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# compression
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[![NPM Version][npm-image]][npm-url]
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[![NPM Downloads][downloads-image]][downloads-url]
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[![Build Status][github-actions-ci-image]][github-actions-ci-url]
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[![OpenSSF Scorecard Badge][ossf-scorecard-badge]][ossf-scorecard-visualizer]
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[![Funding][funding-image]][funding-url]
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Node.js compression middleware.
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The following compression codings are supported:
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- deflate
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- gzip
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- br (brotli)
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**Note** Brotli is supported only since Node.js versions v11.7.0 and v10.16.0.
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## Install
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This is a [Node.js](https://nodejs.org/en/) module available through the
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[npm registry](https://www.npmjs.com/). Installation is done using the
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[`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):
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```bash
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$ npm install compression
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```
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## API
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```js
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var compression = require('compression')
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```
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### compression([options])
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Returns the compression middleware using the given `options`. The middleware
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will attempt to compress response bodies for all requests that traverse through
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the middleware, based on the given `options`.
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This middleware will never compress responses that include a `Cache-Control`
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header with the [`no-transform` directive](https://tools.ietf.org/html/rfc7234#section-5.2.2.4),
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as compressing will transform the body.
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#### Options
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`compression()` accepts these properties in the options object. In addition to
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those listed below, [zlib](http://nodejs.org/api/zlib.html) options may be
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passed in to the options object or
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[brotli](https://nodejs.org/api/zlib.html#zlib_class_brotlioptions) options.
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##### chunkSize
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Type: `Number`<br>
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Default: `zlib.constants.Z_DEFAULT_CHUNK`, or `16384`.
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See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
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regarding the usage.
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##### filter
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Type: `Function`
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A function to decide if the response should be considered for compression.
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This function is called as `filter(req, res)` and is expected to return
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`true` to consider the response for compression, or `false` to not compress
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the response.
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The default filter function uses the [compressible](https://www.npmjs.com/package/compressible)
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module to determine if `res.getHeader('Content-Type')` is compressible.
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##### level
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Type: `Number`<br>
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Default: `zlib.constants.Z_DEFAULT_COMPRESSION`, or `-1`
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The level of zlib compression to apply to responses. A higher level will result
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in better compression, but will take longer to complete. A lower level will
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result in less compression, but will be much faster.
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This is an integer in the range of `0` (no compression) to `9` (maximum
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compression). The special value `-1` can be used to mean the "default
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compression level", which is a default compromise between speed and
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compression (currently equivalent to level 6).
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- `-1` Default compression level (also `zlib.constants.Z_DEFAULT_COMPRESSION`).
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- `0` No compression (also `zlib.constants.Z_NO_COMPRESSION`).
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- `1` Fastest compression (also `zlib.constants.Z_BEST_SPEED`).
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- `2`
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- `3`
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- `4`
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- `5`
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- `6` (currently what `zlib.constants.Z_DEFAULT_COMPRESSION` points to).
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- `7`
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- `8`
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- `9` Best compression (also `zlib.constants.Z_BEST_COMPRESSION`).
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**Note** in the list above, `zlib` is from `zlib = require('zlib')`.
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##### memLevel
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Type: `Number`<br>
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Default: `zlib.constants.Z_DEFAULT_MEMLEVEL`, or `8`
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This specifies how much memory should be allocated for the internal compression
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state and is an integer in the range of `1` (minimum level) and `9` (maximum
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level).
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See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
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regarding the usage.
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##### brotli
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Type: `Object`
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This specifies the options for configuring Brotli. See [Node.js documentation](https://nodejs.org/api/zlib.html#class-brotlioptions) for a complete list of available options.
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##### strategy
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Type: `Number`<br>
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Default: `zlib.constants.Z_DEFAULT_STRATEGY`
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This is used to tune the compression algorithm. This value only affects the
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compression ratio, not the correctness of the compressed output, even if it
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is not set appropriately.
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- `zlib.constants.Z_DEFAULT_STRATEGY` Use for normal data.
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- `zlib.constants.Z_FILTERED` Use for data produced by a filter (or predictor).
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Filtered data consists mostly of small values with a somewhat random
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distribution. In this case, the compression algorithm is tuned to
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compress them better. The effect is to force more Huffman coding and less
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string matching; it is somewhat intermediate between `zlib.constants.Z_DEFAULT_STRATEGY`
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and `zlib.constants.Z_HUFFMAN_ONLY`.
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- `zlib.constants.Z_FIXED` Use to prevent the use of dynamic Huffman codes, allowing
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for a simpler decoder for special applications.
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- `zlib.constants.Z_HUFFMAN_ONLY` Use to force Huffman encoding only (no string match).
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- `zlib.constants.Z_RLE` Use to limit match distances to one (run-length encoding).
