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Node.js v5.11.0 Documentation
Table of Contents
- Zlib
- Examples
- Memory Usage Tuning
- Flushing
- Constants
- Class Options
- Class: zlib.Deflate
- Class: zlib.DeflateRaw
- Class: zlib.Gunzip
- Class: zlib.Gzip
- Class: zlib.Inflate
- Class: zlib.InflateRaw
- Class: zlib.Unzip
- Class: zlib.Zlib
- zlib.createDeflate([options])
- zlib.createDeflateRaw([options])
- zlib.createGunzip([options])
- zlib.createGzip([options])
- zlib.createInflate([options])
- zlib.createInflateRaw([options])
- zlib.createUnzip([options])
- Convenience Methods
- zlib.deflate(buf[, options], callback)
- zlib.deflateSync(buf[, options])
- zlib.deflateRaw(buf[, options], callback)
- zlib.deflateRawSync(buf[, options])
- zlib.gunzip(buf[, options], callback)
- zlib.gunzipSync(buf[, options])
- zlib.gzip(buf[, options], callback)
- zlib.gzipSync(buf[, options])
- zlib.inflate(buf[, options], callback)
- zlib.inflateSync(buf[, options])
- zlib.inflateRaw(buf[, options], callback)
- zlib.inflateRawSync(buf[, options])
- zlib.unzip(buf[, options], callback)
- zlib.unzipSync(buf[, options])
Zlib#
Stability: 2 - Stable
You can access this module with:
const zlib = require('zlib');
This provides bindings to Gzip/Gunzip, Deflate/Inflate, and DeflateRaw/InflateRaw classes. Each class takes the same options, and is a readable/writable Stream.
Examples#
Compressing or decompressing a file can be done by piping an fs.ReadStream into a zlib stream, then into an fs.WriteStream.
const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');
inp.pipe(gzip).pipe(out);
Compressing or decompressing data in one step can be done by using the convenience methods.
const input = '.................................';
zlib.deflate(input, (err, buffer) => {
if (!err) {
console.log(buffer.toString('base64'));
} else {
// handle error
}
});
const buffer = new Buffer('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
if (!err) {
console.log(buffer.toString());
} else {
// handle error
}
});
To use this module in an HTTP client or server, use the accept-encoding on requests, and the content-encoding header on responses.
Note: these examples are drastically simplified to show the basic concept. 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: 'izs.me',
path: '/',
port: 80,
headers: { 'accept-encoding': 'gzip,deflate' } });
request.on('response', (response) => {
var output = fs.createWriteStream('izs.me_index.html');
switch (response.headers['content-encoding']) {
// or, just use zlib.createUnzip() to handle both cases
case 'gzip':
response.pipe(zlib.createGunzip()).pipe(output);
break;
case 'deflate':
response.pipe(zlib.createInflate()).pipe(output);
break;
default:
response.pipe(output);
break;
}
});
// 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 http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (acceptEncoding.match(/\bdeflate\b/)) {
response.writeHead(200, { 'content-encoding': 'deflate' });
raw.pipe(zlib.createDeflate()).pipe(response);
} else if (acceptEncoding.match(/\bgzip\b/)) {
response.writeHead(200, { 'content-encoding': 'gzip' });
raw.pipe(zlib.createGzip()).pipe(response);
} else {
response.writeHead(200, {});
raw.pipe(response);
}
}).listen(1337);
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 = new Buffer('eJzT0yMA', 'base64');
zlib.unzip(buffer, { finishFlush: zlib.Z_SYNC_FLUSH }, (err, buffer) => {
if (!err) {
console.log(buffer.toString());
} 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, if you want to reduce the default memory requirements from 256K to 128K, set the options to:
{ windowBits: 14, memLevel: 7 }
Of course this will generally degrade compression (there's no free lunch).
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, since it'll be able to process more data in a
single write
operation. So, this is another factor that affects the
speed, at the cost of memory usage.
Flushing#
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();
output.pipe(response);
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.
output.flush();
});
}, 1000);
}).listen(1337);
Constants#
All of the constants defined in zlib.h are also defined on
require('zlib')
.
In the normal course of operations, you will not need to ever set any of
these. They are documented here so that their presence is not
surprising. This section is taken almost directly from the
zlib documentation. See http://zlib.net/manual.html#Constants for more
details.
Allowed flush values.
zlib.Z_NO_FLUSH
zlib.Z_PARTIAL_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_STREAM_ERROR
zlib.Z_DATA_ERROR
zlib.Z_MEM_ERROR
zlib.Z_BUF_ERROR
zlib.Z_VERSION_ERROR
Compression levels.
zlib.Z_NO_COMPRESSION
zlib.Z_BEST_SPEED
zlib.Z_BEST_COMPRESSION
zlib.Z_DEFAULT_COMPRESSION
Compression strategy.
zlib.Z_FILTERED
zlib.Z_HUFFMAN_ONLY
zlib.Z_RLE
zlib.Z_FIXED
zlib.Z_DEFAULT_STRATEGY
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
http://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.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.
zlib.reset()#
Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.
zlib.createDeflate(options)#
Returns a new Deflate object with an options.
zlib.createDeflateRaw(options)#
Returns a new DeflateRaw object with an options.
zlib.createGunzip(options)#
Returns a new Gunzip object with an options.
zlib.createGzip(options)#
Returns a new Gzip object with an options.
zlib.createInflate(options)#
Returns a new Inflate object with an options.
zlib.createInflateRaw(options)#
Returns a new InflateRaw object with an options.
zlib.createUnzip(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.