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When.js is a rock solid, battle-testedPromises/A+ andwhen() implementation, including a completeES6 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 ofcujoJS, the JavaScript Architectural Toolkit.

Check it out:

• What's new
• API docs
• Read more about howpromises simplify async programming

Available aswhen throughbower, or just clone the repo and loadwhen.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 usesrest to make an ajax request to a (fictional) external service.

varrest=require('rest');fetchRemoteGreeting().then(addExclamation).catch(handleError).done(function(greeting){console.log(greeting);});functionfetchRemoteGreeting(){// convert native Promise to a when.js promise for 'hello world'varresult=rest('http://example.com/greeting');returnwhen(result)}functionaddExclamation(greeting){returngreeting+'!!!!'}functionhandleError(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 print150 if all went well, and if there was a problem will print a full stack trace.

varwhen=require('when');varrest=require('rest');when.reduce(when.map(getRemoteNumberList(),times10),sum).done(function(result){console.log(result);});functiongetRemoteNumberList(){// Get a remote array [1, 2, 3, 4, 5]returnrest('http://example.com/numbers').then(JSON.parse);}functionsum(x,y){returnx+y;}functiontimes10(x){returnx*10;}

Licensed under MIT.Full license here »

Please see thecontributing guide for more information on running tests, opening issues, and contributing code to the project.

Much of this code was inspired by the async innards ofwire.js, and has been influenced by the great work inQ,Dojo's Deferred, anduber.js.