yield, ?Await]
HoistableDeclaration
[Yield, Await, Default]
FunctionDeclaration
[?Yield, ?Await, ?Default]
GeneratorDeclaration
[?Yield, ?Await, ?Default]
AsyncFunctionDeclaration
[?Yield, ?Await, ?Default]
AsyncGeneratorDeclaration
[?Yield, ?Await, ?Default]
BreakableStatement
[Yield, Await, Return]
IterationStatement
[?Yield, ?Await, ?Return]
SwitchStatement
[?Yield, ?Await, ?Return]
BlockStatement
[Yield, Await, Return]
Block
[?Yield, ?Await, ?Return]
Block
[Yield, Await, Return]
StatementList
[?Yield, ?Await, ?Return]
opt
StatementList
[Yield, Await, Return]
StatementListItem
[?Yield, ?Await, ?Return]
StatementList
[?Yield, ?Await, ?Return]
StatementListItem
[?Yield, ?Await, ?Return]
StatementListItem
[Yield, Await, Return]
Statement
[?Yield, ?Await, ?Return]
Declaration
[?Yield, ?Await]
LexicalDeclaration
[In, Yield, Await]
LetOrConst
BindingList
[?In, ?Yield, ?Await]
LetOrConst
let
const
BindingList
[In, Yield, Await]
LexicalBinding
[?In, ?Yield, ?Await]
BindingList
[?In, ?Yield, ?Await]
LexicalBinding
[?In, ?Yield, ?Await]
LexicalBinding
[In, Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
opt
BindingPattern
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
VariableStatement
[Yield, Await]
var
VariableDeclarationList
[+In, ?Yield, ?Await]
VariableDeclarationList
[In, Yield, Await]
VariableDeclaration
[?In, ?Yield, ?Await]
VariableDeclarationList
[?In, ?Yield, ?Await]
VariableDeclaration
[?In, ?Yield, ?Await]
VariableDeclaration
[In, Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
opt
BindingPattern
[?Yield, ?Await]
Initializer
[?In, ?Yield, ?Await]
BindingPattern
[Yield, Await]
ObjectBindingPattern
[?Yield, ?Await]
ArrayBindingPattern
[?Yield, ?Await]
ObjectBindingPattern
[Yield, Await]
BindingRestProperty
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingRestProperty
[?Yield, ?Await]
opt
ArrayBindingPattern
[Yield, Await]
Elision
opt
BindingRestElement
[?Yield, ?Await]
opt
BindingElementList
[?Yield, ?Await]
BindingElementList
[?Yield, ?Await]
Elision
opt
BindingRestElement
[?Yield, ?Await]
opt
BindingRestProperty
[Yield, Await]
...
BindingIdentifier
[?Yield, ?Await]
BindingPropertyList
[Yield, Await]
BindingProperty
[?Yield, ?Await]
BindingPropertyList
[?Yield, ?Await]
BindingProperty
[?Yield, ?Await]
BindingElementList
[Yield, Await]
BindingElisionElement
[?Yield, ?Await]
BindingElementList
[?Yield, ?Await]
BindingElisionElement
[?Yield, ?Await]
BindingElisionElement
[Yield, Await]
Elision
opt
BindingElement
[?Yield, ?Await]
BindingProperty
[Yield, Await]
SingleNameBinding
[?Yield, ?Await]
PropertyName
[?Yield, ?Await]
BindingElement
[?Yield, ?Await]
BindingElement
[Yield, Await]
SingleNameBinding
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
SingleNameBinding
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
Initializer
[+In, ?Yield, ?Await]
opt
BindingRestElement
[Yield, Await]
...
BindingIdentifier
[?Yield, ?Await]
...
BindingPattern
[?Yield, ?Await]
EmptyStatement
ExpressionStatement
[Yield, Await]
[lookahead ∉ {
function
async
[no
LineTerminator
here]
function
class
let
}]
Expression
[+In, ?Yield, ?Await]
IfStatement
[Yield, Await, Return]
if
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
else
Statement
[?Yield, ?Await, ?Return]
if
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
IterationStatement
[Yield, Await, Return]
do
Statement
[?Yield, ?Await, ?Return]
while
Expression
[+In, ?Yield, ?Await]
while
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ∉ {
let
}]
Expression
[~In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
var
VariableDeclarationList
[~In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
LexicalDeclaration
[~In, ?Yield, ?Await]
Expression
[+In, ?Yield, ?Await]
opt
Expression
[+In, ?Yield, ?Await]
opt
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ∉ {
let
}]
LeftHandSideExpression
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
var
ForBinding
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
ForDeclaration
[?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
[lookahead ≠
let
LeftHandSideExpression
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
var
ForBinding
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
for
ForDeclaration
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
[lookahead ≠
let
LeftHandSideExpression
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
var
ForBinding
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
[+Await]
for
await
ForDeclaration
[?Yield, ?Await]
of
AssignmentExpression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
ForDeclaration
[Yield, Await]
LetOrConst
ForBinding
[?Yield, ?Await]
ForBinding
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
ContinueStatement
[Yield, Await]
continue
continue
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
BreakStatement
[Yield, Await]
break
break
[no
LineTerminator
here]
LabelIdentifier
[?Yield, ?Await]
ReturnStatement
[Yield, Await]
return
return
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
WithStatement
[Yield, Await, Return]
with
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
SwitchStatement
[Yield, Await, Return]
switch
Expression
[+In, ?Yield, ?Await]
CaseBlock
[?Yield, ?Await, ?Return]
CaseBlock
[Yield, Await, Return]
CaseClauses
[?Yield, ?Await, ?Return]
opt
CaseClauses
[?Yield, ?Await, ?Return]
opt
DefaultClause
[?Yield, ?Await, ?Return]
CaseClauses
[?Yield, ?Await, ?Return]
opt
CaseClauses
[Yield, Await, Return]
CaseClause
[?Yield, ?Await, ?Return]
CaseClauses
[?Yield, ?Await, ?Return]
CaseClause
[?Yield, ?Await, ?Return]
CaseClause
[Yield, Await, Return]
case
Expression
[+In, ?Yield, ?Await]
StatementList
[?Yield, ?Await, ?Return]
opt
DefaultClause
[Yield, Await, Return]
default
StatementList
[?Yield, ?Await, ?Return]
opt
LabelledStatement
[Yield, Await, Return]
LabelIdentifier
[?Yield, ?Await]
LabelledItem
[?Yield, ?Await, ?Return]
LabelledItem
[Yield, Await, Return]
Statement
[?Yield, ?Await, ?Return]
FunctionDeclaration
[?Yield, ?Await, ~Default]
ThrowStatement
[Yield, Await]
throw
[no
LineTerminator
here]
Expression
[+In, ?Yield, ?Await]
TryStatement
[Yield, Await, Return]
try
Block
[?Yield, ?Await, ?Return]
Catch
[?Yield, ?Await, ?Return]
try
Block
[?Yield, ?Await, ?Return]
Finally
[?Yield, ?Await, ?Return]
try
Block
[?Yield, ?Await, ?Return]
Catch
[?Yield, ?Await, ?Return]
Finally
[?Yield, ?Await, ?Return]
Catch
[Yield, Await, Return]
catch
CatchParameter
[?Yield, ?Await]
Block
[?Yield, ?Await, ?Return]
catch
Block
[?Yield, ?Await, ?Return]
Finally
[Yield, Await, Return]
finally
Block
[?Yield, ?Await, ?Return]
CatchParameter
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
BindingPattern
[?Yield, ?Await]
DebuggerStatement
debugger
A.4
Functions and Classes
FunctionDeclaration
[Yield, Await, Default]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
[+Default]
function
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
FunctionExpression
function
BindingIdentifier
[~Yield, ~Await]
opt
FormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
UniqueFormalParameters
[Yield, Await]
FormalParameters
[?Yield, ?Await]
FormalParameters
[Yield, Await]
[empty]
FunctionRestParameter
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FunctionRestParameter
[?Yield, ?Await]
FormalParameterList
[Yield, Await]
FormalParameter
[?Yield, ?Await]
FormalParameterList
[?Yield, ?Await]
FormalParameter
[?Yield, ?Await]
FunctionRestParameter
[Yield, Await]
BindingRestElement
[?Yield, ?Await]
FormalParameter
[Yield, Await]
BindingElement
[?Yield, ?Await]
FunctionBody
[Yield, Await]
FunctionStatementList
[?Yield, ?Await]
FunctionStatementList
[Yield, Await]
StatementList
[?Yield, ?Await, +Return]
opt
ArrowFunction
[In, Yield, Await]
ArrowParameters
[?Yield, ?Await]
[no
LineTerminator
here]
=>
ConciseBody
[?In]
ArrowParameters
[Yield, Await]
BindingIdentifier
[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
ConciseBody
[In]
[lookahead ≠
AssignmentExpression
[?In, ~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
When the production
ArrowParameters
[Yield, Await]
CoverParenthesizedExpressionAndArrowParameterList
[?Yield, ?Await]
is recognized the following grammar is used to refine the interpretation of
CoverParenthesizedExpressionAndArrowParameterList
ArrowFormalParameters
[Yield, Await]
UniqueFormalParameters
[?Yield, ?Await]
AsyncArrowFunction
[In, Yield, Await]
async
[no
LineTerminator
here]
AsyncArrowBindingIdentifier
[?Yield]
[no
LineTerminator
here]
=>
AsyncConciseBody
[?In]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
[no
LineTerminator
here]
=>
AsyncConciseBody
[?In]
AsyncConciseBody
[In]
[lookahead ≠
AssignmentExpression
[?In, ~Yield, +Await]
AsyncFunctionBody
When the production
AsyncArrowFunction
[In, Yield, Await]
CoverCallExpressionAndAsyncArrowHead
[?Yield, ?Await]
[no
LineTerminator
here]
=>
AsyncConciseBody
[?In]
is recognized the following grammar is used to refine the interpretation of
CoverParenthesizedExpressionAndArrowParameterList
AsyncArrowHead
async
[no
LineTerminator
here]
ArrowFormalParameters
[~Yield, +Await]
MethodDefinition
[Yield, Await]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[~Yield, ~Await]
FunctionBody
[~Yield, ~Await]
GeneratorMethod
[?Yield, ?Await]
AsyncMethod
[?Yield, ?Await]
AsyncGeneratorMethod
[?Yield, ?Await]
get
PropertyName
[?Yield, ?Await]
