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Editing: README.md

# compression

[![NPM Version][npm-image]][npm-url]
[![NPM Downloads][downloads-image]][downloads-url]
[![Build Status][travis-image]][travis-url]
[![Test Coverage][coveralls-image]][coveralls-url]

Node.js compression middleware.

The following compression codings are supported:

- deflate
  - gzip

## Install

This is a [Node.js](https://nodejs.org/en/) module available through the
[npm registry](https://www.npmjs.com/). Installation is done using the
[`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):

```bash
$ npm install compression
```

## API

```js
var compression = require('compression')
```

### compression([options])

Returns the compression middleware using the given `options`. The middleware
will attempt to compress response bodies for all request that traverse through
the middleware, based on the given `options`.

This middleware will never compress responses that include a `Cache-Control`
header with the [`no-transform` directive](https://tools.ietf.org/html/rfc7234#section-5.2.2.4),
as compressing will transform the body.

#### Options

`compression()` accepts these properties in the options object. In addition to
those listed below, [zlib](http://nodejs.org/api/zlib.html) options may be
passed in to the options object.

##### chunkSize

The default value is `zlib.Z_DEFAULT_CHUNK`, or `16384`.

See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
regarding the usage.

##### filter

A function to decide if the response should be considered for compression.
This function is called as `filter(req, res)` and is expected to return
`true` to consider the response for compression, or `false` to not compress
the response.

The default filter function uses the [compressible](https://www.npmjs.com/package/compressible)
module to determine if `res.getHeader('Content-Type')` is compressible.

##### level

The level of zlib compression to apply to responses. 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.

This is an integer in the range of `0` (no compression) to `9` (maximum
compression). The special value `-1` can be used to mean the "default
compression level", which is a default compromise between speed and
compression (currently equivalent to level 6).

- `-1` Default compression level (also `zlib.Z_DEFAULT_COMPRESSION`).
  - `0` No compression (also `zlib.Z_NO_COMPRESSION`).
  - `1` Fastest compression (also `zlib.Z_BEST_SPEED`).
  - `2`
  - `3`
  - `4`
  - `5`
  - `6` (currently what `zlib.Z_DEFAULT_COMPRESSION` points to).
  - `7`
  - `8`
  - `9` Best compression (also `zlib.Z_BEST_COMPRESSION`).

The default value is `zlib.Z_DEFAULT_COMPRESSION`, or `-1`.

**Note** in the list above, `zlib` is from `zlib = require('zlib')`.

##### memLevel

This specifies how much memory should be allocated for the internal compression
state and is an integer in the range of `1` (minimum level) and `9` (maximum
level).

The default value is `zlib.Z_DEFAULT_MEMLEVEL`, or `8`.

See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
regarding the usage.

##### strategy

This is used to tune the compression algorithm. This value only affects the
compression ratio, not the correctness of the compressed output, even if it
is not set appropriately.

- `zlib.Z_DEFAULT_STRATEGY` Use for normal data.
  - `zlib.Z_FILTERED` Use for data produced by a filter (or predictor).
    Filtered data consists mostly of small values with a somewhat random
    distribution. In this case, the compression algorithm is tuned to
    compress them better. The effect is to force more Huffman coding and less
    string matching; it is somewhat intermediate between `zlib.Z_DEFAULT_STRATEGY`
    and `zlib.Z_HUFFMAN_ONLY`.
  - `zlib.Z_FIXED` Use to prevent the use of dynamic Huffman codes, allowing
    for a simpler decoder for special applications.
  - `zlib.Z_HUFFMAN_ONLY` Use to force Huffman encoding only (no string match).
  - `zlib.Z_RLE` Use to limit match distances to one (run-length encoding).
    This is designed to be almost as fast as `zlib.Z_HUFFMAN_ONLY`, but give
    better compression for PNG image data.

**Note** in the list above, `zlib` is from `zlib = require('zlib')`.

##### threshold

The byte threshold for the response body size before compression is considered
for the response, defaults to `1kb`. This is a number of bytes or any string
accepted by the [bytes](https://www.npmjs.com/package/bytes) module.

**Note** this is only an advisory setting; if the response size cannot be determined
at the time the response headers are written, then it is assumed the response is
_over_ the threshold. To guarantee the response size can be determined, be sure
set a `Content-Length` response header.

##### windowBits

The default value is `zlib.Z_DEFAULT_WINDOWBITS`, or `15`.

See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
regarding the usage.

#### .filter

The default `filter` function. This is used to construct a custom filter
function that is an extension of the default function.

```js
var compression = require('compression')
var express = require('express')

var app = express()
app.use(compression({ filter: shouldCompress }))

function shouldCompress (req, res) {
  if (req.headers['x-no-compression']) {
    // don't compress responses with this request header
    return false
  }

// fallback to standard filter function
  return compression.filter(req, res)
}
```

### res.flush

This module adds a `res.flush()` method to force the partially-compressed
response to be flushed to the client.

## Examples

### express/connect

When using this module with express or connect, simply `app.use` the module as
high as you like. Requests that pass through the middleware will be compressed.

```js
var compression = require('compression')
var express = require('express')

var app = express()

// compress all responses
app.use(compression())

// add all routes
```

### Server-Sent Events

Because of the nature of compression this module does not work out of the box
with server-sent events. To compress content, a window of the output needs to
be buffered up in order to get good compression. Typically when using server-sent
events, there are certain block of data that need to reach the client.

You can achieve this by calling `res.flush()` when you need the data written to
actually make it to the client.

```js
var compression = require('compression')
var express = require('express')

var app = express()

// compress responses
app.use(compression())

// server-sent event stream
app.get('/events', function (req, res) {
  res.setHeader('Content-Type', 'text/event-stream')
  res.setHeader('Cache-Control', 'no-cache')

// send a ping approx every 2 seconds
  var timer = setInterval(function () {
    res.write('data: ping\n\n')

// !!! this is the important part
    res.flush()
  }, 2000)

res.on('close', function () {
    clearInterval(timer)
  })
})
```

## License

[MIT](LICENSE)

[npm-image]: https://img.shields.io/npm/v/compression.svg
[npm-url]: https://npmjs.org/package/compression
[travis-image]: https://img.shields.io/travis/expressjs/compression/master.svg
[travis-url]: https://travis-ci.org/expressjs/compression
[coveralls-image]: https://img.shields.io/coveralls/expressjs/compression/master.svg
[coveralls-url]: https://coveralls.io/r/expressjs/compression?branch=master
[downloads-image]: https://img.shields.io/npm/dm/compression.svg
[downloads-url]: https://npmjs.org/package/compression

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