Compression Standard Compression Living Standard — Last Updated 20 April 2026 Participate: GitHub whatwg/compression (new issue, open issues) Chat on Matrix Commits: GitHub whatwg/compression/commits Snapshot as of this commit @compressionapi Tests: web-platform-tests compression/ (ongoing work) Translations (non-normative): 简体中文 日本語 한국어 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] A 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. A 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; A 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 Uint8Arrays. 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 Uint8Arrays. For each Uint8Array array of arrays, enqueue array in cs’s transform. 5. Interface DecompressionStream [Exposed=*] interface DecompressionStream { constructor(CompressionFormat format); }; DecompressionStream includes GenericTransformStream; A 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 Uint8Arrays. 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 Uint8Arrays. 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 = 0; for (const chunk of cs.readable) { output.push(value); totalSize += value.byteLength; } const concatenated = new Uint8Array(totalSize); let offset = 0; 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, [email protected]). 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.