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# on-headers
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[![NPM Version][npm-version-image]][npm-url]
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[![NPM Downloads][npm-downloads-image]][npm-url]
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[![Node.js Version][node-version-image]][node-version-url]
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[![Build Status][travis-image]][travis-url]
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[![Test Coverage][coveralls-image]][coveralls-url]
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Execute a listener when a response is about to write headers.
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## Installation
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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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```sh
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$ npm install on-headers
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```
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## API
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<!-- eslint-disable no-unused-vars -->
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```js
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var onHeaders = require('on-headers')
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```
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### onHeaders(res, listener)
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This will add the listener `listener` to fire when headers are emitted for `res`.
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The listener is passed the `response` object as it's context (`this`). Headers are
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considered to be emitted only once, right before they are sent to the client.
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When this is called multiple times on the same `res`, the `listener`s are fired
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in the reverse order they were added.
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## Examples
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```js
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var http = require('http')
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var onHeaders = require('on-headers')
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http
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.createServer(onRequest)
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.listen(3000)
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function addPoweredBy () {
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// set if not set by end of request
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if (!this.getHeader('X-Powered-By')) {
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this.setHeader('X-Powered-By', 'Node.js')
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}
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}
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function onRequest (req, res) {
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onHeaders(res, addPoweredBy)
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res.setHeader('Content-Type', 'text/plain')
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res.end('hello!')
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}
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```
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## Testing
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```sh
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$ npm test
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```
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## License
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[MIT](LICENSE)
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[coveralls-image]: https://badgen.net/coveralls/c/github/jshttp/on-headers/master
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[coveralls-url]: https://coveralls.io/r/jshttp/on-headers?branch=master
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[node-version-image]: https://badgen.net/npm/node/on-headers
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[node-version-url]: https://nodejs.org/en/download
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[npm-downloads-image]: https://badgen.net/npm/dm/on-headers
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[npm-url]: https://npmjs.org/package/on-headers
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[npm-version-image]: https://badgen.net/npm/v/on-headers
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[travis-image]: https://badgen.net/travis/jshttp/on-headers/master
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[travis-url]: https://travis-ci.org/jshttp/on-headers
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