# first-chunk-stream [![Build Status](https://travis-ci.org/sindresorhus/first-chunk-stream.svg?branch=master)](https://travis-ci.org/sindresorhus/first-chunk-stream)

> Buffer and transform the n first bytes of a stream


## Install

```
$ npm install --save first-chunk-stream
```


## Usage

```js
const fs = require('fs');
const concatStream = require('concat-stream');
const firstChunkStream = require('first-chunk-stream');

// unicorn.txt => unicorn rainbow
fs.createReadStream('unicorn.txt')
	.pipe(firstChunkStream({chunkLength: 7}, function (chunk, enc, cb) {
		this.push(chunk.toUpperCase());
		cb();
	}))
	.pipe(concatStream(function (data) {
		if (data.length < 7) {
			throw new Error('Couldn\'t get the minimum required first chunk length');
		}

		console.log(data);
		//=> 'UNICORN rainbow'
	}));
```


## API

### firstChunkStream([options], transform)

Returns a `FirstChunkStream` instance.

#### options

The options object is passed to the [`Duplex` stream](https://nodejs.org/api/stream.html#stream_class_stream_duplex) constructor allowing you to customize your stream behavior. In addition you can specify the following option:

##### chunkLength

Type: `number`

How many bytes you want to buffer.

#### transform(chunk, encoding, callback)

Type: `function`

The function that gets the required `options.chunkLength` bytes.

Note that the buffer can have a smaller length than the required one. In that case, it will be due to the fact that the complete stream contents has a length less than the `òptions.chunkLength` value. You should check for this yourself if you strictly depend on the length.


## License

MIT © [Sindre Sorhus](http://sindresorhus.com)