Elep

# Zlib

> Stability: 2 - Stable

The `zlib` module provides compression functionality implemented using Gzip and
Deflate/Inflate, as well as Brotli. It can be accessed using:

```js
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:

```js
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);
```

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

```js
const input = '.................................';
zlib.deflate(input, (err, buffer) => {
  if (!err) {
    console.log(buffer.toString('base64'));
  } else {
    // handle error
  }
});

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

## Threadpool Usage

Note that all zlib APIs except those that are explicitly synchronous use libuv's
threadpool. This can lead to surprising effects in some applications, such as
subpar performance (which can be mitigated by adjusting the [pool size][])
and/or unrecoverable and catastrophic memory fragmentation.

## Compressing HTTP requests and responses

The `zlib` module can be used to implement support for the `gzip`, `deflate`
and `br` content-encoding mechanisms defined by
[HTTP](https://tools.ietf.org/html/rfc7230#section-4.2).

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.

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.

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

switch (response.headers['content-encoding']) {
    case 'br':
      response.pipe(zlib.createBrotliDecompress()).pipe(output);
      break;
    // Or, just use zlib.createUnzip() to handle both of the following cases:
    case 'gzip':
      response.pipe(zlib.createGunzip()).pipe(output);
      break;
    case 'deflate':
      response.pipe(zlib.createInflate()).pipe(output);
      break;
    default:
      response.pipe(output);
      break;
  }
});
```

```js
// 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) => {
  const raw = fs.createReadStream('index.html');
  let acceptEncoding = request.headers['accept-encoding'];
  if (!acceptEncoding) {
    acceptEncoding = '';
  }

// Note: This is not a conformant accept-encoding parser.
  // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (/\bdeflate\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    raw.pipe(zlib.createDeflate()).pipe(response);
  } else if (/\bgzip\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    raw.pipe(zlib.createGzip()).pipe(response);
  } else if (/\bbr\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'br' });
    raw.pipe(zlib.createBrotliCompress()).pipe(response);
  } else {
    response.writeHead(200, {});
    raw.pipe(response);
  }
}).listen(1337);
```

By default, the `zlib` methods will 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 decompress the last chunk of input data:

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

zlib.unzip(
  buffer,
  // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
  { finishFlush: zlib.constants.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

### For zlib-based streams

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

The memory requirements for deflate are (in bytes):

```js
(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:

```js
const 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.

### For Brotli-based streams

There are equivalents to the zlib options for Brotli-based streams, although
these options have different ranges than the zlib ones:

- zlib’s `level` option matches Brotli’s `BROTLI_PARAM_QUALITY` option.
- zlib’s `windowBits` option matches Brotli’s `BROTLI_PARAM_LGWIN` option.

See [below][Brotli parameters] for more details on Brotli-specific options.

## 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:
```js
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

### zlib constants

All of the constants defined in `zlib.h` are also defined on
`require('zlib').constants`. 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 <https://zlib.net/manual.html#Constants> for more
details.

Previously, the constants were available directly from `require('zlib')`, for
instance `zlib.Z_NO_FLUSH`. Accessing the constants directly from the module is
currently still possible but is deprecated.

Allowed flush values.

* `zlib.constants.Z_NO_FLUSH`
* `zlib.constants.Z_PARTIAL_FLUSH`
* `zlib.constants.Z_SYNC_FLUSH`
* `zlib.constants.Z_FULL_FLUSH`
* `zlib.constants.Z_FINISH`
* `zlib.constants.Z_BLOCK`
* `zlib.constants.Z_TREES`

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

* `zlib.constants.Z_OK`
* `zlib.constants.Z_STREAM_END`
* `zlib.constants.Z_NEED_DICT`
* `zlib.constants.Z_ERRNO`
* `zlib.constants.Z_STREAM_ERROR`
* `zlib.constants.Z_DATA_ERROR`
* `zlib.constants.Z_MEM_ERROR`
* `zlib.constants.Z_BUF_ERROR`
* `zlib.constants.Z_VERSION_ERROR`

Compression levels.

* `zlib.constants.Z_NO_COMPRESSION`
* `zlib.constants.Z_BEST_SPEED`
* `zlib.constants.Z_BEST_COMPRESSION`
* `zlib.constants.Z_DEFAULT_COMPRESSION`

Compression strategy.

