Source Code: lib/zlib.js
\nThe zlib module provides compression functionality implemented using Gzip,\nDeflate/Inflate, and Brotli.
To access it:
\nconst zlib = require('zlib');\nCompression and decompression are built around the Node.js Streams API.
\nCompressing or decompressing a stream (such as a file) can be accomplished by\npiping the source stream through a zlib Transform stream into a destination\nstream:
const { createGzip } = require('zlib');\nconst { pipeline } = require('stream');\nconst {\n createReadStream,\n createWriteStream\n} = require('fs');\n\nconst gzip = createGzip();\nconst source = createReadStream('input.txt');\nconst destination = createWriteStream('input.txt.gz');\n\npipeline(source, gzip, destination, (err) => {\n if (err) {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n }\n});\n\n// Or, Promisified\n\nconst { promisify } = require('util');\nconst pipe = promisify(pipeline);\n\nasync function do_gzip(input, output) {\n const gzip = createGzip();\n const source = createReadStream(input);\n const destination = createWriteStream(output);\n await pipe(source, gzip, destination);\n}\n\ndo_gzip('input.txt', 'input.txt.gz')\n .catch((err) => {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n });\nIt is also possible to compress or decompress data in a single step:
\nconst { deflate, unzip } = require('zlib');\n\nconst input = '.................................';\ndeflate(input, (err, buffer) => {\n if (err) {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n }\n console.log(buffer.toString('base64'));\n});\n\nconst buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');\nunzip(buffer, (err, buffer) => {\n if (err) {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n }\n console.log(buffer.toString());\n});\n\n// Or, Promisified\n\nconst { promisify } = require('util');\nconst do_unzip = promisify(unzip);\n\ndo_unzip(buffer)\n .then((buf) => console.log(buf.toString()))\n .catch((err) => {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n });\nAll zlib APIs, except those that are explicitly synchronous, use the Node.js\ninternal threadpool. This can lead to surprising effects and performance\nlimitations in some applications.
Creating and using a large number of zlib objects simultaneously can cause\nsignificant memory fragmentation.
\nconst zlib = require('zlib');\n\nconst payload = Buffer.from('This is some data');\n\n// WARNING: DO NOT DO THIS!\nfor (let i = 0; i < 30000; ++i) {\n zlib.deflate(payload, (err, buffer) => {});\n}\nIn the preceding example, 30,000 deflate instances are created concurrently.\nBecause of how some operating systems handle memory allocation and\ndeallocation, this may lead to to significant memory fragmentation.
\nIt is strongly recommended that the results of compression\noperations be cached to avoid duplication of effort.
", "type": "module", "displayName": "Threadpool usage and performance considerations" }, { "textRaw": "Compressing HTTP requests and responses", "name": "compressing_http_requests_and_responses", "desc": "The zlib module can be used to implement support for the gzip, deflate\nand br content-encoding mechanisms defined by\nHTTP.
The HTTP Accept-Encoding header is used within an http request to identify\nthe compression encodings accepted by the client. The Content-Encoding\nheader is used to identify the compression encodings actually applied to a\nmessage.
The examples given below are drastically simplified to show the basic concept.\nUsing zlib encoding can be expensive, and the results ought to be cached.\nSee Memory usage tuning for more information on the speed/memory/compression\ntradeoffs involved in zlib usage.
