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https://caolan.github.io/highland/

High-level streams library for Node.js and the browser
https://caolan.github.io/highland/

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High-level streams library for Node.js and the browser

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# Highland



The high-level streams library for Node.js and the browser.

View the [Highland website](https://caolan.github.io/highland) for more in-depth

documentation.



[![build status](https://secure.travis-ci.org/caolan/highland.png)](http://travis-ci.org/caolan/highland)

[![Join the chat at https://gitter.im/caolan/highland](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/caolan/highland?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)



## Introduction



Re-thinking the [JavaScript](http://underscorejs.org)

[utility](http://lodash.com) [belt](https://github.com/caolan/async),

Highland manages synchronous and asynchronous code easily, using nothing more than

standard JavaScript and Node-like Streams.

You may be familiar with Promises, EventEmitters and callbacks, but moving

between them is far from seamless. Thankfully, there exists a deeper abstraction

which can free our code. By updating the tools we use on Arrays, and applying them

to values distributed in time instead of space, we can discard plumbing and

focus on the important things. With Highland, you can switch between

synchronous and asynchronous data sources at will, without having to

re-write your code. Time to dive in!



Made by @caolan, with help and patience from friends :)



## Highland v3

This branch tracks the ongoing development of version 3.0, which will feature a

rewritten Highland core implementation, extensibility support, limited stream

lifecycle, and some breaking changes to certain transforms.  See

[#179](https://github.com/caolan/highland/issues/179) and the [3.x

label](https://github.com/caolan/highland/issues?utf8=%E2%9C%93&q=label%3A3.x%20)

for more details. New features will only be added to this branch. However,

until 3.0 is released, we will still be doing bug fixes for the 2.x releases.

See the [2.x branch](https://github.com/caolan/highland/tree/2.x) for those

files.



Currently, the code is in a semi-stable state. The only major missing feature

is [`onDestroy` for higher-level

transforms](https://github.com/caolan/highland/issues/412). To try out the new

goodness, install the `next` tag from NPM.



```

npm install --save highland@next

```



## Interoperability



Highland implements the [Fantasy

Land](https://github.com/fantasyland/fantasy-land)

[Alternative](https://github.com/fantasyland/fantasy-land#alternative),

[Filterable](https://github.com/fantasyland/fantasy-land#filterable), and

[Monad](https://github.com/fantasyland/fantasy-land#monad)

specifications.



Highland supports

[Transducers](https://github.com/cognitect-labs/transducers-js) via the

`transduce` transform.



## Examples



Usage as a Node.js module



```javascript

var _ = require('highland');

```



Converting to/from Highland Streams



```javascript

_([1,2,3,4]).toArray(function (xs) {

    // xs is [1,2,3,4]

});

```



Mapping over a Stream



```javascript

var doubled = _([1,2,3,4]).map(function (x) {

    return x * 2;

});

```



Reading files in parallel (4 at once)



```javascript

var data = _(filenames).map(readFile).parallel(4);

```



Handling errors



```javascript

data.errors(function (err, rethrow) {

    // handle or rethrow error

});

```



Piping to a Node Stream



```javascript

data.pipe(output);

```



Piping in data from Node Streams



```javascript

var output = fs.createWriteStream('output');

var docs = db.createReadStream();



// wrap a node stream and pipe to file

_(docs).filter(isBlogpost).pipe(output);



// or, pipe in a node stream directly:

var through = _.pipeline(_.filter(isBlogpost));

docs.pipe(through).pipe(output);

```



Handling events



```javascript

var clicks = _('click', btn).map(1);

var counter = clicks.scan(0, _.add);



counter.each(function (n) {

    $('#count').text(n);

});

```



Learn more at [https://caolan.github.io/highland/](https://caolan.github.io/highland/)