FunctionBody
[~Yield, ~Await]
set
PropertyName
[?Yield, ?Await]
PropertySetParameterList
FunctionBody
[~Yield, ~Await]
PropertySetParameterList
FormalParameter
[~Yield, ~Await]
GeneratorMethod
[Yield, Await]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorDeclaration
[Yield, Await, Default]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[+Yield, ~Await]
GeneratorBody
[+Default]
function
FormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorExpression
function
BindingIdentifier
[+Yield, ~Await]
opt
FormalParameters
[+Yield, ~Await]
GeneratorBody
GeneratorBody
FunctionBody
[+Yield, ~Await]
YieldExpression
[In, Await]
yield
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
yield
[no
LineTerminator
here]
AssignmentExpression
[?In, +Yield, ?Await]
AsyncGeneratorMethod
[Yield, Await]
async
[no
LineTerminator
here]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorDeclaration
[Yield, Await, Default]
async
[no
LineTerminator
here]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
[+Default]
async
[no
LineTerminator
here]
function
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorExpression
async
[no
LineTerminator
here]
function
BindingIdentifier
[+Yield, +Await]
opt
FormalParameters
[+Yield, +Await]
AsyncGeneratorBody
AsyncGeneratorBody
FunctionBody
[+Yield, +Await]
AsyncMethod
[Yield, Await]
async
[no
LineTerminator
here]
PropertyName
[?Yield, ?Await]
UniqueFormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncFunctionDeclaration
[Yield, Await, Default]
async
[no
LineTerminator
here]
function
BindingIdentifier
[?Yield, ?Await]
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
[+Default]
async
[no
LineTerminator
here]
function
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncFunctionExpression
async
[no
LineTerminator
here]
function
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
async
[no
LineTerminator
here]
function
BindingIdentifier
[~Yield, +Await]
FormalParameters
[~Yield, +Await]
AsyncFunctionBody
AsyncFunctionBody
FunctionBody
[~Yield, +Await]
AwaitExpression
[Yield]
await
UnaryExpression
[?Yield, +Await]
ClassDeclaration
[Yield, Await, Default]
class
BindingIdentifier
[?Yield, ?Await]
ClassTail
[?Yield, ?Await]
[+Default]
class
ClassTail
[?Yield, ?Await]
ClassExpression
[Yield, Await]
class
BindingIdentifier
[?Yield, ?Await]
opt
ClassTail
[?Yield, ?Await]
ClassTail
[Yield, Await]
ClassHeritage
[?Yield, ?Await]
opt
ClassBody
[?Yield, ?Await]
opt
ClassHeritage
[Yield, Await]
extends
LeftHandSideExpression
[?Yield, ?Await]
ClassBody
[Yield, Await]
ClassElementList
[?Yield, ?Await]
ClassElementList
[Yield, Await]
ClassElement
[?Yield, ?Await]
ClassElementList
[?Yield, ?Await]
ClassElement
[?Yield, ?Await]
ClassElement
[Yield, Await]
MethodDefinition
[?Yield, ?Await]
static
MethodDefinition
[?Yield, ?Await]
A.5
Scripts and Modules
Script
ScriptBody
opt
ScriptBody
StatementList
[~Yield, ~Await, ~Return]
Module
ModuleBody
opt
ModuleBody
ModuleItemList
ModuleItemList
ModuleItem
ModuleItemList
ModuleItem
ModuleItem
ImportDeclaration
ExportDeclaration
StatementListItem
[~Yield, ~Await, ~Return]
ImportDeclaration
import
ImportClause
FromClause
import
ModuleSpecifier
ImportClause
ImportedDefaultBinding
NameSpaceImport
NamedImports
ImportedDefaultBinding
NameSpaceImport
ImportedDefaultBinding
NamedImports
ImportedDefaultBinding
ImportedBinding
NameSpaceImport
as
ImportedBinding
NamedImports
ImportsList
ImportsList
FromClause
from
ModuleSpecifier
ImportsList
ImportSpecifier
ImportsList
ImportSpecifier
ImportSpecifier
ImportedBinding
IdentifierName
as
ImportedBinding
ModuleSpecifier
StringLiteral
ImportedBinding
BindingIdentifier
[~Yield, ~Await]
ExportDeclaration
export
FromClause
export
ExportClause
FromClause
export
ExportClause
export
VariableStatement
[~Yield, ~Await]
export
Declaration
[~Yield, ~Await]
export
default
HoistableDeclaration
[~Yield, ~Await, +Default]
export
default
ClassDeclaration
[~Yield, ~Await, +Default]
export
default
[lookahead ∉ {
function
async
[no
LineTerminator
here]
function
class
}]
AssignmentExpression
[+In, ~Yield, ~Await]
ExportClause
ExportsList
ExportsList
ExportsList
ExportSpecifier
ExportsList
ExportSpecifier
ExportSpecifier
IdentifierName
IdentifierName
as
IdentifierName
A.6
Number Conversions
StringNumericLiteral
:::
StrWhiteSpace
opt
StrWhiteSpace
opt
StrNumericLiteral
StrWhiteSpace
opt
StrWhiteSpace
:::
StrWhiteSpaceChar
StrWhiteSpace
opt
StrWhiteSpaceChar
:::
WhiteSpace
LineTerminator
StrNumericLiteral
:::
StrDecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
StrDecimalLiteral
:::
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral
:::
Infinity
DecimalDigits
DecimalDigits
opt
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
ExponentPart
opt
DecimalDigits
::
DecimalDigit
DecimalDigits
DecimalDigit
DecimalDigit
::
one of
ExponentPart
::
ExponentIndicator
SignedInteger
ExponentIndicator
::
one of
SignedInteger
::
DecimalDigits
DecimalDigits
DecimalDigits
HexIntegerLiteral
::
0x
HexDigits
0X
HexDigits
HexDigit
::
one of
All grammar symbols not explicitly defined by the
StringNumericLiteral
grammar have the definitions used in the
Lexical Grammar for numeric literals
A.7
Universal Resource Identifier Character Classes
uri
:::
uriCharacters
opt
uriCharacters
:::
uriCharacter
uriCharacters
opt
uriCharacter
:::
uriReserved
uriUnescaped
uriEscaped
uriReserved
:::
one of
uriUnescaped
:::
uriAlpha
DecimalDigit
uriMark
uriEscaped
:::
HexDigit
HexDigit
uriAlpha
:::
one of
uriMark
:::
one of
A.8
Regular Expressions
Pattern
[U, N]
::
Disjunction
[?U, ?N]
Disjunction
[U, N]
::
Alternative
[?U, ?N]
Alternative
[?U, ?N]
Disjunction
[?U, ?N]
Alternative
[U, N]
::
[empty]
Alternative
[?U, ?N]
Term
[?U, ?N]
Term
[U, N]
::
Assertion
[?U, ?N]
Atom
[?U, ?N]
Atom
[?U, ?N]
Quantifier
Assertion
[U, N]
::
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
<=
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
Quantifier
::
QuantifierPrefix
QuantifierPrefix
QuantifierPrefix
::
DecimalDigits
DecimalDigits
DecimalDigits
DecimalDigits
Atom
[U, N]
::
PatternCharacter
AtomEscape
[?U, ?N]
CharacterClass
[?U]
GroupSpecifier
[?U]
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
SyntaxCharacter
::
one of
PatternCharacter
::
SourceCharacter
but not
SyntaxCharacter
AtomEscape
[U, N]
::
DecimalEscape
CharacterClassEscape
[?U]
CharacterEscape
[?U]
[+N]
GroupName
[?U]
CharacterEscape
[U]
::
ControlEscape
ControlLetter
[lookahead ∉
DecimalDigit
HexEscapeSequence
RegExpUnicodeEscapeSequence
[?U]
IdentityEscape
[?U]
ControlEscape
::
one of
ControlLetter
::
one of
GroupSpecifier
[U]
::
[empty]
GroupName
[?U]
GroupName
[U]
::
RegExpIdentifierName
[?U]
RegExpIdentifierName
[U]
::
RegExpIdentifierStart
[?U]
RegExpIdentifierName
[?U]
RegExpIdentifierPart
[?U]
RegExpIdentifierStart
[U]
::
UnicodeIDStart
RegExpUnicodeEscapeSequence
[?U]
RegExpIdentifierPart
[U]
::
UnicodeIDContinue
RegExpUnicodeEscapeSequence
[?U]


RegExpUnicodeEscapeSequence
[U]
::
[+U]
LeadSurrogate
\u
TrailSurrogate
[+U]
LeadSurrogate
[+U]
TrailSurrogate
[+U]
NonSurrogate
[~U]
Hex4Digits
[+U]
u{
CodePoint
Each
\u
TrailSurrogate
for which the choice of associated
LeadSurrogate
is ambiguous shall be associated with the nearest possible
LeadSurrogate
that would otherwise have no corresponding
\u
TrailSurrogate
LeadSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is in the inclusive range 0xD800 to 0xDBFF
TrailSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is in the inclusive range 0xDC00 to 0xDFFF
NonSurrogate
::
Hex4Digits
but only if the SV of
Hex4Digits
is not in the inclusive range 0xD800 to 0xDFFF
IdentityEscape
[U]
::
[+U]
SyntaxCharacter
[+U]
[~U]
SourceCharacter
but not
UnicodeIDContinue
DecimalEscape
::
NonZeroDigit
DecimalDigits
opt
[lookahead ∉
DecimalDigit
CharacterClassEscape
[U]
::
[+U]
p{
UnicodePropertyValueExpression
[+U]
P{
UnicodePropertyValueExpression
UnicodePropertyValueExpression
::
UnicodePropertyName
UnicodePropertyValue
LoneUnicodePropertyNameOrValue
UnicodePropertyName
::
UnicodePropertyNameCharacters
UnicodePropertyNameCharacters
::
UnicodePropertyNameCharacter
UnicodePropertyNameCharacters
opt
UnicodePropertyValue
::
UnicodePropertyValueCharacters
LoneUnicodePropertyNameOrValue
::
UnicodePropertyValueCharacters
UnicodePropertyValueCharacters
::
UnicodePropertyValueCharacter
UnicodePropertyValueCharacters
opt
UnicodePropertyValueCharacter
::
UnicodePropertyNameCharacter
UnicodePropertyNameCharacter
::
ControlLetter
CharacterClass
[U]
::
[lookahead ∉ {
}]
ClassRanges
[?U]
ClassRanges
[?U]
ClassRanges
[U]
::
[empty]
NonemptyClassRanges
[?U]
NonemptyClassRanges
[U]
::
ClassAtom
[?U]
ClassAtom
[?U]
NonemptyClassRangesNoDash
[?U]
ClassAtom
[?U]
ClassAtom
[?U]
ClassRanges
[?U]
NonemptyClassRangesNoDash
[U]
::
ClassAtom
[?U]
ClassAtomNoDash
[?U]
NonemptyClassRangesNoDash
[?U]
ClassAtomNoDash
[?U]
ClassAtom
[?U]
ClassRanges
[?U]
ClassAtom
[U]
::
ClassAtomNoDash
[?U]
ClassAtomNoDash
[U]
::
SourceCharacter
but not one of
or
or
ClassEscape
[?U]
ClassEscape
[U]
::
[+U]
CharacterClassEscape
[?U]
CharacterEscape
[?U]
Additional ECMAScript Features for Web Browsers
The ECMAScript language syntax and semantics defined in this annex
are required when the ECMAScript host is a web browser. The content of
this annex is normative but optional if the ECMAScript host is not a web
browser.
