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A solid, fast Promises/A+ and when() implementation, plus other async goodies.

Octocat-spinner-32 benchmark Use more sensible names for cycle error and forever pending promise
Octocat-spinner-32 build Add when.node alias to browserify build. Update README
Octocat-spinner-32 docs Update docs. PrettyMonitor compatibility ftw
Octocat-spinner-32 es6-shim Rebuild es6 shim
Octocat-spinner-32 lib Skip cycle checking in resolve() when not needed
Octocat-spinner-32 monitor Make PromiseMonitor call reporter.configurePromiseMonitor if availabl…
Octocat-spinner-32 node Close #249. Move when/node/function to when/node. Add when/node/funct…
Octocat-spinner-32 test Make PromiseMonitor call reporter.configurePromiseMonitor if availabl…
Octocat-spinner-32 unfold Add Object.create shim so that es6-shim/Promise is ES3 compatible. Ad…
Octocat-spinner-32 .editorconfig Allow developers to use the tabstop they prefer
Octocat-spinner-32 .gitignore Add test browserify build. Update README with instructions
Octocat-spinner-32 .jshintignore Add es6 Promise shim. Appease jshint. Remove commented out code
Octocat-spinner-32 .jshintrc Allow custom rejection reason for promise.timeout()
Octocat-spinner-32 .npmignore Bump version, add changelog. Clean up npmignore
Octocat-spinner-32 .travis.yml Remove when3 branch from travis. Temporarily disable saucelabs testin…
Octocat-spinner-32 Update changelog
Octocat-spinner-32 run-script > run
Octocat-spinner-32 LICENSE.txt Adding license file
Octocat-spinner-32 Add note about debuggability to es6-shim docs. Add note about browser…
Octocat-spinner-32 bower.json Improve long stack handling in v8. Bump version.
Octocat-spinner-32 callbacks.js Update README. Simplify internal API
Octocat-spinner-32 cancelable.js Clean up umd
Octocat-spinner-32 delay.js Cleanup boilerplate, use newer APIs
Octocat-spinner-32 function.js Clean up and reformat in general. Add iterate and reduceRight tests. …
Octocat-spinner-32 generator.js Make es6-shim work even when loaded in a module system, eg curl prelo…
Octocat-spinner-32 guard.js Dont capture arguments object
Octocat-spinner-32 keys.js Dont capture pending in keys.all
Octocat-spinner-32 node.js Update README. Simplify internal API
Octocat-spinner-32 package.json Improve long stack handling in v8. Bump version.
Octocat-spinner-32 parallel.js Minor cleanup
Octocat-spinner-32 pipeline.js Minor cleanup
Octocat-spinner-32 poll.js Add liftAll variants
Octocat-spinner-32 sequence.js Minor cleanup
Octocat-spinner-32 timeout.js Cleanup boilerplate, use newer APIs
Octocat-spinner-32 unfold.js Move unfold-test to generate-test, add stub for iterate tests
Octocat-spinner-32 when.js Update file header comment

Promises/A+ logo

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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:



Availble 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');

    .done(function(greeting) {

function fetchRemoteGreeting() {
    // returns a when.js promise for 'hello world'
    return rest('');

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(, times10), sum)
    .done(function(result) {

function getRemoteNumberList() {
    // Get a remote array [1, 2, 3, 4, 5]
    return rest('').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 code 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.

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