Compression Standard

Compression Standard
Compression
Living Standard — Last Updated
20 April 2026
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Abstract
This document defines a set of JavaScript APIs to compress and decompress streams of binary data.
1.
Introduction
This section is non-normative.
The APIs specified in this specification are used to compress and decompress streams of data. They support "deflate", "deflate-raw" and "gzip" as compression algorithms. They are widely used by web developers.
2.
Infrastructure
This specification depends on
Infra
[INFRA]
A chunk is a piece of data. In the case of CompressionStream and DecompressionStream, the output chunk type is Uint8Array. They accept any
BufferSource
type as input.
A stream represents an ordered sequence of chunks. The terms
ReadableStream
and
WritableStream
are defined in
Streams
[STREAMS]
compression context
is the internal state maintained by a compression or decompression algorithm. The contents of a
compression context
depend on the format, algorithm and implementation in use. From the point of view of this specification, it is an opaque object. A
compression context
is initially in a start state such that it anticipates the first byte of input.
3.
Supported formats
brotli
"Brotli Compressed Data Format"
[RFC7932]
Implementation must be "compliant" as described in
[RFC7932]
section 1.4.
Non-
[RFC7932]
-conforming blocks must not be created by
CompressionStream
, and are errors for
DecompressionStream
deflate
"ZLIB Compressed Data Format"
[RFC1950]
Note:
This format is referred to as "deflate" for consistency with HTTP Content-Encodings. See
[RFC7230]
section 4.2.2.
Implementations must be "compliant" as described in
[RFC1950]
section 2.3.
Field values described as invalid in
[RFC1950]
must not be created by
CompressionStream
, and are errors for
DecompressionStream
The only valid value of the
CM
(Compression method) part of the
CMF
field is 8.
The
FDICT
flag is not supported by these APIs, and will error the stream if set.
The
FLEVEL
flag is ignored by
DecompressionStream
It is an error for
DecompressionStream
if the
ADLER32
checksum is not correct.
It is an error if there is additional input data after the
ADLER32
checksum.
deflate-raw
"The DEFLATE algorithm"
[RFC1951]
Implementations must be "compliant" as described in
[RFC1951]
section 1.4.
Non-
[RFC1951]
-conforming blocks must not be created by
CompressionStream
, and are errors for
DecompressionStream
It is an error if there is additional input data after the final block indicated by the
BFINAL
flag.
gzip
"GZIP file format"
[RFC1952]
Implementations must be "compliant" as described in
[RFC1952]
section 2.3.1.2.
Field values described as invalid in
[RFC1952]
must not be created by
CompressionStream
, and are errors for
DecompressionStream
The only valid value of the
CM
(Compression Method) field is 8.
The
FTEXT
flag must be ignored by
DecompressionStream
If the
FHCRC
field is present, it is an error for it to be incorrect.
The contents of any
FEXTRA
FNAME
and
FCOMMENT
fields must be ignored by
DecompressionStream
, except to verify that they are terminated correctly.
The contents of the
MTIME
XFL
and
OS
fields must be ignored by
DecompressionStream
It is an error if
CRC32
or
ISIZE
do not match the decompressed data.
gzip
stream may only contain one "member".
It is an error if there is additional input data after the end of the "member".
4.
Interface
CompressionStream
enum
CompressionFormat
"brotli"
"deflate"
"deflate-raw"
"gzip"
};

[Exposed=*]
interface
CompressionStream
constructor
CompressionFormat
format
);
};
CompressionStream
includes
GenericTransformStream
CompressionStream
has an associated
format
and
compression context
context
The
new CompressionStream(
format
steps are:
If
format
is unsupported in
CompressionStream
, then throw a
TypeError
Set
this
’s
format
to
format
Let
transformAlgorithm
be an algorithm which takes a
chunk
argument and runs the
compress and enqueue a chunk
algorithm with
this
and
chunk
Let
flushAlgorithm
be an algorithm which takes no argument and runs the
compress flush and enqueue
algorithm with
this
Set
this
’s
transform
to a
new
TransformStream
Set up
this
’s
transform
with
transformAlgorithm
set to
transformAlgorithm
and
flushAlgorithm
set to
flushAlgorithm
The
compress and enqueue a chunk
algorithm, given a
CompressionStream
object
cs
and a
chunk
, runs these steps:
If
chunk
is not a
BufferSource
type, then throw a
TypeError
Let
buffer
be the result of compressing
chunk
with
cs
’s
format
and
context
If
buffer
is empty, return.