Note
This annex describes various legacy features and other
characteristics of web browser based ECMAScript implementations. All of
the language features and behaviours specified in this annex have one or
more undesirable characteristics and in the absence of legacy usage
would be removed from this specification. However, the usage of these
features by large numbers of existing web pages means that web browsers
must continue to support them. The specifications in this annex define
the requirements for interoperable implementations of these legacy
features.
These features are not considered part of the core ECMAScript
language. Programmers should not use or assume the existence of these
features and behaviours when writing new ECMAScript code. ECMAScript
implementations are discouraged from implementing these features unless
the implementation is part of a web browser or is required to run the
same legacy ECMAScript code that web browsers encounter.
B.1
Additional Syntax
B.1.1
Numeric Literals
The syntax and semantics of
11.8.3
is extended as follows except that this extension is not allowed for
strict mode code
Syntax
NumericLiteral
::
DecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
LegacyOctalIntegerLiteral
LegacyOctalIntegerLiteral
::
OctalDigit
LegacyOctalIntegerLiteral
OctalDigit
DecimalIntegerLiteral
::
NonZeroDigit
DecimalDigits
opt
NonOctalDecimalIntegerLiteral
NonOctalDecimalIntegerLiteral
::
NonOctalDigit
LegacyOctalLikeDecimalIntegerLiteral
NonOctalDigit
NonOctalDecimalIntegerLiteral
DecimalDigit
LegacyOctalLikeDecimalIntegerLiteral
::
OctalDigit
LegacyOctalLikeDecimalIntegerLiteral
OctalDigit
NonOctalDigit
::
one of
B.1.1.1
Static Semantics
The MV of
LegacyOctalIntegerLiteral
::
OctalDigit
is the MV of
OctalDigit
The MV of
LegacyOctalIntegerLiteral
::
LegacyOctalIntegerLiteral
OctalDigit
is (the MV of
LegacyOctalIntegerLiteral
times 8) plus the MV of
OctalDigit
The MV of
DecimalIntegerLiteral
::
NonOctalDecimalIntegerLiteral
is the MV of
NonOctalDecimalIntegerLiteral
The MV of
NonOctalDecimalIntegerLiteral
::
NonOctalDigit
is the MV of
NonOctalDigit
The MV of
NonOctalDecimalIntegerLiteral
::
LegacyOctalLikeDecimalIntegerLiteral
NonOctalDigit
is (the MV of
LegacyOctalLikeDecimalIntegerLiteral
times 10) plus the MV of
NonOctalDigit
The MV of
NonOctalDecimalIntegerLiteral
::
NonOctalDecimalIntegerLiteral
DecimalDigit
is (the MV of
NonOctalDecimalIntegerLiteral
times 10) plus the MV of
DecimalDigit
The MV of
LegacyOctalLikeDecimalIntegerLiteral
::
OctalDigit
is the MV of
OctalDigit
The MV of
LegacyOctalLikeDecimalIntegerLiteral
::
LegacyOctalLikeDecimalIntegerLiteral
OctalDigit
is (the MV of
LegacyOctalLikeDecimalIntegerLiteral
times 10) plus the MV of
OctalDigit
The MV of
NonOctalDigit
::
is 8.
The MV of
NonOctalDigit
::
is 9.
B.1.2
String Literals
The syntax and semantics of
11.8.4
is extended as follows except that this extension is not allowed for
strict mode code
Syntax
EscapeSequence
::
CharacterEscapeSequence
LegacyOctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
LegacyOctalEscapeSequence
::
OctalDigit
[lookahead ∉
OctalDigit
ZeroToThree
OctalDigit
[lookahead ∉
OctalDigit
FourToSeven
OctalDigit
ZeroToThree
OctalDigit
OctalDigit
ZeroToThree
::
one of
FourToSeven
::
one of
This definition of
EscapeSequence
is not used in strict mode or when parsing
TemplateCharacter
B.1.2.1
Static Semantics
The SV of
EscapeSequence
::
LegacyOctalEscapeSequence
is the SV of the
LegacyOctalEscapeSequence
The SV of
LegacyOctalEscapeSequence
::
OctalDigit
is the code unit whose value is the MV of the
OctalDigit
The SV of
LegacyOctalEscapeSequence
::
ZeroToThree
OctalDigit
is the code unit whose value is (8 times the MV of the
ZeroToThree
) plus the MV of the
OctalDigit
The SV of
LegacyOctalEscapeSequence
::
FourToSeven
OctalDigit
is the code unit whose value is (8 times the MV of the
FourToSeven
) plus the MV of the
OctalDigit
The SV of
LegacyOctalEscapeSequence
::
ZeroToThree
OctalDigit
OctalDigit
is the code unit whose value is (64 (that is, 8
) times the MV of the
ZeroToThree
) plus (8 times the MV of the first
OctalDigit
) plus the MV of the second
OctalDigit
The MV of
ZeroToThree
::
is 0.
The MV of
ZeroToThree
::
is 1.
The MV of
ZeroToThree
::
is 2.
The MV of
ZeroToThree
::
is 3.
The MV of
FourToSeven
::
is 4.
The MV of
FourToSeven
::
is 5.
The MV of
FourToSeven
::
is 6.
The MV of
FourToSeven
::
is 7.
B.1.3
HTML-like Comments
The syntax and semantics of
11.4
is extended as follows except that this extension is not allowed when parsing source code using the
goal symbol
Module
Syntax
Comment
::
MultiLineComment
SingleLineComment
SingleLineHTMLOpenComment
SingleLineHTMLCloseComment
SingleLineDelimitedComment
MultiLineComment
::
/*
FirstCommentLine
opt
LineTerminator
MultiLineCommentChars
opt
*/
HTMLCloseComment
opt
FirstCommentLine
::
SingleLineDelimitedCommentChars
SingleLineHTMLOpenComment
::

SingleLineCommentChars
opt
SingleLineDelimitedCommentChars
::
SingleLineNotAsteriskChar
SingleLineDelimitedCommentChars
opt
SingleLinePostAsteriskCommentChars
opt
SingleLineNotAsteriskChar
::
SourceCharacter
but not one of
or
LineTerminator
SingleLinePostAsteriskCommentChars
::
SingleLineNotForwardSlashOrAsteriskChar
SingleLineDelimitedCommentChars
opt
SingleLinePostAsteriskCommentChars
opt
SingleLineNotForwardSlashOrAsteriskChar
::
SourceCharacter
but not one of
or
or
LineTerminator
WhiteSpaceSequence
::
WhiteSpace
WhiteSpaceSequence
opt
SingleLineDelimitedCommentSequence
::
SingleLineDelimitedComment
WhiteSpaceSequence
opt
SingleLineDelimitedCommentSequence
opt
Similar to a
MultiLineComment
that contains a line terminator code point, a
SingleLineHTMLCloseComment
is considered to be a
LineTerminator
for purposes of parsing by the syntactic grammar.
B.1.4
Regular Expressions Patterns
The syntax of
21.2.1
is modified and extended as follows. These changes introduce
ambiguities that are broken by the ordering of grammar productions and
by contextual information. When parsing using the following grammar,
each alternative is considered only if previous production alternatives
do not match.
This alternative pattern grammar and semantics only changes the
syntax and semantics of BMP patterns. The following grammar extensions
include productions parameterized with the [U] parameter. However, none
of these extensions change the syntax of Unicode patterns recognized
when parsing with the [U] parameter present on the
goal symbol
Syntax
Term
[U, N]
::
[+U]
Assertion
[+U, ?N]
[+U]
Atom
[+U, ?N]
[+U]
Atom
[+U, ?N]
Quantifier
[~U]
QuantifiableAssertion
[?N]
Quantifier
[~U]
Assertion
[~U, ?N]
[~U]
ExtendedAtom
[?N]
Quantifier
[~U]
ExtendedAtom
[?N]
Assertion
[U, N]
::
[+U]
Disjunction
[+U, ?N]
[+U]
Disjunction
[+U, ?N]
[~U]
QuantifiableAssertion
[?N]
<=
Disjunction
[?U, ?N]
Disjunction
[?U, ?N]
QuantifiableAssertion
[N]
::
Disjunction
[~U, ?N]
Disjunction
[~U, ?N]
ExtendedAtom
[N]
::
AtomEscape
[~U, ?N]
[lookahead =
CharacterClass
[~U]
Disjunction
[~U, ?N]
Disjunction
[~U, ?N]
InvalidBracedQuantifier
ExtendedPatternCharacter
InvalidBracedQuantifier
::
DecimalDigits
DecimalDigits
DecimalDigits
DecimalDigits
ExtendedPatternCharacter
::
SourceCharacter
but not one of
AtomEscape
[U, N]
::
[+U]
DecimalEscape
[~U]
DecimalEscape
but only if the CapturingGroupNumber of
DecimalEscape
is <= _NcapturingParens_
CharacterClassEscape
[?U]
CharacterEscape
[~U, ?N]
[+N]
GroupName
[?U]
CharacterEscape
[U, N]
::
ControlEscape
ControlLetter
[lookahead ∉
DecimalDigit
HexEscapeSequence
RegExpUnicodeEscapeSequence
[?U]
[~U]
LegacyOctalEscapeSequence
IdentityEscape
[?U, ?N]
IdentityEscape
[U, N]
::
[+U]
SyntaxCharacter
[+U]
[~U]
SourceCharacterIdentityEscape
[?N]
SourceCharacterIdentityEscape
[N]
::
[~N]
SourceCharacter
but not
[+N]
SourceCharacter
but not one of
or
ClassAtomNoDash
[U, N]
::
SourceCharacter
but not one of
or
or
ClassEscape
[?U, ?N]
[lookahead =
ClassEscape
[U, N]
::
[+U]
[~U]
ClassControlLetter
CharacterClassEscape
[?U]
CharacterEscape
[?U, ?N]
ClassControlLetter
::
DecimalDigit
Note
When the same left hand sides occurs with both [+U] and [~U] guards it is to control the disambiguation priority.
