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[](https://travis-ci.org/cujojs/when)
[](http://inch-ci.org/github/cujojs/when)
when.js
=======
When.js is a rock solid, battle-tested [Promises/A+](http://promises-aplus.github.com/promises-spec) and `when()` implementation, including a complete [ES6 Promise shim](docs/es6-promise-shim.md). It's a powerful combination of small size, high performance, debuggability, and rich features:
* Resolve arrays and hashes of promises, as well as infinite promise sequences
* Execute tasks in parallel or sequentially
* Transform Node-style and other callback-based APIs into promise-based APIs
When.js is one of the many stand-alone components of [cujoJS](http://cujojs.com), the JavaScript Architectural Toolkit.
Check it out:
- [What's new](CHANGES.md)
- [API docs](docs/api.md#api)
- Read more about how [promises simplify async programming](http://know.cujojs.com/tutorials/async/simplifying-async-with-promises)
Installation
------------
#### AMD
Available as `when` through [bower](http://bower.io), or just clone the repo and load `when.js` from the root.
```
bower install --save when
```
#### CommonJS/Node
```
npm install --save when
```
[More help & other environments »](docs/installation.md)
Usage
-----
Promises can be used to help manage complex and/or nested callback flows in a simple manner. To get a better handle on how promise flows look and how they can be helpful, there are a couple examples below (using commonjs).
This first example will print `"hello world!!!!"` if all went well, or `"drat!"` if there was a problem. It also uses [rest](https://github.com/cujojs/rest) to make an ajax request to a (fictional) external service.
```js
var rest = require('rest');
fetchRemoteGreeting()
.then(addExclamation)
.catch(handleError)
.done(function(greeting) {
console.log(greeting);
});
function fetchRemoteGreeting() {
// returns a when.js promise for 'hello world'
return rest('http://example.com/greeting');
}
function addExclamation(greeting) {
return greeting + '!!!!'
}
function handleError(e) {
return 'drat!';
}
```
The second example shows off the power that comes with when's promise logic. Here, we get an array of numbers from a remote source and reduce them. The example will print `150` if all went well, and if there was a problem will print a full stack trace.
```js
var when = require('when');
var rest = require('rest');
when.reduce(when.map(getRemoteNumberList(), times10), sum)
.done(function(result) {
console.log(result);
});
function getRemoteNumberList() {
// Get a remote array [1, 2, 3, 4, 5]
return rest('http://example.com/numbers').then(JSON.parse);
}
function sum(x, y) { return x + y; }
function times10(x) {return x * 10; }
```
License
-------
Licensed under MIT. [Full license here »](LICENSE.txt)
Contributing
------------
Please see the [contributing guide](CONTRIBUTING.md) for more information on running tests, opening issues, and contributing code to the project.
References
----------
Much of this code was inspired by the async innards of [wire.js](https://github.com/cujojs/wire), and has been influenced by the great work in [Q](https://github.com/kriskowal/q), [Dojo's Deferred](https://github.com/dojo/dojo), and [uber.js](https://github.com/phiggins42/uber.js).
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