* `zlib.constants.Z_FILTERED`
* `zlib.constants.Z_HUFFMAN_ONLY`
* `zlib.constants.Z_RLE`
* `zlib.constants.Z_FIXED`
* `zlib.constants.Z_DEFAULT_STRATEGY`

### Brotli constants

There are several options and other constants available for Brotli-based
streams:

#### Flush operations

The following values are valid flush operations for Brotli-based streams:

* `zlib.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`
  * This particular operation may be hard to use in a Node.js context,
     as the streaming layer makes it hard to know which data will end up
     in this frame. Also, there is currently no way to consume this data through
     the Node.js API.

#### Compressor options

There are several options that can be set on Brotli encoders, affecting
compression efficiency and speed. Both the keys and the values can be accessed
as properties of the `zlib.constants` object.

The most important options are:

* `BROTLI_PARAM_MODE`
  * `BROTLI_MODE_GENERIC` (default)
  * `BROTLI_MODE_TEXT`, adjusted for UTF-8 text
  * `BROTLI_MODE_FONT`, adjusted for WOFF 2.0 fonts
* `BROTLI_PARAM_QUALITY`
  * Ranges from `BROTLI_MIN_QUALITY` to `BROTLI_MAX_QUALITY`,
    with a default of `BROTLI_DEFAULT_QUALITY`.
* `BROTLI_PARAM_SIZE_HINT`
  * Integer value representing the expected input size;
    defaults to `0` for an unknown input size.

The following flags can be set for advanced control over the compression
algorithm and memory usage tuning:

* `BROTLI_PARAM_LGWIN`
  * Ranges from `BROTLI_MIN_WINDOW_BITS` to `BROTLI_MAX_WINDOW_BITS`,
    with a default of `BROTLI_DEFAULT_WINDOW`, or up to
    `BROTLI_LARGE_MAX_WINDOW_BITS` if the `BROTLI_PARAM_LARGE_WINDOW` flag
    is set.
* `BROTLI_PARAM_LGBLOCK`
  * Ranges from `BROTLI_MIN_INPUT_BLOCK_BITS` to `BROTLI_MAX_INPUT_BLOCK_BITS`.
* `BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING`
  * Boolean flag that decreases compression ratio in favour of
    decompression speed.
* `BROTLI_PARAM_LARGE_WINDOW`
  * Boolean flag enabling “Large Window Brotli” mode (not compatible with the
    Brotli format as standardized in [RFC 7932][]).
* `BROTLI_PARAM_NPOSTFIX`
  * Ranges from `0` to `BROTLI_MAX_NPOSTFIX`.
* `BROTLI_PARAM_NDIRECT`
  * Ranges from `0` to `15 << NPOSTFIX` in steps of `1 << NPOSTFIX`.

#### Decompressor options

These advanced options are available for controlling decompression:

* `BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION`
  * Boolean flag that affects internal memory allocation patterns.
* `BROTLI_DECODER_PARAM_LARGE_WINDOW`
  * Boolean flag enabling “Large Window Brotli” mode (not compatible with the
    Brotli format as standardized in [RFC 7932][]).

## Class: Options

Each zlib-based 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: BrotliOptions

Each Brotli-based class takes an `options` object. All options are optional.

* `flush` {integer} **Default:** `zlib.constants.BROTLI_OPERATION_PROCESS`
* `finishFlush` {integer} **Default:** `zlib.constants.BROTLI_OPERATION_FINISH`
* `chunkSize` {integer} **Default:** `16 * 1024`
* `params` {Object} Key-value object containing indexed [Brotli parameters][].

For example:

```js
const stream = zlib.createBrotliCompress({
  chunkSize: 32 * 1024,
  params: {
    [zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,
    [zlib.constants.BROTLI_PARAM_QUALITY]: 4,
    [zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size
  }
});
```

## Class: zlib.BrotliCompress

Compress data using the Brotli algorithm.

## Class: zlib.BrotliDecompress

Decompress data using the Brotli algorithm.

## 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.ZlibBase

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

This class inherits from [`stream.Transform`][], allowing `zlib` objects to be
used in pipes and similar stream operations.

### zlib.bytesRead

> Stability: 0 - Deprecated: Use [`zlib.bytesWritten`][] instead.