B.1.4.1
Static Semantics: Early Errors
The semantics of
21.2.1.1
is extended as follows:
ExtendedAtom
::
InvalidBracedQuantifier
It is a Syntax Error if any source text matches this rule.
NonemptyClassRanges
::
ClassAtom
ClassAtom
ClassRanges
It is a Syntax Error if IsCharacterClass of the first
ClassAtom
is
true
or IsCharacterClass of the second
ClassAtom
is
true
and this production has a
[U]
parameter
NonemptyClassRangesNoDash
::
ClassAtomNoDash
ClassAtom
ClassRanges
It is a Syntax Error if IsCharacterClass of
ClassAtomNoDash
is
true
or IsCharacterClass of
ClassAtom
is
true
and this production has a
[U]
parameter
B.1.4.2
Static Semantics: IsCharacterClass
The semantics of
21.2.1.3
is extended as follows:
ClassAtomNoDash
::
[lookahead =
Return
false
B.1.4.3
Static Semantics: CharacterValue
The semantics of
21.2.1.4
is extended as follows:
ClassAtomNoDash
::
[lookahead =
Return the code point value of U+005C (REVERSE SOLIDUS).
ClassEscape
::
ClassControlLetter
Let
ch
be the code point matched by
ClassControlLetter
Let
be
ch
's code point value.
Return the remainder of dividing
by 32.
CharacterEscape
::
LegacyOctalEscapeSequence
Evaluate the SV of the
LegacyOctalEscapeSequence
(see
B.1.2
) to obtain a code unit
cu
Return the numeric value of
cu
B.1.4.4
Pattern Semantics
The semantics of
21.2.2
is extended as follows:
Within
21.2.2.5
reference to “
Atom
::
GroupSpecifier
Disjunction
” are to be interpreted as meaning “
Atom
::
GroupSpecifier
Disjunction
” or “
ExtendedAtom
::
Disjunction
”.
Term (
21.2.2.5
) includes the following additional evaluation rules:
The production
Term
::
QuantifiableAssertion
Quantifier
evaluates the same as the production
Term
::
Atom
Quantifier
but with
QuantifiableAssertion
substituted for
Atom
The production
Term
::
ExtendedAtom
Quantifier
evaluates the same as the production
Term
::
Atom
Quantifier
but with
ExtendedAtom
substituted for
Atom
The production
Term
::
ExtendedAtom
evaluates the same as the production
Term
::
Atom
but with
ExtendedAtom
substituted for
Atom
Assertion (
21.2.2.6
) includes the following additional evaluation rule:
The production
Assertion
::
QuantifiableAssertion
evaluates as follows:
Evaluate
QuantifiableAssertion
to obtain a Matcher
Return
Assertion (
21.2.2.6
) evaluation rules for the
Assertion
::
Disjunction
and
Assertion
::
Disjunction
productions are also used for the
QuantifiableAssertion
productions, but with
QuantifiableAssertion
substituted for
Assertion
Atom (
21.2.2.8
) evaluation rules for the
Atom
productions except for
Atom
::
PatternCharacter
are also used for the
ExtendedAtom
productions, but with
ExtendedAtom
substituted for
Atom
. The following evaluation rules are also added:
The production
ExtendedAtom
::
[lookahead =
evaluates as follows:
Let
be the CharSet containing the single character
U+005C (REVERSE SOLIDUS).
Call
CharacterSetMatcher
false
) and return its Matcher result.
The production
ExtendedAtom
::
ExtendedPatternCharacter
evaluates as follows:
Let
ch
be the character represented by
ExtendedPatternCharacter
Let
be a one-element CharSet containing the character
ch
Call
CharacterSetMatcher
false
) and return its Matcher result.
CharacterEscape (
21.2.2.10
) includes the following additional evaluation rule:
The production
CharacterEscape
::
LegacyOctalEscapeSequence
evaluates as follows:
Let
cv
be the CharacterValue of this
CharacterEscape
Return the character whose character value is
cv
NonemptyClassRanges (
21.2.2.15
) modifies the following evaluation rule:
The production
NonemptyClassRanges
::
ClassAtom
ClassAtom
ClassRanges
evaluates as follows:
Evaluate the first
ClassAtom
to obtain a CharSet
Evaluate the second
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRangeOrUnion
) and let
be the resulting CharSet.
Return the union of CharSets
and
NonemptyClassRangesNoDash (
21.2.2.16
) modifies the following evaluation rule:
The production
NonemptyClassRangesNoDash
::
ClassAtomNoDash
ClassAtom
ClassRanges
evaluates as follows:
Evaluate
ClassAtomNoDash
to obtain a CharSet
Evaluate
ClassAtom
to obtain a CharSet
Evaluate
ClassRanges
to obtain a CharSet
Call
CharacterRangeOrUnion
) and let
be the resulting CharSet.
Return the union of CharSets
and
ClassEscape (
21.2.2.19
) includes the following additional evaluation rule:
The production
ClassEscape
::
ClassControlLetter
evaluates as follows:
Let
cv
be the CharacterValue of this
ClassEscape
Let
be the character whose character value is
cv
Return the CharSet containing the single character
ClassAtomNoDash (
21.2.2.18
) includes the following additional evaluation rule:
The production
ClassAtomNoDash
::
[lookahead =
evaluates as follows:
Return the CharSet containing the single character
U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence
\c
within a character class where it is not followed by an acceptable control character.
B.1.4.4.1
Runtime Semantics: CharacterRangeOrUnion (
The abstract operation CharacterRangeOrUnion takes two CharSet parameters
and
and performs the following steps:
If
Unicode
is
false
, then
If
does not contain exactly one character or
does not contain exactly one character, then
Let
be the CharSet containing the single character
U+002D (HYPHEN-MINUS).
Return the union of CharSets
and
Return
CharacterRange
).
B.2
Additional Built-in Properties
When the ECMAScript host is a web browser the following additional properties of the standard built-in objects are defined.
B.2.1
Additional Properties of the Global Object
The entries in
Table 83
are added to
Table 7
Table 83: Additional Well-known Intrinsic Objects
Intrinsic Name
Global Name
ECMAScript Language Association
%escape%
escape
The
escape
function (
B.2.1.1
%unescape%
unescape
The
unescape
function (
B.2.1.2
B.2.1.1
escape (
string
The
escape
function is a property of the
global object
. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
For those code units being replaced whose value is
0x00FF
or less, a two-digit escape sequence of the form
xx
is used. For those characters being replaced whose code unit value is greater than
0x00FF
, a four-digit escape sequence of the form
%u
xxxx
is used.
The
escape
function is the
%escape%
intrinsic object. When the
escape
function is called with one argument
string
, the following steps are taken:
Set
string
to ?
ToString
string
).
Let
length
be the number of code units in
string
Let
be the empty string.
Let
be 0.
Repeat, while
length
Let
char
be the code unit (represented as a 16-bit unsigned integer) at index
within
string
If
char
is one of the code units in
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./"
, then
Let
be the String value containing the single code unit
char
Else if
char
≥ 256, then
Let
be the numeric value of
char
Let
be the
string-concatenation
of:
"%u"
the String representation of
, formatted as a four-digit uppercase hexadecimal number, padded to the left with zeroes if necessary
Else
char
< 256,
Let
be the numeric value of
char
Let
be the
string-concatenation
of:
"%"
the String representation of
, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
Set
to the
string-concatenation
of the previous value of
and
Increase
by 1.
Return
Note
The encoding is partly based on the encoding described in
RFC 1738, but the entire encoding specified in this standard is
described above without regard to the contents of RFC 1738. This
encoding does not reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2
unescape (
string
The
unescape
function is a property of the
global object
. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the
escape
function is replaced with the code unit that it represents.
The
unescape
function is the
%unescape%
intrinsic object. When the
unescape
function is called with one argument
string
, the following steps are taken:
Set
string
to ?
ToString
string
).
Let
length
be the number of code units in
string
Let
be the empty String.
Let
be 0.
Repeat, while
length
Let
be the code unit at index
within
string
If
is the code unit 0x0025 (PERCENT SIGN), then
If
length
- 6 and the code unit at index
+ 1 within
string
is the code unit 0x0075 (LATIN SMALL LETTER U) and the four code units at indices
+ 2,
+ 3,
+ 4, and
+ 5 within
string
are all hexadecimal digits, then
Set
to the code unit whose value is the integer represented by the four hexadecimal digits at indices
+ 2,
+ 3,
+ 4, and
+ 5 within
string
Increase
by 5.
Else if
length
- 3 and the two code units at indices
+ 1 and
+ 2 within
string
are both hexadecimal digits, then
Set
to the code unit whose value is the integer represented by two zeroes plus the two hexadecimal digits at indices
+ 1 and
+ 2 within
string
Increase
by 2.
Set
to the
string-concatenation
of the previous value of
and
Increase
by 1.
Return
B.2.2
Additional Properties of the Object.prototype Object
B.2.2.1
Object.prototype.__proto__
Object.prototype.__proto__ is an
accessor property
with attributes { [[Enumerable]]:
false
, [[Configurable]]:
true
}. The [[Get]] and [[Set]] attributes are defined as follows:
B.2.2.1.1
get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps:
Let
be ?
ToObject
this
value).
Return ?
.[[GetPrototypeOf]]().
B.2.2.1.2
set Object.prototype.__proto__
The value of the [[Set]] attribute is a built-in function that takes an argument
proto
. It performs the following steps:
Let
be ?
RequireObjectCoercible
this
value).
If
Type
proto
) is neither Object nor Null, return
undefined
If
Type
) is not Object, return
undefined
Let
status
be ?
.[[SetPrototypeOf]](
proto
).
If
status
is
false
, throw a
TypeError
exception.
Return
undefined
B.2.2.2
Object.prototype.__defineGetter__ (
getter
When the
__defineGetter__
method is called with arguments
and
getter
, the following steps are taken:
Let
be ?
ToObject
this
value).
If
IsCallable
getter
) is
false
, throw a
TypeError
exception.
Let
desc
be PropertyDescriptor { [[Get]]:
getter
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
Let
key
be ?
ToPropertyKey
).
Perform ?
DefinePropertyOrThrow
key
desc
).
Return
undefined
B.2.2.3
Object.prototype.__defineSetter__ (
setter
When the
__defineSetter__
method is called with arguments
and
setter
, the following steps are taken:
Let
be ?
ToObject
this
value).
If
IsCallable
setter
) is
false
, throw a
TypeError
exception.
Let
desc
be PropertyDescriptor { [[Set]]:
setter
, [[Enumerable]]:
true
, [[Configurable]]:
true
}.
Let
key
be ?
ToPropertyKey
).
Perform ?
DefinePropertyOrThrow
key
desc
).