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This is designed to be almost as fast as `zlib.constants.Z_HUFFMAN_ONLY`, but give
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better compression for PNG image data.
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**Note** in the list above, `zlib` is from `zlib = require('zlib')`.
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##### threshold
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Type: `Number` or `String`<br>
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Default: `1kb`
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The byte threshold for the response body size before compression is considered
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for the response. This is a number of bytes or any string
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accepted by the [bytes](https://www.npmjs.com/package/bytes) module.
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**Note** this is only an advisory setting; if the response size cannot be determined
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at the time the response headers are written, then it is assumed the response is
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_over_ the threshold. To guarantee the response size can be determined, be sure
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set a `Content-Length` response header.
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##### windowBits
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Type: `Number`<br>
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Default: `zlib.constants.Z_DEFAULT_WINDOWBITS`, or `15`
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See [Node.js documentation](http://nodejs.org/api/zlib.html#zlib_memory_usage_tuning)
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regarding the usage.
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##### enforceEncoding
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Type: `String`<br>
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Default: `identity`
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This is the default encoding to use when the client does not specify an encoding in the request's [Accept-Encoding](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding) header.
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#### .filter
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The default `filter` function. This is used to construct a custom filter
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function that is an extension of the default function.
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```js
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var compression = require('compression')
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var express = require('express')
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var app = express()
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app.use(compression({ filter: shouldCompress }))
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function shouldCompress (req, res) {
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if (req.headers['x-no-compression']) {
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// don't compress responses with this request header
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return false
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}
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// fallback to standard filter function
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return compression.filter(req, res)
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}
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```
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### res.flush
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This module adds a `res.flush()` method to force the partially-compressed
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response to be flushed to the client.
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## Examples
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### express
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When using this module with express, simply `app.use` the module as
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high as you like. Requests that pass through the middleware will be compressed.
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```js
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var compression = require('compression')
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var express = require('express')
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var app = express()
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// compress all responses
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app.use(compression())
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// add all routes
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```
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### Node.js HTTP server
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```js
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var compression = require('compression')({ threshold: 0 })
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var http = require('http')
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function createServer (fn) {
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return http.createServer(function (req, res) {
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compression(req, res, function (err) {
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if (err) {
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res.statusCode = err.status || 500
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res.end(err.message)
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return
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}
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fn(req, res)
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})
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})
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}
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var server = createServer(function (req, res) {
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res.setHeader('Content-Type', 'text/plain')
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res.end('hello world!')
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})
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server.listen(3000, () => {
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console.log('> Listening at http://localhost:3000')
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})
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```
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### Server-Sent Events
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Because of the nature of compression this module does not work out of the box
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with server-sent events. To compress content, a window of the output needs to
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be buffered up in order to get good compression. Typically when using server-sent
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events, there are certain block of data that need to reach the client.
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You can achieve this by calling `res.flush()` when you need the data written to
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actually make it to the client.
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```js
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var compression = require('compression')
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var express = require('express')
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var app = express()
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// compress responses
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app.use(compression())
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// server-sent event stream
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app.get('/events', function (req, res) {
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res.setHeader('Content-Type', 'text/event-stream')
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res.setHeader('Cache-Control', 'no-cache')
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// send a ping approx every 2 seconds
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var timer = setInterval(function () {
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res.write('data: ping\n\n')
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// !!! this is the important part
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res.flush()
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}, 2000)
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res.on('close', function () {
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clearInterval(timer)
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})
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})
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```
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## Contributing
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The Express.js project welcomes all constructive contributions. Contributions take many forms,
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from code for bug fixes and enhancements, to additions and fixes to documentation, additional
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tests, triaging incoming pull requests and issues, and more!
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See the [Contributing Guide](https://github.com/expressjs/express/blob/master/Contributing.md) for more technical details on contributing.
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## License
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[MIT](LICENSE)
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[npm-image]: https://badgen.net/npm/v/compression
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[npm-url]: https://npmjs.org/package/compression
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[downloads-image]: https://badgen.net/npm/dm/compression
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[downloads-url]: https://npmcharts.com/compare/compression?minimal=true
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[github-actions-ci-image]: https://badgen.net/github/checks/expressjs/compression/master?label=CI
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[github-actions-ci-url]: https://github.com/expressjs/compression/actions?query=workflow%3Aci
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[ossf-scorecard-badge]: https://api.scorecard.dev/projects/github.com/expressjs/compression/badge
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[ossf-scorecard-visualizer]: https://ossf.github.io/scorecard-visualizer/#/projects/github.com/expressjs/compression
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[funding-url]: https://opencollective.com/express
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[funding-image]: https://badgen.net/badge/icon/sponsor/pink?icon=github&label=Open%20Collective
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