// Client request example\nconst zlib = require('zlib');\nconst http = require('http');\nconst fs = require('fs');\nconst { pipeline } = require('stream');\n\nconst request = http.get({ host: 'example.com',\n path: '/',\n port: 80,\n headers: { 'Accept-Encoding': 'br,gzip,deflate' } });\nrequest.on('response', (response) => {\n const output = fs.createWriteStream('example.com_index.html');\n\n const onError = (err) => {\n if (err) {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n }\n };\n\n switch (response.headers['content-encoding']) {\n case 'br':\n pipeline(response, zlib.createBrotliDecompress(), output, onError);\n break;\n // Or, just use zlib.createUnzip() to handle both of the following cases:\n case 'gzip':\n pipeline(response, zlib.createGunzip(), output, onError);\n break;\n case 'deflate':\n pipeline(response, zlib.createInflate(), output, onError);\n break;\n default:\n pipeline(response, output, onError);\n break;\n }\n});\n// server example\n// Running a gzip operation on every request is quite expensive.\n// It would be much more efficient to cache the compressed buffer.\nconst zlib = require('zlib');\nconst http = require('http');\nconst fs = require('fs');\nconst { pipeline } = require('stream');\n\nhttp.createServer((request, response) => {\n const raw = fs.createReadStream('index.html');\n // Store both a compressed and an uncompressed version of the resource.\n response.setHeader('Vary', 'Accept-Encoding');\n let acceptEncoding = request.headers['accept-encoding'];\n if (!acceptEncoding) {\n acceptEncoding = '';\n }\n\n const onError = (err) => {\n if (err) {\n // If an error occurs, there's not much we can do because\n // the server has already sent the 200 response code and\n // some amount of data has already been sent to the client.\n // The best we can do is terminate the response immediately\n // and log the error.\n response.end();\n console.error('An error occurred:', err);\n }\n };\n\n // Note: This is not a conformant accept-encoding parser.\n // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3\n if (/\\bdeflate\\b/.test(acceptEncoding)) {\n response.writeHead(200, { 'Content-Encoding': 'deflate' });\n pipeline(raw, zlib.createDeflate(), response, onError);\n } else if (/\\bgzip\\b/.test(acceptEncoding)) {\n response.writeHead(200, { 'Content-Encoding': 'gzip' });\n pipeline(raw, zlib.createGzip(), response, onError);\n } else if (/\\bbr\\b/.test(acceptEncoding)) {\n response.writeHead(200, { 'Content-Encoding': 'br' });\n pipeline(raw, zlib.createBrotliCompress(), response, onError);\n } else {\n response.writeHead(200, {});\n pipeline(raw, response, onError);\n }\n}).listen(1337);\nBy default, the zlib methods will throw an error when decompressing\ntruncated data. However, if it is known that the data is incomplete, or\nthe desire is to inspect only the beginning of a compressed file, it is\npossible to suppress the default error handling by changing the flushing\nmethod that is used to decompress the last chunk of input data:
// This is a truncated version of the buffer from the above examples\nconst buffer = Buffer.from('eJzT0yMA', 'base64');\n\nzlib.unzip(\n buffer,\n // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.\n { finishFlush: zlib.constants.Z_SYNC_FLUSH },\n (err, buffer) => {\n if (err) {\n console.error('An error occurred:', err);\n process.exitCode = 1;\n }\n console.log(buffer.toString());\n });\nThis will not change the behavior in other error-throwing situations, e.g.\nwhen the input data has an invalid format. Using this method, it will not be\npossible to determine whether the input ended prematurely or lacks the\nintegrity checks, making it necessary to manually check that the\ndecompressed result is valid.
", "type": "module", "displayName": "Compressing HTTP requests and responses" }, { "textRaw": "Flushing", "name": "flushing", "desc": "Calling .flush() on a compression stream will make zlib return as much\noutput as currently possible. This may come at the cost of degraded compression\nquality, but can be useful when data needs to be available as soon as possible.
In the following example, flush() is used to write a compressed partial\nHTTP response to the client:
const zlib = require('zlib');\nconst http = require('http');\nconst { pipeline } = require('stream');\n\nhttp.createServer((request, response) => {\n // For the sake of simplicity, the Accept-Encoding checks are omitted.\n response.writeHead(200, { 'content-encoding': 'gzip' });\n const output = zlib.createGzip();\n let i;\n\n pipeline(output, response, (err) => {\n if (err) {\n // If an error occurs, there's not much we can do because\n // the server has already sent the 200 response code and\n // some amount of data has already been sent to the client.\n // The best we can do is terminate the response immediately\n // and log the error.\n clearInterval(i);\n response.end();\n console.error('An error occurred:', err);\n }\n });\n\n i = setInterval(() => {\n output.write(`The current time is ${Date()}\\n`, () => {\n // The data has been passed to zlib, but the compression algorithm may\n // have decided to buffer the data for more efficient compression.\n // Calling .flush() will make the data available as soon as the client\n // is ready to receive it.\n output.flush();\n });\n }, 1000);\n}).listen(1337);\nFrom zlib/zconf.h, modified for Node.js usage:
The memory requirements for deflate are (in bytes):
\n\n(1 << (windowBits + 2)) + (1 << (memLevel + 9))\nThat is: 128K for windowBits = 15 + 128K for memLevel = 8\n(default values) plus a few kilobytes for small objects.
For example, to reduce the default memory requirements from 256K to 128K, the\noptions should be set to:
\nconst options = { windowBits: 14, memLevel: 7 };\nThis will, however, generally degrade compression.