Return
undefined
B.2.2.4
Object.prototype.__lookupGetter__ (
When the
__lookupGetter__
method is called with argument
, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
key
be ?
ToPropertyKey
).
Repeat,
Let
desc
be ?
.[[GetOwnProperty]](
key
).
If
desc
is not
undefined
, then
If
IsAccessorDescriptor
desc
) is
true
, return
desc
.[[Get]].
Return
undefined
Set
to ?
.[[GetPrototypeOf]]().
If
is
null
, return
undefined
B.2.2.5
Object.prototype.__lookupSetter__ (
When the
__lookupSetter__
method is called with argument
, the following steps are taken:
Let
be ?
ToObject
this
value).
Let
key
be ?
ToPropertyKey
).
Repeat,
Let
desc
be ?
.[[GetOwnProperty]](
key
).
If
desc
is not
undefined
, then
If
IsAccessorDescriptor
desc
) is
true
, return
desc
.[[Set]].
Return
undefined
Set
to ?
.[[GetPrototypeOf]]().
If
is
null
, return
undefined
B.2.3
Additional Properties of the String.prototype Object
B.2.3.1
String.prototype.substr (
start
length
The
substr
method takes two arguments,
start
and
length
, and returns a substring of the result of converting the
this
object to a String, starting from index
start
and running for
length
code units (or through the end of the String if
length
is
undefined
). If
start
is negative, it is treated as
sourceLength
start
where
sourceLength
is the length of the String. The result is a String value, not a String object. The following steps are taken:
Let
be ?
RequireObjectCoercible
this
value).
Let
be ?
ToString
).
Let
intStart
be ?
ToInteger
start
).
If
length
is
undefined
, let
end
be
+∞
; otherwise let
end
be ?
ToInteger
length
).
Let
size
be the number of code units in
If
intStart
< 0, set
intStart
to
max
size
intStart
, 0).
Let
resultLength
be
min
max
end
, 0),
size
intStart
).
If
resultLength
≤ 0, return the empty String
""
Return the String value containing
resultLength
consecutive code units from
beginning with the code unit at index
intStart
Note
The
substr
function is intentionally generic; it does not require that its
this
value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.3.2
String.prototype.anchor (
name
When the
anchor
method is called with argument
name
, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"a"
"name"
name
).
B.2.3.2.1
Runtime Semantics: CreateHTML (
string
tag
attribute
value
The abstract operation CreateHTML is called with arguments
string
tag
attribute
, and
value
. The arguments
tag
and
attribute
must be String values. The following steps are taken:
Let
str
be ?
RequireObjectCoercible
string
).
Let
be ?
ToString
str
).
Let
p1
be the
string-concatenation
of
"<"
and
tag
If
attribute
is not the empty String, then
Let
be ?
ToString
value
).
Let
escapedV
be the String value that is the same as
except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in
has been replaced with the six code unit sequence
"""
Set
p1
to the
string-concatenation
of:
p1
the code unit 0x0020 (SPACE)
attribute
the code unit 0x003D (EQUALS SIGN)
the code unit 0x0022 (QUOTATION MARK)
escapedV
the code unit 0x0022 (QUOTATION MARK)
Let
p2
be the
string-concatenation
of
p1
and
">"
Let
p3
be the
string-concatenation
of
p2
and
Let
p4
be the
string-concatenation
of
p3
"tag
, and
">"
Return
p4
B.2.3.3
String.prototype.big ( )
When the
big
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"big"
""
""
).
B.2.3.4
String.prototype.blink ( )
When the
blink
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"blink"
""
""
).
B.2.3.5
String.prototype.bold ( )
When the
bold
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"b"
""
""
).
B.2.3.6
String.prototype.fixed ( )
When the
fixed
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"tt"
""
""
).
B.2.3.7
String.prototype.fontcolor (
color
When the
fontcolor
method is called with argument
color
, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"font"
"color"
color
).
B.2.3.8
String.prototype.fontsize (
size
When the
fontsize
method is called with argument
size
, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"font"
"size"
size
).
B.2.3.9
String.prototype.italics ( )
When the
italics
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"i"
""
""
).
B.2.3.10
String.prototype.link (
url
When the
link
method is called with argument
url
, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"a"
"href"
url
).
B.2.3.11
String.prototype.small ( )
When the
small
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"small"
""
""
).
B.2.3.12
String.prototype.strike ( )
When the
strike
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"strike"
""
""
).
B.2.3.13
String.prototype.sub ( )
When the
sub
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"sub"
""
""
).
B.2.3.14
String.prototype.sup ( )
When the
sup
method is called with no arguments, the following steps are taken:
Let
be the
this
value.
Return ?
CreateHTML
"sup"
""
""
).
B.2.3.15
String.prototype.trimLeft ( )
Note
The property
trimStart
is preferred. The
trimLeft
property is provided principally for compatibility with old code. It is recommended that the
trimStart
property be used in new ECMAScript code.
The initial value of the
trimLeft
property is the same
function object
as the initial value of the
String.prototype.trimStart
property.
B.2.3.16
String.prototype.trimRight ( )
Note
The property
trimEnd
is preferred. The
trimRight
property is provided principally for compatibility with old code. It is recommended that the
trimEnd
property be used in new ECMAScript code.
The initial value of the
trimRight
property is the same
function object
as the initial value of the
String.prototype.trimEnd
property.
B.2.4
Additional Properties of the Date.prototype Object
B.2.4.1
Date.prototype.getYear ( )
Note
The
getFullYear
method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
When the
getYear
method is called with no arguments, the following steps are taken:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, return
NaN
Return
YearFromTime
LocalTime
)) - 1900.
B.2.4.2
Date.prototype.setYear (
year
Note
The
setFullYear
method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
When the
setYear
method is called with one argument
year
, the following steps are taken:
Let
be ?
thisTimeValue
this
value).
If
is
NaN
, set
to
+0
; otherwise, set
to
LocalTime
).
Let
be ?
ToNumber
year
).
If
is
NaN
, then
Set the [[DateValue]] internal slot of
this Date object
to
NaN
Return
NaN
Let
yi
be !
ToInteger
).
If 0 ≤
yi
≤ 99, let
yyyy
be
yi
+ 1900.
Else, let
yyyy
be
Let
be
MakeDay
yyyy
MonthFromTime
),
DateFromTime
)).
Let
date
be
UTC
MakeDate
TimeWithinDay
))).
Set the [[DateValue]] internal slot of
this Date object
to
TimeClip
date
).
Return the value of the [[DateValue]] internal slot of
this Date object
B.2.4.3
Date.prototype.toGMTString ( )
Note
The property
toUTCString
is preferred. The
toGMTString
property is provided principally for compatibility with old code. It is recommended that the
toUTCString
property be used in new ECMAScript code.
The
function object
that is the initial value of
Date.prototype.toGMTString
is the same
function object
that is the initial value of
Date.prototype.toUTCString
B.2.5
Additional Properties of the RegExp.prototype Object
B.2.5.1
RegExp.prototype.compile (
pattern
flags
When the
compile
method is called with arguments
pattern
and
flags
, the following steps are taken:
Let
be the
this
value.
If
Type
) is not Object or
Type
) is Object and
does not have a [[RegExpMatcher]] internal slot, then
Throw a
TypeError
exception.
If
Type
pattern
) is Object and
pattern
has a [[RegExpMatcher]] internal slot, then
If
flags
is not
undefined
, throw a
TypeError
exception.
Let
be
pattern
.[[OriginalSource]].
Let
be
pattern
.[[OriginalFlags]].
Else,
Let
be
pattern
Let
be
flags
Return ?
RegExpInitialize
).
Note
The
compile
method completely reinitializes the
this
object RegExp with a new pattern and flags. An implementation may
interpret use of this method as an assertion that the resulting RegExp
object will be used multiple times and hence is a candidate for extra
optimization.
B.3
Other Additional Features
B.3.1
__proto__ Property Names in Object Initializers
The following Early Error rule is added to those in
12.2.6.1
. When
ObjectLiteral
appears in a context where
ObjectAssignmentPattern
is required the Early Error rule is
not
applied. In addition, it is not applied when initially parsing a
CoverParenthesizedExpressionAndArrowParameterList
or a
CoverCallExpressionAndAsyncArrowHead
ObjectLiteral
PropertyDefinitionList
ObjectLiteral
PropertyDefinitionList
It is a Syntax Error if PropertyNameList of
PropertyDefinitionList
contains any duplicate entries for
"__proto__"
and at least two of those entries were obtained from productions of the form
PropertyDefinition
PropertyName
AssignmentExpression
Note
The
List
returned by PropertyNameList does not include string literal property names defined as using a
ComputedPropertyName
In
12.2.6.8
the PropertyDefinitionEvaluation algorithm for the production
PropertyDefinition
PropertyName
AssignmentExpression
is replaced with the following definition:
PropertyDefinition
PropertyName
AssignmentExpression
Let
propKey
be the result of evaluating
PropertyName
ReturnIfAbrupt
propKey
).
If
propKey
is the String value
"__proto__"
and if IsComputedPropertyKey(
PropertyName
) is
false
, then
Let
isProtoSetter
be
true
Else,
Let
isProtoSetter
be
false
If
IsAnonymousFunctionDefinition
AssignmentExpression
) is
true
and
isProtoSetter
is
false
, then
Let
propValue
be the result of performing NamedEvaluation for
AssignmentExpression
with argument
propKey
Else,
Let
exprValueRef
be the result of evaluating
AssignmentExpression
Let
propValue
be ?
GetValue
exprValueRef
).
If
isProtoSetter
is
true
, then
If
Type
propValue
) is either Object or Null, then
Return
object
.[[SetPrototypeOf]](
propValue
).
Return
NormalCompletion
empty
).
Assert
enumerable
is
true
Assert
object
is an ordinary, extensible object with no non-configurable properties.
Return !
CreateDataPropertyOrThrow
object
propKey
propValue
).
B.3.2
Labelled Function Declarations
Prior to ECMAScript 2015, the specification of
LabelledStatement
did not allow for the association of a statement label with a
FunctionDeclaration
. However, a labelled
FunctionDeclaration
was an allowable extension for
non-strict code
and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015, the grammar productions for
LabelledStatement
permits use of
FunctionDeclaration
as a
LabelledItem
but
13.13.1
includes an Early Error rule that produces a Syntax Error if that
occurs. For web browser compatibility, that rule is modified with the
addition of the
highlighted
text:
LabelledItem
FunctionDeclaration
It is a Syntax Error if any
strict mode
source code matches this rule.
