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Zlib

Stability: 2 - Stable

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

var 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:

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

inp.pipe(gzip).pipe(out);

Threadpool Usage

Note that all zlib APIs except those that are explicitly synchronous use libuv's threadpool, which can have surprising and negative performance implications for some applications, see the [UV_THREADPOOL_SIZE][] documentation for more information.

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 should be set to:

var options = { 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.

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 {integer} (default: zlib.constants.Z_NO_FLUSH)
  • finishFlush {integer} (default: zlib.constants.Z_FINISH)
  • chunkSize {integer} (default: 16*1024)
  • windowBits {integer}
  • level {integer} (compression only)
  • memLevel {integer} (compression only)
  • strategy {integer} (compression only)
  • dictionary {Buffer|TypedArray|DataView|ArrayBuffer} (deflate/inflate only, empty dictionary by default)
  • info {boolean} (If true, returns an object with buffer and engine)

See the description of deflateInit2 and inflateInit2 at https://zlib.net/manual.html#Advanced 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.bytesRead

  • {number}

The zlib.bytesRead property specifies the number of bytes read by the engine before the bytes are processed (compressed or decompressed, as appropriate for the derived class).

zlib.close([callback])

Close the underlying handle.

zlib.flush([kind], callback)

kind defaults to zlib.constants.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.

zlib.reset()

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

zlib.constants

Provides an object enumerating Zlib-related constants.

zlib.createDeflate([options])

Creates and returns a new [Deflate][] object with the given [options][].

zlib.createDeflateRaw([options])

Creates and returns a new [DeflateRaw][] object with the given [options][].

Note: An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits is set to 8 for raw deflate streams. zlib would automatically set windowBits to 9 if was initially set to 8. Newer versions of zlib will throw an exception, so Node.js restored the original behavior of upgrading a value of 8 to 9, since passing windowBits = 9 to zlib actually results in a compressed stream that effectively uses an 8-bit window only.

zlib.createGunzip([options])

Creates and returns a new [Gunzip][] object with the given [options][].

zlib.createGzip([options])

Creates and returns a new [Gzip][] object with the given [options][].

zlib.createInflate([options])

Creates and returns a new [Inflate][] object with the given [options][].

zlib.createInflateRaw([options])

Creates and returns a new [InflateRaw][] object with the given [options][].

zlib.createUnzip([options])

Creates and returns a new [Unzip][] object with the given [options][].