Let
arrays
be the result of splitting
buffer
into one or more non-empty pieces and converting them into
Uint8Array
s.
For each
Uint8Array
array
of
arrays
enqueue
array
in
cs
’s
transform
The
compress flush and enqueue
algorithm, which handles the end of data from the input
ReadableStream
object, given a
CompressionStream
object
cs
, runs these steps:
Let
buffer
be the result of compressing an empty input with
cs
’s
format
and
context
, with the finish flag.
If
buffer
is empty, return.
Let
arrays
be the result of splitting
buffer
into one or more non-empty pieces and converting them into
Uint8Array
s.
For each
Uint8Array
array
of
arrays
enqueue
array
in
cs
’s
transform
5.
Interface
DecompressionStream
[Exposed=*]
interface
DecompressionStream
constructor
CompressionFormat
format
);
};
DecompressionStream
includes
GenericTransformStream
DecompressionStream
has an associated
format
and
compression context
context
The
new DecompressionStream(
format
steps are:
If
format
is unsupported in
DecompressionStream
, then throw a
TypeError
Set
this
’s
format
to
format
Let
transformAlgorithm
be an algorithm which takes a
chunk
argument and runs the
decompress and enqueue a chunk
algorithm with
this
and
chunk
Let
flushAlgorithm
be an algorithm which takes no argument and runs the
decompress flush and enqueue
algorithm with
this
Set
this
’s
transform
to a
new
TransformStream
Set up
this
’s
transform
with
transformAlgorithm
set to
transformAlgorithm
and
flushAlgorithm
set to
flushAlgorithm
The
decompress and enqueue a chunk
algorithm, given a
DecompressionStream
object
ds
and a
chunk
, runs these steps:
If
chunk
is not a
BufferSource
type, then throw a
TypeError
Let
buffer
be the result of decompressing
chunk
with
ds
’s
format
and
context
. If this results in an error, then throw a
TypeError
If
buffer
is empty, return.
Let
arrays
be the result of splitting
buffer
into one or more non-empty pieces and converting them into
Uint8Array
s.
For each
Uint8Array
array
of
arrays
enqueue
array
in
ds
’s
transform
If the end of the compressed input has been reached, and
ds
’s
context
has not fully consumed
chunk
, then throw a
TypeError
The
decompress flush and enqueue
algorithm, which handles the end of data from the input
ReadableStream
object, given a
DecompressionStream
object
ds
, runs these steps:
Let
buffer
be the result of decompressing an empty input with
ds
’s
format
and
context
, with the finish flag.
If
buffer
is not empty:
Let
arrays
be the result of splitting
buffer
into one or more non-empty pieces and converting them into
Uint8Array
s.
For each
Uint8Array
array
of
arrays
enqueue
array
in
ds
’s
transform
If the end of the compressed input has not been reached, then throw a
TypeError
6.
Privacy and security considerations
The API doesn’t add any new privileges to the web platform.
However, web developers have to pay attention to the situation when attackers can get the length of the data. If so, they may be able to guess the contents of the data.
7.
Examples
7.1.
Gzip-compress a stream
const
compressedReadableStream
inputReadableStream
pipeThrough
new
CompressionStream
'gzip'
));
7.2.
Deflate-compress an ArrayBuffer to a Uint8Array
async
function
compressArrayBuffer
input
const
cs
new
CompressionStream
'deflate'
);
const
writer
cs
writable
getWriter
();
writer
write
input
);
writer
close
();
const
output
[];
let
totalSize
for
const
chunk
of
cs
readable
output
push
value
);
totalSize
+=
value
byteLength
const
concatenated
new
Uint8Array
totalSize
);
let
offset
for
const
array
of
output
concatenated
set
array
offset
);
offset
+=
array
byteLength
return
concatenated
7.3.