Note
The
early error
rules for
WithStatement
IfStatement
, and
IterationStatement
prevent these statements from containing a labelled
FunctionDeclaration
in
non-strict code
B.3.3
Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a
FunctionDeclaration
as an element of a
Block
statement's
StatementList
. However, support for that form of
FunctionDeclaration
was an allowable extension and most browser-hosted ECMAScript
implementations permitted them. Unfortunately, the semantics of such
declarations differ among those implementations. Because of these
semantic differences, existing web ECMAScript code that uses
Block
level function declarations is only portable among browser
implementation if the usage only depends upon the semantic intersection
of all of the browser implementations for such declarations. The
following are the use cases that fall within that intersection
semantics:
A function is declared and only referenced within a single block
One or more
FunctionDeclaration
s whose
BindingIdentifier
is the name
occur within the function code of an enclosing function
and that declaration is nested within a
Block
No other declaration of
that is not a
var
declaration occurs within the function code of
All occurrences of
as an
IdentifierReference
are within the
StatementList
of the
Block
containing the declaration of
A function is declared and possibly used within a single
Block
but also referenced by an inner function definition that is not contained within that same
Block
One or more
FunctionDeclaration
s whose
BindingIdentifier
is the name
occur within the function code of an enclosing function
and that declaration is nested within a
Block
No other declaration of
that is not a
var
declaration occurs within the function code of
There may be occurrences of
as an
IdentifierReference
within the
StatementList
of the
Block
containing the declaration of
There is at least one occurrence of
as an
IdentifierReference
within another function
that is nested within
and no other declaration of
shadows the references to
from within
All invocations of
occur after the declaration of
has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
One or more
FunctionDeclaration
whose
BindingIdentifier
is the name
occur within the function code of an enclosing function
and that declaration is nested within a
Block
No other declaration of
that is not a
var
declaration occurs within the function code of
There may be occurrences of
as an
IdentifierReference
within the
StatementList
of the
Block
containing the declaration of
There is at least one occurrence of
as an
IdentifierReference
within the function code of
that lexically follows the
Block
containing the declaration of
The first use case is interoperable with the semantics of
Block
level function declarations provided by ECMAScript 2015. Any
pre-existing ECMAScript code that employs that use case will operate
using the Block level function declarations semantics defined by clauses
9, 13, and 14 of this specification.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause
, clause
14
, clause
18.2.1
and clause
15.1.11
semantics.
If an ECMAScript implementation has a mechanism for reporting
diagnostic warning messages, a warning should be produced when code
contains a
FunctionDeclaration
for which these compatibility semantics are applied and introduce
observable differences from non-compatibility semantics. For example, if
a var binding is not introduced because its introduction would create
an
early error
, a warning message should not be produced.
B.3.3.1
Changes to FunctionDeclarationInstantiation
During
FunctionDeclarationInstantiation
the following steps are performed in place of step 28:
If
strict
is
false
, then
For each
FunctionDeclaration
that is directly contained in the
StatementList
of a
Block
CaseClause
, or
DefaultClause
, do
Let
be StringValue of the
BindingIdentifier
of
FunctionDeclaration
If replacing the
FunctionDeclaration
with a
VariableStatement
that has
as a
BindingIdentifier
would not produce any Early Errors for
func
and
is not an element of
parameterNames
, then
NOTE: A var binding for
is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another
FunctionDeclaration
If
initializedBindings
does not contain
and
is not
"arguments"
, then
Perform !
varEnvRec
.CreateMutableBinding(
false
).
Perform
varEnvRec
.InitializeBinding(
undefined
).
Append
to
instantiatedVarNames
When the
FunctionDeclaration
is evaluated, perform the following steps in place of the
FunctionDeclaration
Evaluation algorithm provided in
14.1.22
Let
fenv
be the
running execution context
's VariableEnvironment.
Let
fenvRec
be
fenv
's
EnvironmentRecord
Let
benv
be the
running execution context
's LexicalEnvironment.
Let
benvRec
be
benv
's
EnvironmentRecord
Let
fobj
be !
benvRec
.GetBindingValue(
false
).
Perform !
fenvRec
.SetMutableBinding(
fobj
false
).
Return
NormalCompletion
empty
).
B.3.3.2
Changes to GlobalDeclarationInstantiation
During
GlobalDeclarationInstantiation
the following steps are performed in place of step 14:
Let
strict
be IsStrict of
script
If
strict
is
false
, then
Let
declaredFunctionOrVarNames
be a new empty
List
Append to
declaredFunctionOrVarNames
the elements of
declaredFunctionNames
Append to
declaredFunctionOrVarNames
the elements of
declaredVarNames
For each
FunctionDeclaration
that is directly contained in the
StatementList
of a
Block
CaseClause
, or
DefaultClause
Contained within
script
, do
Let
be StringValue of the
BindingIdentifier
of
FunctionDeclaration
If replacing the
FunctionDeclaration
with a
VariableStatement
that has
as a
BindingIdentifier
would not produce any Early Errors for
script
, then
If
envRec
.HasLexicalDeclaration(
) is
false
, then
Let
fnDefinable
be ?
envRec
.CanDeclareGlobalVar(
).
If
fnDefinable
is
true
, then
NOTE: A var binding for
is only instantiated here if it is neither a VarDeclaredName nor the name of another
FunctionDeclaration
If
declaredFunctionOrVarNames
does not contain
, then
Perform ?
envRec
.CreateGlobalVarBinding(
false
).
Append
to
declaredFunctionOrVarNames
When the
FunctionDeclaration
is evaluated, perform the following steps in place of the
FunctionDeclaration
Evaluation algorithm provided in
14.1.22
Let
genv
be the
running execution context
's VariableEnvironment.
Let
genvRec
be
genv
's
EnvironmentRecord
Let
benv
be the
running execution context
's LexicalEnvironment.
Let
benvRec
be
benv
's
EnvironmentRecord
Let
fobj
be !
benvRec
.GetBindingValue(
false
).
Perform ?
genvRec
.SetMutableBinding(
fobj
false
).
Return
NormalCompletion
empty
).
B.3.3.3
Changes to EvalDeclarationInstantiation
During
EvalDeclarationInstantiation
the following steps are performed in place of step 9:
If
strict
is
false
, then
Let
declaredFunctionOrVarNames
be a new empty
List
Append to
declaredFunctionOrVarNames
the elements of
declaredFunctionNames
Append to
declaredFunctionOrVarNames
the elements of
declaredVarNames
For each
FunctionDeclaration
that is directly contained in the
StatementList
of a
Block
CaseClause
, or
DefaultClause
Contained within
body
, do
Let
be StringValue of the
BindingIdentifier
of
FunctionDeclaration
If replacing the
FunctionDeclaration
with a
VariableStatement
that has
as a
BindingIdentifier
would not produce any Early Errors for
body
, then
Let
bindingExists
be
false
Let
thisLex
be
lexEnv
Assert
: The following loop will terminate.
Repeat, while
thisLex
is not the same as
varEnv
Let
thisEnvRec
be
thisLex
's
EnvironmentRecord
If
thisEnvRec
is not an object
Environment Record
, then
If
thisEnvRec
.HasBinding(
) is
true
, then
Let
bindingExists
be
true
Set
thisLex
to
thisLex
's outer environment reference.
If
bindingExists
is
false
and
varEnvRec
is a global
Environment Record
, then
If
varEnvRec
.HasLexicalDeclaration(
) is
false
, then
Let
fnDefinable
be ?
varEnvRec
.CanDeclareGlobalVar(
).
Else,
Let
fnDefinable
be
false
Else,
Let
fnDefinable
be
true
If
bindingExists
is
false
and
fnDefinable
is
true
, then
If
declaredFunctionOrVarNames
does not contain
, then
If
varEnvRec
is a global
Environment Record
, then
Perform ?
varEnvRec
.CreateGlobalVarBinding(
true
).
Else,
Let
bindingExists
be
varEnvRec
.HasBinding(
).
If
bindingExists
is
false
, then
Perform !
varEnvRec
.CreateMutableBinding(
true
).
Perform !
varEnvRec
.InitializeBinding(
undefined
).
Append
to
declaredFunctionOrVarNames
When the
FunctionDeclaration
is evaluated, perform the following steps in place of the
FunctionDeclaration
Evaluation algorithm provided in
14.1.22
Let
genv
be the
running execution context
's VariableEnvironment.
Let
genvRec
be
genv
's
EnvironmentRecord
Let
benv
be the
running execution context
's LexicalEnvironment.
Let
benvRec
be
benv
's
EnvironmentRecord
Let
fobj
be !
benvRec
.GetBindingValue(
false
).
Perform ?
genvRec
.SetMutableBinding(
fobj
false
).
Return
NormalCompletion
empty
).
B.3.3.4
Changes to Block Static Semantics: Early Errors
For web browser compatibility, that rule is modified with the addition of the
highlighted
text:
Block
StatementList
It is a Syntax Error if the LexicallyDeclaredNames of
StatementList
contains any duplicate entries,
unless the source code matching this production is not
strict mode code
and the duplicate entries are only bound by FunctionDeclarations.
B.3.3.5
Changes to
switch
Statement Static Semantics: Early Errors
For web browser compatibility, that rule is modified with the addition of the
highlighted
text:
SwitchStatement
switch
Expression
CaseBlock
It is a Syntax Error if the LexicallyDeclaredNames of
CaseBlock
contains any duplicate entries,
unless the source code matching this production is not
strict mode code
and the duplicate entries are only bound by FunctionDeclarations.
B.3.3.6
Changes to BlockDeclarationInstantiation
During
BlockDeclarationInstantiation
the following steps are performed in place of step 4.a.ii.1:
If
envRec
.HasBinding(
dn
) is
false
, then
Perform !
envRec
.CreateMutableBinding(
dn
false
).
During
BlockDeclarationInstantiation
the following steps are performed in place of step 4.b.iii:
If
envRec
.HasBinding(
fn
) is
false
, then
Perform
envRec
.InitializeBinding(
fn
fo
).
Else,
Assert
is a
FunctionDeclaration
Perform
envRec
.SetMutableBinding(
fn
fo
false
).