\nThe memory requirements for inflate are (in bytes) 1 << windowBits.\nThat is, 32K for windowBits = 15 (default value) plus a few kilobytes\nfor small objects.
This is in addition to a single internal output slab buffer of size\nchunkSize, which defaults to 16K.
The speed of zlib compression is affected most dramatically by the\nlevel setting. A higher level will result in better compression, but\nwill take longer to complete. A lower level will result in less\ncompression, but will be much faster.
In general, greater memory usage options will mean that Node.js has to make\nfewer calls to zlib because it will be able to process more data on\neach write operation. So, this is another factor that affects the\nspeed, at the cost of memory usage.
There are equivalents to the zlib options for Brotli-based streams, although\nthese options have different ranges than the zlib ones:
\nlevel option matches Brotli’s BROTLI_PARAM_QUALITY option.windowBits option matches Brotli’s BROTLI_PARAM_LGWIN option.See below for more details on Brotli-specific options.
", "type": "misc", "displayName": "For Brotli-based streams" } ] }, { "textRaw": "Constants", "name": "Constants", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "type": "misc", "miscs": [ { "textRaw": "zlib constants", "name": "zlib_constants", "desc": "All of the constants defined in zlib.h are also defined on\nrequire('zlib').constants. In the normal course of operations, it will not be\nnecessary to use these constants. They are documented so that their presence is\nnot surprising. This section is taken almost directly from the\nzlib documentation.
Previously, the constants were available directly from require('zlib'), for\ninstance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is\ncurrently still possible but is deprecated.
Allowed flush values.
\nzlib.constants.Z_NO_FLUSHzlib.constants.Z_PARTIAL_FLUSHzlib.constants.Z_SYNC_FLUSHzlib.constants.Z_FULL_FLUSHzlib.constants.Z_FINISHzlib.constants.Z_BLOCKzlib.constants.Z_TREESReturn codes for the compression/decompression functions. Negative\nvalues are errors, positive values are used for special but normal\nevents.
\nzlib.constants.Z_OKzlib.constants.Z_STREAM_ENDzlib.constants.Z_NEED_DICTzlib.constants.Z_ERRNOzlib.constants.Z_STREAM_ERRORzlib.constants.Z_DATA_ERRORzlib.constants.Z_MEM_ERRORzlib.constants.Z_BUF_ERRORzlib.constants.Z_VERSION_ERRORCompression levels.
\nzlib.constants.Z_NO_COMPRESSIONzlib.constants.Z_BEST_SPEEDzlib.constants.Z_BEST_COMPRESSIONzlib.constants.Z_DEFAULT_COMPRESSIONCompression strategy.
\nzlib.constants.Z_FILTEREDzlib.constants.Z_HUFFMAN_ONLYzlib.constants.Z_RLEzlib.constants.Z_FIXEDzlib.constants.Z_DEFAULT_STRATEGYThere are several options and other constants available for Brotli-based\nstreams:
", "modules": [ { "textRaw": "Flush operations", "name": "flush_operations", "desc": "The following values are valid flush operations for Brotli-based streams:
\nzlib.constants.BROTLI_OPERATION_PROCESS (default for all operations)zlib.constants.BROTLI_OPERATION_FLUSH (default when calling .flush())zlib.constants.BROTLI_OPERATION_FINISH (default for the last chunk)zlib.constants.BROTLI_OPERATION_EMIT_METADATA
There are several options that can be set on Brotli encoders, affecting\ncompression efficiency and speed. Both the keys and the values can be accessed\nas properties of the zlib.constants object.
The most important options are:
\nBROTLI_PARAM_MODE
BROTLI_MODE_GENERIC (default)BROTLI_MODE_TEXT, adjusted for UTF-8 textBROTLI_MODE_FONT, adjusted for WOFF 2.0 fontsBROTLI_PARAM_QUALITY
BROTLI_MIN_QUALITY to BROTLI_MAX_QUALITY,\nwith a default of BROTLI_DEFAULT_QUALITY.BROTLI_PARAM_SIZE_HINT
0 for an unknown input size.The following flags can be set for advanced control over the compression\nalgorithm and memory usage tuning:
\nBROTLI_PARAM_LGWIN
BROTLI_MIN_WINDOW_BITS to BROTLI_MAX_WINDOW_BITS,\nwith a default of BROTLI_DEFAULT_WINDOW, or up to\nBROTLI_LARGE_MAX_WINDOW_BITS if the BROTLI_PARAM_LARGE_WINDOW flag\nis set.BROTLI_PARAM_LGBLOCK
BROTLI_MIN_INPUT_BLOCK_BITS to BROTLI_MAX_INPUT_BLOCK_BITS.BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING
BROTLI_PARAM_LARGE_WINDOW
BROTLI_PARAM_NPOSTFIX
0 to BROTLI_MAX_NPOSTFIX.BROTLI_PARAM_NDIRECT
0 to 15 << NPOSTFIX in steps of 1 << NPOSTFIX.These advanced options are available for controlling decompression:
\nBROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION
BROTLI_DECODER_PARAM_LARGE_WINDOW
Each zlib-based class takes an options object. No options are required.
