Node.js v7.0.0-nightly20160601f81f0c351a Documentation


Stability: 2 - Stable

The zlib module provides compression functionality implemented using Gzip and Deflate/Inflate. It can be accessed using:

const zlib = require('zlib');

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream data through a zlib stream into a destination stream:

const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');


It is also possible to compress or decompress data in a single step:

const input = '.................................';
zlib.deflate(input, (err, buffer) => {
  if (!err) {
  } else {
    // handle error

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
  if (!err) {
  } else {
    // handle error

Compressing HTTP requests and responses#

The zlib module can be used to implement support for the gzip and deflate content-encoding mechanisms defined by HTTP.

The HTTP Accept-Encoding header is used within an http request to identify the compression encodings accepted by the client. The Content-Encoding header is used to identify the compression encodings actually applied to a message.

Note: the examples given below are drastically simplified to show the basic concept. Using zlib encoding can be expensive, and the results ought to be cached. See Memory Usage Tuning for more information on the speed/memory/compression tradeoffs involved in zlib usage.

// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: '',
                         path: '/',
                         port: 80,
                         headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
  var output = fs.createWriteStream('example.com_index.html');

  switch (response.headers['content-encoding']) {
    // or, just use zlib.createUnzip() to handle both cases
    case 'gzip':
    case 'deflate':

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
http.createServer((request, response) => {
  var raw = fs.createReadStream('index.html');
  var acceptEncoding = request.headers['accept-encoding'];
  if (!acceptEncoding) {
    acceptEncoding = '';

  // Note: this is not a conformant accept-encoding parser.
  // See
  if (acceptEncoding.match(/\bdeflate\b/)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
  } else if (acceptEncoding.match(/\bgzip\b/)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
  } else {
    response.writeHead(200, {});

By default, the zlib methods with throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to compressed the last chunk of input data:

// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');

zlib.unzip(buffer, { finishFlush: zlib.Z_SYNC_FLUSH }, (err, buffer) => {
  if (!err) {
  } else {
    // handle error

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

Memory Usage Tuning#

From zlib/zconf.h, modified to node.js's usage:

The memory requirements for deflate are (in bytes):

(1 << (windowBits+2)) +  (1 << (memLevel+9))

That is: 128K for windowBits=15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options shoud be set to:

{ windowBits: 14, memLevel: 7 }

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes)

1 << windowBits

That is, 32K for windowBits=15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size chunkSize, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to zlib because it will be able to process more data on each write operation. So, this is another factor that affects the speed, at the cost of memory usage.


Calling .flush() on a compression stream will make zlib return as much output as currently possible. This may come at the cost of degraded compression quality, 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 HTTP response to the client:

const zlib = require('zlib');
const http = require('http');

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();

  setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
  }, 1000);


All of the constants defined in zlib.h are also defined on require('zlib'). In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation. See for more details.

Allowed flush values.

  • zlib.Z_NO_FLUSH
  • zlib.Z_SYNC_FLUSH
  • zlib.Z_FULL_FLUSH
  • zlib.Z_FINISH
  • zlib.Z_BLOCK
  • zlib.Z_TREES

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

  • zlib.Z_OK
  • zlib.Z_STREAM_END
  • zlib.Z_NEED_DICT
  • zlib.Z_ERRNO
  • zlib.Z_DATA_ERROR
  • zlib.Z_MEM_ERROR
  • zlib.Z_BUF_ERROR

Compression levels.

  • zlib.Z_BEST_SPEED

Compression strategy.

  • zlib.Z_FILTERED
  • zlib.Z_RLE
  • zlib.Z_FIXED

Possible values of the data_type field.

  • zlib.Z_BINARY
  • zlib.Z_TEXT
  • zlib.Z_ASCII
  • zlib.Z_UNKNOWN

The deflate compression method (the only one supported in this version).

  • zlib.Z_DEFLATED

For initializing zalloc, zfree, opaque.

  • zlib.Z_NULL

Class Options#

Each class takes an options object. All options are optional.

Note that some options are only relevant when compressing, and are ignored by the decompression classes.

  • flush (default: zlib.Z_NO_FLUSH)
  • finishFlush (default: zlib.Z_FINISH)
  • chunkSize (default: 16*1024)
  • windowBits
  • level (compression only)
  • memLevel (compression only)
  • strategy (compression only)
  • dictionary (deflate/inflate only, empty dictionary by default)

See the description of deflateInit2 and inflateInit2 at for more information on these.

Class: zlib.Deflate#

Compress data using deflate.

Class: zlib.DeflateRaw#

Compress data using deflate, and do not append a zlib header.

Class: zlib.Gunzip#

Decompress a gzip stream.

Class: zlib.Gzip#

Compress data using gzip.

Class: zlib.Inflate#

Decompress a deflate stream.

Class: zlib.InflateRaw#

Decompress a raw deflate stream.

Class: zlib.Unzip#

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

Class: zlib.Zlib#

Not exported by the zlib module. It is documented here because it is the base class of the compressor/decompressor classes.

zlib.flush([kind], callback)#

kind defaults to zlib.Z_FULL_FLUSH.

Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal zlib state, and does not perform flushing of any kind on the streams level. Rather, it behaves like a normal call to .write(), i.e. it will be queued up behind other pending writes and will only produce output when data is being read from the stream.

zlib.params(level, strategy, callback)#

Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.


Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.


Returns a new Deflate object with an options.


Returns a new DeflateRaw object with an options.


Returns a new Gunzip object with an options.


Returns a new Gzip object with an options.


Returns a new Inflate object with an options.


Returns a new InflateRaw object with an options.


Returns a new Unzip object with an options.

Convenience Methods#

All of these take a Buffer or string as the first argument, an optional second argument to supply options to the zlib classes and will call the supplied callback with callback(error, result).

Every method has a *Sync counterpart, which accept the same arguments, but without a callback.

zlib.deflate(buf[, options], callback)#

zlib.deflateSync(buf[, options])#

Compress a Buffer or string with Deflate.

zlib.deflateRaw(buf[, options], callback)#

zlib.deflateRawSync(buf[, options])#

Compress a Buffer or string with DeflateRaw.

zlib.gunzip(buf[, options], callback)#

zlib.gunzipSync(buf[, options])#

Decompress a Buffer or string with Gunzip.

zlib.gzip(buf[, options], callback)#

zlib.gzipSync(buf[, options])#

Compress a Buffer or string with Gzip.

zlib.inflate(buf[, options], callback)#

zlib.inflateSync(buf[, options])#

Decompress a Buffer or string with Inflate.

zlib.inflateRaw(buf[, options], callback)#

zlib.inflateRawSync(buf[, options])#

Decompress a Buffer or string with InflateRaw.

zlib.unzip(buf[, options], callback)#

zlib.unzipSync(buf[, options])#

Decompress a Buffer or string with Unzip.