B.3.4
FunctionDeclarations in IfStatement Statement Clauses
The following augments the
IfStatement
production in
13.6
IfStatement
[Yield, Await, Return]
if
Expression
[+In, ?Yield, ?Await]
FunctionDeclaration
[?Yield, ?Await, ~Default]
else
Statement
[?Yield, ?Await, ?Return]
if
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
else
FunctionDeclaration
[?Yield, ?Await, ~Default]
if
Expression
[+In, ?Yield, ?Await]
FunctionDeclaration
[?Yield, ?Await, ~Default]
else
FunctionDeclaration
[?Yield, ?Await, ~Default]
if
Expression
[+In, ?Yield, ?Await]
FunctionDeclaration
[?Yield, ?Await, ~Default]
This production only applies when parsing
non-strict code
. Code matching this production is processed as if each matching occurrence of
FunctionDeclaration
[?Yield, ?Await, ~Default]
was the sole
StatementListItem
of a
BlockStatement
occupying that position in the source code. The semantics of such a synthetic
BlockStatement
includes the web legacy compatibility semantics specified in
B.3.3
B.3.5
VariableStatements in Catch Blocks
The content of subclause
13.15.1
is replaced with the following:
Catch
catch
CatchParameter
Block
It is a Syntax Error if BoundNames of
CatchParameter
contains any duplicate elements.
It is a Syntax Error if any element of the BoundNames of
CatchParameter
also occurs in the LexicallyDeclaredNames of
Block
It is a Syntax Error if any element of the BoundNames of
CatchParameter
also occurs in the VarDeclaredNames of
Block
unless
CatchParameter
is
CatchParameter
BindingIdentifier
Note
The
Block
of a
Catch
clause may contain
var
declarations that bind a name that is also bound by the
CatchParameter
At runtime, such bindings are instantiated in the
VariableDeclarationEnvironment. They do not shadow the same-named
bindings introduced by the
CatchParameter
and hence the
Initializer
for such
var
declarations will assign to the corresponding catch parameter rather than the
var
binding.
This modified behaviour also applies to
var
and
function
declarations introduced by
direct eval
calls contained within the
Block
of a
Catch
clause. This change is accomplished by modifying the algorithm of
18.2.1.3
as follows:
Step 5.d.ii.2.a.i is replaced by:
If
thisEnvRec
is not the
Environment Record
for a
Catch
clause, throw a
SyntaxError
exception.
Step 9.d.ii.4.b.i.i is replaced by:
If
thisEnvRec
is not the
Environment Record
for a
Catch
clause, let
bindingExists
be
true
B.3.6
Initializers in ForIn Statement Heads
The following augments the
IterationStatement
production in
13.7
IterationStatement
[Yield, Await, Return]
for
var
BindingIdentifier
[?Yield, ?Await]
Initializer
[~In, ?Yield, ?Await]
in
Expression
[+In, ?Yield, ?Await]
Statement
[?Yield, ?Await, ?Return]
This production only applies when parsing
non-strict code
The
static semantics
of ContainsDuplicateLabels in
13.7.5.3
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Return ContainsDuplicateLabels of
Statement
with argument
labelSet
The
static semantics
of ContainsUndefinedBreakTarget in
13.7.5.4
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Return ContainsUndefinedBreakTarget of
Statement
with argument
labelSet
The
static semantics
of ContainsUndefinedContinueTarget in
13.7.5.5
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Return ContainsUndefinedContinueTarget of
Statement
with arguments
iterationSet
and « ».
The
static semantics
of IsDestructuring in
13.7.5.6
are augmented with the following:
BindingIdentifier
Identifier
yield
await
Return
false
The
static semantics
of VarDeclaredNames in
13.7.5.7
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Let
names
be the BoundNames of
BindingIdentifier
Append to
names
the elements of the VarDeclaredNames of
Statement
Return
names
The
static semantics
of VarScopedDeclarations in
13.7.5.8
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Let
declarations
be a
List
containing
BindingIdentifier
Append to
declarations
the elements of the VarScopedDeclarations of
Statement
Return
declarations
The
runtime semantics
of LabelledEvaluation in
13.7.5.11
are augmented with the following:
IterationStatement
for
var
BindingIdentifier
Initializer
in
Expression
Statement
Let
bindingId
be StringValue of
BindingIdentifier
Let
lhs
be ?
ResolveBinding
bindingId
).
If
IsAnonymousFunctionDefinition
Initializer
) is
true
, then
Let
value
be the result of performing NamedEvaluation for
Initializer
with argument
bindingId
Else,
Let
rhs
be the result of evaluating
Initializer
Let
value
be ?
GetValue
rhs
).
Perform ?
PutValue
lhs
value
).
Let
keyResult
be ?
ForIn/OfHeadEvaluation
(« »,
Expression
enumerate
).
Return ?
ForIn/OfBodyEvaluation
BindingIdentifier
Statement
keyResult
enumerate
varBinding
labelSet
).
B.3.7
The [[IsHTMLDDA]] Internal Slot
An
[[IsHTMLDDA]] internal slot
may exist on implementation-defined objects. Objects with an [[IsHTMLDDA]] internal slot behave like
undefined
in the
ToBoolean
and
Abstract Equality Comparison
abstract operations
and when used as an operand for the
typeof
operator
Note
Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the
document.all
object
in web browsers is a host-created
exotic object
with this slot that exists for web compatibility purposes. There are no
other known examples of this type of object and implementations should
not create any with the exception of
document.all
B.3.7.1
Changes to ToBoolean
The result column in
Table 9
for an argument type of Object is replaced with the following algorithm:
If
argument
has an
[[IsHTMLDDA]] internal slot
, return
false
Return
true
B.3.7.2
Changes to Abstract Equality Comparison
The following steps are inserted after step 3 of the
Abstract Equality Comparison
algorithm:
If
Type
) is Object and
has an
[[IsHTMLDDA]] internal slot
and
is either
null
or
undefined
, return
true
If
is either
null
or
undefined
and
Type
) is Object and
has an
[[IsHTMLDDA]] internal slot
, return
true
B.3.7.3
Changes to the
typeof
Operator
The following table entry is inserted into
Table 35
immediately preceeding the entry for "Object (implements [[Call]])":
Table 84:
Additional
typeof
Operator Results
Type of
val
Result
Object (has an
[[IsHTMLDDA]] internal slot
"undefined"
The Strict Mode of ECMAScript
The strict mode restriction and exceptions
implements
interface
let
package
private
protected
public
static
, and
yield
are reserved words within
strict mode code
. (
11.6.2
).
A conforming implementation, when processing
strict mode code
, must not extend, as described in
B.1.1
, the syntax of
NumericLiteral
to include
LegacyOctalIntegerLiteral
, nor extend the syntax of
DecimalIntegerLiteral
to include
NonOctalDecimalIntegerLiteral
A conforming implementation, when processing
strict mode code
, may not extend the syntax of
EscapeSequence
to include
LegacyOctalEscapeSequence
as described in
B.1.2
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the
global object
. When a simple assignment occurs within
strict mode code
, its
LeftHandSideExpression
must not evaluate to an unresolvable
Reference
. If it does a
ReferenceError
exception is thrown (
6.2.4.9
). The
LeftHandSideExpression
also may not be a reference to a
data property
with the attribute value { [[Writable]]:
false
}, to an
accessor property
with the attribute value { [[Set]]:
undefined
}, nor to a non-existent property of an object whose [[Extensible]] internal slot has the value
false
. In these cases a
TypeError
exception is thrown (
12.15
).
An
IdentifierReference
with the StringValue
"eval"
or
"arguments"
may not appear as the
LeftHandSideExpression
of an Assignment operator (
12.15
) or of an
UpdateExpression
12.4
) or as the
UnaryExpression
operated upon by a Prefix Increment (
12.4.6
) or a Prefix Decrement (
12.4.7
) operator.
Arguments objects for strict functions define a non-configurable
accessor property
"callee"
which throws a
TypeError
exception on access (
9.4.4.6
).
Arguments objects for strict functions do not dynamically share their
array-indexed
property values with the corresponding formal parameter bindings of their functions. (
9.4.4
).
For strict functions, if an arguments object is created the binding of the local identifier
arguments
to the arguments object is immutable and hence may not be the target of an assignment expression. (
9.2.15
).
It is a
SyntaxError
if the StringValue of a
BindingIdentifier
is
"eval"
or
"arguments"
within
strict mode code
12.1.1
).
Strict mode eval code cannot instantiate variables or functions in
the variable environment of the caller to eval. Instead, a new variable
environment is created and that environment is used for declaration
binding instantiation for the eval code (
18.2.1
).
If
this
is evaluated within
strict mode code
, then the
this
value is not coerced to an object. A
this
value of
undefined
or
null
is not converted to the
global object
and primitive values are not converted to wrapper objects. The
this
value passed via a function call (including calls made using
Function.prototype.apply
and
Function.prototype.call
) do not coerce the passed this value to an object (
9.2.1.2
19.2.3.1
19.2.3.3
).
When a
delete
operator occurs within
strict mode code
, a
SyntaxError
is thrown if its
UnaryExpression
is a direct reference to a variable, function argument, or function name (
12.5.3.1
).
When a
delete
operator occurs within
strict mode code
, a
TypeError
is thrown if the property to be deleted has the attribute { [[Configurable]]:
false
} (
12.5.3.2
).
Strict mode code
may not include a
WithStatement
. The occurrence of a
WithStatement
in such a context is a
SyntaxError
13.11.1
).
It is a
SyntaxError
if a
CatchParameter
occurs within
strict mode code
and BoundNames of
CatchParameter
contains either
eval
or
arguments
13.15.1
).
It is a
SyntaxError
if the same
BindingIdentifier
appears more than once in the
FormalParameters
of a
strict function
. An attempt to create such a function using a
Function
Generator
, or
AsyncFunction
constructor
is a
SyntaxError
14.1.2
19.2.1.1.1
).
An implementation may not extend, beyond that defined in this
specification, the meanings within strict functions of properties named
caller
or
arguments
of function instances.
Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
8.1.1.4.15
8.1.1.4.18
Edition 5 and 5.1 used a property existence test to determine whether a
global object
property corresponding to a new global declaration already existed.
ECMAScript 2015 uses an own property existence test. This corresponds to
what has been most commonly implemented by web browsers.
9.4.2.1
: The 5
th
Edition moved the capture of the current array length prior to the integer conversion of the
array index
or new length value. However, the captured length value could become
invalid if the conversion process has the side-effect of changing the
array length. ECMAScript 2015 specifies that the current array length
must be captured after the possible occurrence of such side-effects.
20.3.1.14
: Previous editions permitted the
TimeClip
abstract operation to return either
+0
or
-0
as the representation of a 0
time value
. ECMAScript 2015 specifies that
+0
always returned. This means that for ECMAScript 2015 the
time value
of a Date object is never observably
-0
and methods that return time values never return
-0
20.3.1.15
If a time zone offset is not present, the local time zone is used.