Gzip-decompress a Blob to Blob
function
decompressBlob
blob
const
ds
new
DecompressionStream
'gzip'
);
const
decompressionStream
blob
stream
().
pipeThrough
ds
);
return
new
Response
decompressionStream
).
blob
();
Acknowledgments
Thanks to Canon Mukai, Domenic Denicola, and Yutaka Hirano, for their support.
This standard is written by Adam Rice (
Google
ricea@chromium.org
).
Intellectual property rights
This Living Standard was originally developed in the W3C WICG, where it was available under the
W3C Software and Document License
Copyright © WHATWG (Apple, Google, Mozilla, Microsoft). This work is licensed under a
Creative Commons Attribution 4.0
International License
. To the extent portions of it are incorporated into source code, such
portions in the source code are licensed under the
BSD 3-Clause License
instead.
This is the Living Standard. Those
interested in the patent-review version should view the
Living Standard Review Draft
Index
Terms defined by this specification
"brotli"
, in § 4
compress and enqueue a chunk
, in § 4
compress flush and enqueue
, in § 4
compression context
, in § 2
CompressionFormat
, in § 4
CompressionStream
, in § 4
CompressionStream(format)
, in § 4
constructor(format)
constructor for CompressionStream
, in § 4
constructor for DecompressionStream
, in § 5
context
dfn for CompressionStream
, in § 4
dfn for DecompressionStream
, in § 5
decompress and enqueue a chunk
, in § 5
decompress flush and enqueue
, in § 5
DecompressionStream
, in § 5
DecompressionStream(format)
, in § 5
"deflate"
, in § 4
"deflate-raw"
, in § 4
format
dfn for CompressionStream
, in § 4
dfn for DecompressionStream
, in § 5
"gzip"
, in § 4
Terms defined by reference
[INFRA]
defines the following terms:
for each
[STREAMS]
defines the following terms:
GenericTransformStream
ReadableStream
TransformStream
WritableStream
enqueue
flushAlgorithm
set up
transform
transformAlgorithm
[WEBIDL]
defines the following terms:
BufferSource
TypeError
Uint8Array
new
this
References
Normative References
[INFRA]
Anne van Kesteren; Domenic Denicola.
Infra Standard
. Living Standard. URL:
[RFC1950]
P. Deutsch; J-L. Gailly.
ZLIB Compressed Data Format Specification version 3.3
. May 1996. Informational. URL:
[RFC1951]
P. Deutsch.
DEFLATE Compressed Data Format Specification version 1.3
. May 1996. Informational. URL:
[RFC1952]
P. Deutsch.
GZIP file format specification version 4.3
. May 1996. Informational. URL:
[RFC7932]
J. Alakuijala; Z. Szabadka.
Brotli Compressed Data Format
. July 2016. Informational. URL:
[STREAMS]
Adam Rice; et al.
Streams Standard
. Living Standard. URL:
[WEBIDL]
Edgar Chen; Timothy Gu.
Web IDL Standard
. Living Standard. URL:
Non-Normative References
[RFC7230]
R. Fielding, Ed.; J. Reschke, Ed..
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
. June 2014. Proposed Standard. URL:
IDL Index
enum
CompressionFormat
"brotli"
"deflate"
"deflate-raw"
"gzip"
};

[Exposed=*]
interface
CompressionStream
constructor
CompressionFormat
format
);
};
CompressionStream
includes
GenericTransformStream

[Exposed=*]
interface
DecompressionStream
constructor
CompressionFormat
format
);
};
DecompressionStream
includes
GenericTransformStream
MDN
CompressionStream/CompressionStream
In all current engines.
Firefox
113+
Safari
16.4+
Chrome
80+
Opera
Edge
80+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
Node.js
17.0.0+
MDN
CompressionStream
In all current engines.
Firefox
113+
Safari
16.4+
Chrome
80+
Opera
Edge
80+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
Node.js
18.0.0+
MDN
DecompressionStream/DecompressionStream
In all current engines.
Firefox
113+
Safari
16.4+
Chrome
80+
Opera
Edge
80+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
Node.js
17.0.0+
MDN
DecompressionStream
In all current engines.
Firefox
113+
Safari
16.4+
Chrome
80+
Opera
Edge
80+
Edge (Legacy)
IE
None
Firefox for Android
iOS Safari
Chrome for Android
Android WebView
Samsung Internet
Opera Mobile
Node.js
18.0.0+