* {number}

Deprecated alias for [`zlib.bytesWritten`][]. This original name was chosen
because it also made sense to interpret the value as the number of bytes
read by the engine, but is inconsistent with other streams in Node.js that
expose values under these names.

### zlib.bytesWritten

* {number}

The `zlib.bytesWritten` property specifies the number of bytes written to
the engine, before the bytes are processed (compressed or decompressed,
as appropriate for the derived class).

### zlib.close([callback])

* `callback` {Function}

Close the underlying handle.

### zlib.flush([kind, ]callback)

* `kind` **Default:** `zlib.constants.Z_FULL_FLUSH` for zlib-based streams,
  `zlib.constants.BROTLI_OPERATION_FLUSH` for Brotli-based streams.
* `callback` {Function}

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)

* `level` {integer}
* `strategy` {integer}
* `callback` {Function}

This function is only available for zlib-based streams, i.e. not Brotli.

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.createBrotliCompress([options])

* `options` {brotli options}

Creates and returns a new [`BrotliCompress`][] object.

## zlib.createBrotliDecompress([options])

* `options` {brotli options}

Creates and returns a new [`BrotliDecompress`][] object.

## zlib.createDeflate([options])

* `options` {Object}

Creates and returns a new [`Deflate`][] object.

## zlib.createDeflateRaw([options])

* `options` {Object}

Creates and returns a new [`DeflateRaw`][] object.

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])

* `options` {Object}

Creates and returns a new [`Gunzip`][] object.

## zlib.createGzip([options])

* `options` {Object}

Creates and returns a new [`Gzip`][] object.

## zlib.createInflate([options])

* `options` {Object}

Creates and returns a new [`Inflate`][] object.

## zlib.createInflateRaw([options])

* `options` {Object}

Creates and returns a new [`InflateRaw`][] object.

## zlib.createUnzip([options])

* `options` {Object}

Creates and returns a new [`Unzip`][] object.

## Convenience Methods

All of these take a [`Buffer`][], [`TypedArray`][], [`DataView`][],
[`ArrayBuffer`][] 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.brotliCompress(buffer[, options], callback)

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {brotli options}
* `callback` {Function}

### zlib.brotliCompressSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {brotli options}

Compress a chunk of data with [`BrotliCompress`][].

### zlib.brotliDecompress(buffer[, options], callback)

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {brotli options}
* `callback` {Function}

### zlib.brotliDecompressSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {brotli options}

Decompress a chunk of data with [`BrotliDecompress`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.deflateSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Compress a chunk of data with [`Deflate`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.deflateRawSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Compress a chunk of data with [`DeflateRaw`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.gunzipSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Decompress a chunk of data with [`Gunzip`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.gzipSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Compress a chunk of data with [`Gzip`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.inflateSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Decompress a chunk of data with [`Inflate`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.inflateRawSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Decompress a chunk of data with [`InflateRaw`][].

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

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}
* `callback` {Function}

### zlib.unzipSync(buffer[, options])

* `buffer` {Buffer|TypedArray|DataView|ArrayBuffer|string}
* `options` {Object}

Decompress a chunk of data with [`Unzip`][].

[`.flush()`]: #zlib_zlib_flush_kind_callback
[`Accept-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
[`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
[`BrotliCompress`]: #zlib_class_zlib_brotlicompress
[`BrotliDecompress`]: #zlib_class_zlib_brotlidecompress
[`Buffer`]: buffer.html#buffer_class_buffer
[`Content-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11
[`DataView`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView
[`DeflateRaw`]: #zlib_class_zlib_deflateraw
[`Deflate`]: #zlib_class_zlib_deflate
[`Gunzip`]: #zlib_class_zlib_gunzip
[`Gzip`]: #zlib_class_zlib_gzip
[`InflateRaw`]: #zlib_class_zlib_inflateraw
[`Inflate`]: #zlib_class_zlib_inflate
[`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[`Unzip`]: #zlib_class_zlib_unzip
[`stream.Transform`]: stream.html#stream_class_stream_transform
[`zlib.bytesWritten`]: #zlib_zlib_byteswritten
[Brotli parameters]: #zlib_brotli_constants
[Memory Usage Tuning]: #zlib_memory_usage_tuning
[RFC 7932]: https://www.rfc-editor.org/rfc/rfc7932.txt
[pool size]: cli.html#cli_uv_threadpool_size_size
[zlib documentation]: https://zlib.net/manual.html#Constants