Edition 5.1 incorrectly stated that a missing time zone should be
interpreted as
"z"
20.3.4.36
: If the year cannot be represented using the Date Time String Format specified in
20.3.1.15
a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
20.3.4.41
: Previous editions did not specify the value returned by Date.prototype.toString when
this time value
is
NaN
. ECMAScript 2015 specifies the result to be the String value is
"Invalid Date"
21.2.3.1
21.2.3.2.4
: Any LineTerminator code points in the value of the
source
property of a RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of
"/"
21.2.5.7
21.2.5.9
: In previous editions, the specifications for
String.prototype.match
and
String.prototype.replace
was incorrect for cases where the pattern argument was a RegExp value whose
global
is flag set. The previous specifications stated that for each attempt to match the pattern, if
lastIndex
did not change it should be incremented by 1. The correct behaviour is that
lastIndex
should be incremented by one only if the pattern matched the empty string.
22.1.3.27
22.1.3.27.1
: Previous editions did not specify how a
NaN
value returned by a
comparefn
was interpreted by
Array.prototype.sort
. ECMAScript 2015 specifies that such as value is treated as if
+0
was returned from the
comparefn
. ECMAScript 2015 also specifies that
ToNumber
is applied to the result returned by a
comparefn
. In previous editions, the effect of a
comparefn
result that is not a Number value was implementation-dependent. In practice, implementations call
ToNumber
Additions and Changes That Introduce Incompatibilities with Prior Editions
7.1.3.1
: In ECMAScript 2015,
ToNumber
applied to a String value now recognizes and converts
BinaryIntegerLiteral
and
OctalIntegerLiteral
numeric strings. In previous editions such strings were converted to
NaN
6.2.4
: In ECMAScript 2015, Function calls are not allowed to return a
Reference
value.
11.6
: In ECMAScript 2015, the valid code points for an
IdentifierName
are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid
IdentifierName
or
Identifier
code points were specified by enumerating various Unicode code point categories.
11.9.1
In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at
the end of a do-while statement if the semicolon is missing. This change
aligns the specification with the actual behaviour of most existing
implementations.
12.2.6.1
: In ECMAScript 2015, it is no longer an
early error
to have duplicate property names in Object Initializers.
12.15.1
: In ECMAScript 2015,
strict mode code
containing an assignment to an immutable binding such as the function name of a
FunctionExpression
does not produce an
early error
. Instead it produces a runtime error.
13.2
: In ECMAScript 2015, a
StatementList
beginning with the token let followed by the input elements
LineTerminator
then
Identifier
is the start of a
LexicalDeclaration
. In previous editions, automatic semicolon insertion would always insert a semicolon before the
Identifier
input element.
13.5
: In ECMAScript 2015, a
StatementListItem
beginning with the token
let
followed by the token
is the start of a
LexicalDeclaration
. In previous editions such a sequence would be the start of an
ExpressionStatement
13.6.7
: In ECMAScript 2015, the normal completion value of an
IfStatement
is never the value
empty
. If no
Statement
part is evaluated or if the evaluated
Statement
part produces a normal completion whose value is
empty
, the completion value of the
IfStatement
is
undefined
13.7
: In ECMAScript 2015, if the
token of a for statement is immediately followed by the token sequence
let [
then the
let
is treated as the start of a
LexicalDeclaration
. In previous editions such a token sequence would be the start of an
Expression
13.7
: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence
let [
then the
let
is treated as the start of a
ForDeclaration
. In previous editions such a token sequence would be the start of an
LeftHandSideExpression
13.7
: Prior to ECMAScript 2015, an initialization expression could appear as part of the
VariableDeclaration
that precedes the
in
keyword. In ECMAScript 2015, the
ForBinding
in that same position does not allow the occurrence of such an
initializer. In ECMAScript 2017, such an initializer is permitted only
in
non-strict code
13.7
: In ECMAScript 2015, the completion value of an
IterationStatement
is never the value
empty
. If the
Statement
part of an
IterationStatement
is not evaluated or if the final evaluation of the
Statement
part produces a completion whose value is
empty
, the completion value of the
IterationStatement
is
undefined
13.11.7
: In ECMAScript 2015, the normal completion value of a
WithStatement
is never the value
empty
. If evaluation of the
Statement
part of a
WithStatement
produces a normal completion whose value is
empty
, the completion value of the
WithStatement
is
undefined
13.12.11
: In ECMAScript 2015, the completion value of a
SwitchStatement
is never the value
empty
. If the
CaseBlock
part of a
SwitchStatement
produces a completion whose value is
empty
, the completion value of the
SwitchStatement
is
undefined
13.15
: In ECMAScript 2015, it is an
early error
for a
Catch
clause to contain a
var
declaration for the same
Identifier
that appears as the
Catch
clause parameter. In previous editions, such a variable declaration
would be instantiated in the enclosing variable environment but the
declaration's
Initializer
value would be assigned to the
Catch
parameter.
13.15
18.2.1.3
: In ECMAScript 2015, a runtime
SyntaxError
is thrown if a
Catch
clause evaluates a non-strict direct
eval
whose eval code includes a
var
or
FunctionDeclaration
declaration that binds the same
Identifier
that appears as the
Catch
clause parameter.
13.15.8
: In ECMAScript 2015, the completion value of a
TryStatement
is never the value
empty
. If the
Block
part of a
TryStatement
evaluates to a normal completion whose value is
empty
, the completion value of the
TryStatement
is
undefined
. If the
Block
part of a
TryStatement
evaluates to a throw completion and it has a
Catch
part that evaluates to a normal completion whose value is
empty
, the completion value of the
TryStatement
is
undefined
if there is no
Finally
clause or if its
Finally
clause evalulates to an
empty
normal completion.
14.3.8
In ECMAScript 2015, the function objects that are created as the values
of the [[Get]] or [[Set]] attribute of accessor properties in an
ObjectLiteral
are not
constructor
functions and they do not have a
prototype
own property. In the previous edition, they were constructors and had a
prototype
property.
19.1.2.6
: In ECMAScript 2015, if the argument to
Object.freeze
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.1.2.8
: In ECMAScript 2015, if the argument to
Object.getOwnPropertyDescriptor
is not an object an attempt is made to coerce the argument using
ToObject
If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a
TypeError
to be thrown.
19.1.2.10
: In ECMAScript 2015, if the argument to
Object.getOwnPropertyNames
is not an object an attempt is made to coerce the argument using
ToObject
If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a
TypeError
to be thrown.
19.1.2.12
: In ECMAScript 2015, if the argument to
Object.getPrototypeOf
is not an object an attempt is made to coerce the argument using
ToObject
If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a
TypeError
to be thrown.
19.1.2.14
: In ECMAScript 2015, if the argument to
Object.isExtensible
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.1.2.15
: In ECMAScript 2015, if the argument to
Object.isFrozen
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.1.2.16
: In ECMAScript 2015, if the argument to
Object.isSealed
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.1.2.17
: In ECMAScript 2015, if the argument to
Object.keys
is not an object an attempt is made to coerce the argument using
ToObject
If the coercion is successful the result is used in place of the
original argument value. In the previous edition, a non-object argument
always causes a
TypeError
to be thrown.
19.1.2.18
: In ECMAScript 2015, if the argument to
Object.preventExtensions
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.1.2.20
: In ECMAScript 2015, if the argument to
Object.seal
is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object
argument always causes a
TypeError
to be thrown.
19.2.3.2
: In ECMAScript 2015, the [[Prototype]] internal slot of a
bound function
is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to
%FunctionPrototype%
19.2.4.1
: In ECMAScript 2015, the
"length"
property of function instances is configurable. In previous editions it was non-configurable.
19.5.6.2
: In ECMAScript 2015, the [[Prototype]] internal slot of a
NativeError
constructor
is the Error
constructor
. In previous editions it was the Function prototype object.
20.3.4
In ECMAScript 2015, the Date prototype object is not a Date instance.
In previous editions it was a Date instance whose TimeValue was
NaN
21.1.3.10
In ECMAScript 2015, the
String.prototype.localeCompare
function must treat Strings that are canonically equivalent according
to the Unicode standard as being identical. In previous editions
implementations were permitted to ignore canonical equivalence and could
instead use a bit-wise comparison.
21.1.3.24
and
21.1.3.26
In ECMAScript 2015, lowercase/upper conversion processing operates on
code points. In previous editions such the conversion processing was
only applied to individual code units. The only affected code points are
those in the Deseret block of Unicode.
21.1.3.27
In ECMAScript 2015, the
String.prototype.trim
method is defined to recognize white space code points that may exists
outside of the Unicode BMP. However, as of Unicode 7 no such code points
are defined. In previous editions such code points would not have been
recognized as white space.
21.2.3.1
In ECMAScript 2015, If the
pattern
argument is a RegExp instance and the
flags
argument is not
undefined
, a new RegExp instance is created just like
pattern
except that
pattern
's flags are replaced by the argument
flags
. In previous editions a
TypeError
exception was thrown when
pattern
was a RegExp instance and
flags
was not
undefined
21.2.5
In ECMAScript 2015, the RegExp prototype object is not a RegExp
instance. In previous editions it was a RegExp instance whose pattern is
the empty string.
21.2.5
In ECMAScript 2015,
source
global
ignoreCase
, and
multiline
are accessor properties defined on the RegExp prototype object. In
previous editions they were data properties defined on RegExp instances.
Colophon
This specification is authored on
GitHub
in a plaintext source format called
Ecmarkup
Ecmarkup is an HTML and Markdown dialect that provides a framework and
toolset for authoring ECMAScript specifications in plaintext and
processing the specification into a full-featured HTML rendering that
follows the editorial conventions for this document. Ecmarkup builds on
and integrates a number of other formats and technologies including
Grammarkdown
for defining syntax and
Ecmarkdown
for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the
Ecmarkup source text that formed the basis of this edition was produced
by converting the ECMAScript 2015 Word document to Ecmarkup using an
automated conversion tool.
Bibliography
IEEE Std 754-2008:
IEEE Standard for Floating-Point Arithmetic
. Institute of Electrical and Electronic Engineers, New York (2008)
The Unicode Standard
, available at <
Unicode Technical Note #5: Canonical Equivalence in Applications
, available at <
Unicode Technical Standard #10: Unicode Collation Algorithm
, available at <
Unicode Standard Annex #15, Unicode Normalization Forms
, available at <
Unicode Standard Annex #18: Unicode Regular Expressions
, available at <
Unicode Standard Annex #24: Unicode
Script
Property
, available at <
Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax
, available at <
Unicode Standard Annex #44: Unicode Character Database
, available at <
Unicode Technical Standard #51: Unicode Emoji
, available at <
IANA Time Zone Database
, available at <
ISO 8601:2004(E)
Data elements and interchange formats – Information interchange
Representation of dates and times
RFC 1738 “Uniform Resource Locators (URL)”
, available at <
RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”
, available at <
RFC 3629 “UTF-8, a transformation format of ISO 10646”
, available at <
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