# 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.

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## 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 <a href="http://twitter.com/caolan">@caolan</a>, 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/)