When.js is a rock solid, battle-tested Promises/A+ and when() implementation, including a complete ES6 Promise shim. 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, the JavaScript Architectural Toolkit.
Check it out:
- What's new
- API docs
- Read more about how promises simplify async programming
Available as when through bower, or just clone the repo and load when.js from the root.
bower install --save when
npm install --save when
More help & other environments »
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 to make an ajax request to a (fictional) external service.
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.
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; }Licensed under MIT. Full license here »
Please see the contributing guide for more information on running tests, opening issues, and contributing cod 58BC e to the project.
Much of this code was inspired by the async innards of wire.js, and has been influenced by the great work in Q, Dojo's Deferred, and uber.js.