HORIZON PROJECT

# on-headers

[![NPM Version][npm-image]][npm-url]

[![NPM Downloads][downloads-image]][downloads-url]

[![Node.js Version][node-version-image]][node-version-url]

[![Build Status][travis-image]][travis-url]

[![Test Coverage][coveralls-image]][coveralls-url]

Execute a listener when a response is about to write headers.

## Installation

```sh

$ npm install on-headers

```

## API

```js

var onHeaders = require('on-headers')

```

### onHeaders(res, listener)

This will add the listener `listener` to fire when headers are emitted for `res`.

The listener is passed the `response` object as it's context (`this`). Headers are

considered to be emitted only once, right before they are sent to the client.

When this is called multiple times on the same `res`, the `listener`s are fired

in the reverse order they were added.

## Examples

```js

var http = require('http')

var onHeaders = require('on-headers')

http

.createServer(onRequest)

.listen(3000)

function addPoweredBy() {

  // set if not set by end of request

  if (!this.getHeader('X-Powered-By')) {

    this.setHeader('X-Powered-By', 'Node.js')

  }

}

function onRequest(req, res) {

  onHeaders(res, addPoweredBy)

  res.setHeader('Content-Type', 'text/plain')

  res.end('hello!')

}

```

## Testing

```sh

$ npm test

```

## License

[MIT](LICENSE)

[npm-image]: https://img.shields.io/npm/v/on-headers.svg

[npm-url]: https://npmjs.org/package/on-headers

[node-version-image]: https://img.shields.io/node/v/on-headers.svg

[node-version-url]: http://nodejs.org/download/

[travis-image]: https://img.shields.io/travis/jshttp/on-headers/master.svg

[travis-url]: https://travis-ci.org/jshttp/on-headers

[coveralls-image]: https://img.shields.io/coveralls/jshttp/on-headers/master.svg

[coveralls-url]: https://coveralls.io/r/jshttp/on-headers?branch=master

[downloads-image]: https://img.shields.io/npm/dm/on-headers.svg

[downloads-url]: https://npmjs.org/package/on-headers