[](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).