Some options are only relevant when compressing and are\nignored by the decompression classes.
\nflush <integer> Default: zlib.constants.Z_NO_FLUSHfinishFlush <integer> Default: zlib.constants.Z_FINISHchunkSize <integer> Default: 16 * 1024windowBits <integer>level <integer> (compression only)memLevel <integer> (compression only)strategy <integer> (compression only)dictionary <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> (deflate/inflate only,\nempty dictionary by default)info <boolean> (If true, returns an object with buffer and engine.)maxOutputLength <integer> Limits output size when using\nconvenience methods. Default: buffer.kMaxLengthSee the deflateInit2 and inflateInit2 documentation for more\ninformation.
Each Brotli-based class takes an options object. All options are optional.
flush <integer> Default: zlib.constants.BROTLI_OPERATION_PROCESSfinishFlush <integer> Default: zlib.constants.BROTLI_OPERATION_FINISHchunkSize <integer> Default: 16 * 1024params <Object> Key-value object containing indexed Brotli parameters.maxOutputLength <integer> Limits output size when using\nconvenience methods. Default: buffer.kMaxLengthFor example:
\nconst stream = zlib.createBrotliCompress({\n chunkSize: 32 * 1024,\n params: {\n [zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,\n [zlib.constants.BROTLI_PARAM_QUALITY]: 4,\n [zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size\n }\n});\nAll of these take a Buffer, TypedArray, DataView,\nArrayBuffer or string as the first argument, an optional second argument\nto supply options to the zlib classes and will call the supplied callback\nwith callback(error, result).
Every method has a *Sync counterpart, which accept the same arguments, but\nwithout a callback.
Compress a chunk of data with BrotliCompress.
Decompress a chunk of data with BrotliDecompress.
Compress a chunk of data with Deflate.
Compress a chunk of data with DeflateRaw.
Decompress a chunk of data with Gunzip.
Compress a chunk of data with Gzip.
Decompress a chunk of data with Inflate.
Decompress a chunk of data with InflateRaw.
Decompress a chunk of data with Unzip.
Compress data using the Brotli algorithm.
" }, { "textRaw": "Class: `zlib.BrotliDecompress`", "type": "class", "name": "zlib.BrotliDecompress", "meta": { "added": [ "v11.7.0", "v10.16.0" ], "changes": [] }, "desc": "Decompress data using the Brotli algorithm.
" }, { "textRaw": "Class: `zlib.Deflate`", "type": "class", "name": "zlib.Deflate", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "desc": "Compress data using deflate.
" }, { "textRaw": "Class: `zlib.DeflateRaw`", "type": "class", "name": "zlib.DeflateRaw", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "desc": "Compress data using deflate, and do not append a zlib header.
Decompress a gzip stream.
" }, { "textRaw": "Class: `zlib.Gzip`", "type": "class", "name": "zlib.Gzip", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "desc": "Compress data using gzip.
" }, { "textRaw": "Class: `zlib.Inflate`", "type": "class", "name": "zlib.Inflate", "meta": { "added": [ "v0.5.8" ], "changes": [ { "version": "v5.0.0", "pr-url": "https://github.com/nodejs/node/pull/2595", "description": "A truncated input stream will now result in an `'error'` event." } ] }, "desc": "Decompress a deflate stream.
" }, { "textRaw": "Class: `zlib.InflateRaw`", "type": "class", "name": "zlib.InflateRaw", "meta": { "added": [ "v0.5.8" ], "changes": [ { "version": "v6.8.0", "pr-url": "https://github.com/nodejs/node/pull/8512", "description": "Custom dictionaries are now supported by `InflateRaw`." }, { "version": "v5.0.0", "pr-url": "https://github.com/nodejs/node/pull/2595", "description": "A truncated input stream will now result in an `'error'` event." } ] }, "desc": "Decompress a raw deflate stream.
