1  # on-headers
 2  
 3  [![NPM Version][npm-version-image]][npm-url]
 4  [![NPM Downloads][npm-downloads-image]][npm-url]
 5  [![Node.js Version][node-image]][node-url]
 6  [![Build Status][ci-image]][ci-url]
 7  [![Coverage Status][coveralls-image]][coveralls-url]
 8  
 9  Execute a listener when a response is about to write headers.
10  
11  ## Installation
12  
13  This is a [Node.js](https://nodejs.org/en/) module available through the
14  [npm registry](https://www.npmjs.com/). Installation is done using the
15  [`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):
16  
17  ```sh
18  $ npm install on-headers
19  ```
20  
21  ## API
22  
23  <!-- eslint-disable no-unused-vars -->
24  
25  ```js
26  var onHeaders = require('on-headers')
27  ```
28  
29  ### onHeaders(res, listener)
30  
31  This will add the listener `listener` to fire when headers are emitted for `res`.
32  The listener is passed the `response` object as it's context (`this`). Headers are
33  considered to be emitted only once, right before they are sent to the client.
34  
35  When this is called multiple times on the same `res`, the `listener`s are fired
36  in the reverse order they were added.
37  
38  ## Examples
39  
40  ```js
41  var http = require('http')
42  var onHeaders = require('on-headers')
43  
44  http
45    .createServer(onRequest)
46    .listen(3000)
47  
48  function addPoweredBy () {
49    // set if not set by end of request
50    if (!this.getHeader('X-Powered-By')) {
51      this.setHeader('X-Powered-By', 'Node.js')
52    }
53  }
54  
55  function onRequest (req, res) {
56    onHeaders(res, addPoweredBy)
57  
58    res.setHeader('Content-Type', 'text/plain')
59    res.end('hello!')
60  }
61  ```
62  
63  ## Testing
64  
65  ```sh
66  $ npm test
67  ```
68  
69  ## License
70  
71  [MIT](LICENSE)
72  
73  [ci-image]: https://badgen.net/github/checks/jshttp/on-headers/master?label=ci
74  [ci-url]: https://github.com/jshttp/on-headers/actions/workflows/ci.yml
75  [coveralls-image]: https://badgen.net/coveralls/c/github/jshttp/on-headers/master
76  [coveralls-url]: https://coveralls.io/r/jshttp/on-headers?branch=master
77  [node-image]: https://badgen.net/npm/node/on-headers
78  [node-url]: https://nodejs.org/en/download
79  [npm-downloads-image]: https://badgen.net/npm/dm/on-headers
80  [npm-url]: https://npmjs.org/package/on-headers
81  [npm-version-image]: https://badgen.net/npm/v/on-headers