compression

Node.js compression middleware.

The following compression codings are supported:

• deflate
• gzip

Install

This is a Node.js module available through the
npm registry. Installation is done using the
npm install command:

$ npm install compression

API

var compression = require('compression')

compression([options])

Returns the compression middleware using the given options. The middleware
will attempt to compress response bodies for all requests 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,
as compressing will transform the body.

Options

compression() accepts these properties in the options object. In addition to
those listed below, zlib options may be
passed in to the options object.

chunkSize

The default value is zlib.Z_DEFAULT_CHUNK, or 16384.

See Node.js documentation
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
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
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 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
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.

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.

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.

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