" }, { "textRaw": "Class: `zlib.Unzip`", "type": "class", "name": "zlib.Unzip", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "desc": "Decompress either a Gzip- or Deflate-compressed stream by auto-detecting\nthe header.
" }, { "textRaw": "Class: `zlib.ZlibBase`", "type": "class", "name": "zlib.ZlibBase", "meta": { "added": [ "v0.5.8" ], "changes": [ { "version": [ "v11.7.0", "v10.16.0" ], "pr-url": "https://github.com/nodejs/node/pull/24939", "description": "This class was renamed from `Zlib` to `ZlibBase`." } ] }, "desc": "Not exported by the zlib module. It is documented here because it is the base\nclass of the compressor/decompressor classes.
This class inherits from stream.Transform, allowing zlib objects to be\nused in pipes and similar stream operations.
Deprecated alias for zlib.bytesWritten. This original name was chosen\nbecause it also made sense to interpret the value as the number of bytes\nread by the engine, but is inconsistent with other streams in Node.js that\nexpose values under these names.
The zlib.bytesWritten property specifies the number of bytes written to\nthe engine, before the bytes are processed (compressed or decompressed,\nas appropriate for the derived class).
Close the underlying handle.
" }, { "textRaw": "`zlib.flush([kind, ]callback)`", "type": "method", "name": "flush", "meta": { "added": [ "v0.5.8" ], "changes": [] }, "signatures": [ { "params": [ { "textRaw": "`kind` **Default:** `zlib.constants.Z_FULL_FLUSH` for zlib-based streams, `zlib.constants.BROTLI_OPERATION_FLUSH` for Brotli-based streams.", "name": "kind", "default": "`zlib.constants.Z_FULL_FLUSH` for zlib-based streams, `zlib.constants.BROTLI_OPERATION_FLUSH` for Brotli-based streams" }, { "textRaw": "`callback` {Function}", "name": "callback", "type": "Function" } ] } ], "desc": "Flush pending data. Don't call this frivolously, premature flushes negatively\nimpact the effectiveness of the compression algorithm.
\nCalling this only flushes data from the internal zlib state, and does not\nperform flushing of any kind on the streams level. Rather, it behaves like a\nnormal call to .write(), i.e. it will be queued up behind other pending\nwrites and will only produce output when data is being read from the stream.
This function is only available for zlib-based streams, i.e. not Brotli.
\nDynamically update the compression level and compression strategy.\nOnly applicable to deflate algorithm.
" }, { "textRaw": "`zlib.reset()`", "type": "method", "name": "reset", "meta": { "added": [ "v0.7.0" ], "changes": [] }, "signatures": [ { "params": [] } ], "desc": "Reset the compressor/decompressor to factory defaults. Only applicable to\nthe inflate and deflate algorithms.
" } ] } ], "properties": [ { "textRaw": "`zlib.constants`", "name": "constants", "meta": { "added": [ "v7.0.0" ], "changes": [] }, "desc": "Provides an object enumerating Zlib-related constants.
" } ], "methods": [ { "textRaw": "`zlib.createBrotliCompress([options])`", "type": "method", "name": "createBrotliCompress", "meta": { "added": [ "v11.7.0", "v10.16.0" ], "changes": [] }, "signatures": [ { "params": [ { "textRaw": "`options` {brotli options}", "name": "options", "type": "brotli options" } ] } ], "desc": "Creates and returns a new BrotliCompress object.
Creates and returns a new BrotliDecompress object.
Creates and returns a new Deflate object.
Creates and returns a new DeflateRaw object.
An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits\nis set to 8 for raw deflate streams. zlib would automatically set windowBits\nto 9 if was initially set to 8. Newer versions of zlib will throw an exception,\nso Node.js restored the original behavior of upgrading a value of 8 to 9,\nsince passing windowBits = 9 to zlib actually results in a compressed stream\nthat effectively uses an 8-bit window only.
Creates and returns a new Gunzip object.
Creates and returns a new Gzip object.\nSee example.
Creates and returns a new Inflate object.
Creates and returns a new InflateRaw object.
Creates and returns a new Unzip object.
Compress a chunk of data with BrotliCompress.
Decompress a chunk of data with BrotliDecompress.
Compress a chunk of data with Deflate.
Compress a chunk of data with DeflateRaw.
Decompress a chunk of data with Gunzip.
Compress a chunk of data with Gzip.
Decompress a chunk of data with Inflate.
Decompress a chunk of data with InflateRaw.
Decompress a chunk of data with Unzip.