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Please Note: this project has moved from briancavalier/when to cujojs/when. Any existing forks have been automatically moved to cujojs/when. However, you'll need to update your clone and submodule remotes manually.

Update the url in your .git/config, and also .gitmodules for submodules:


Helpful link for updating submodules: Git Submodules: Adding, Using, Removing, Updating

A lightweight CommonJS Promises/A and when() implementation. It also provides several other useful Promise-related concepts, such as joining and chaining, and has a robust unit test suite.

It's just over 1k when compiled with Google Closure (w/advanced optimizations) and gzipped.

when.js was derived from the async core of wire.js.

What's New?


  • Fix for rejections propagating in some cases when they shouldn't have been (#19)
  • Using buster.js for unit tests now.


  • First official when.js release as a part of cujojs.
  • Added when/cancelable decorator for creating cancelable deferreds
  • Added when/delay and when/timeout helpers for creating delayed promises and promises that timeout and reject if not resolved first.


  • Added when/apply helper module for using arguments-based and variadic callbacks with when.all, when.some,, or any promise that resolves to an array. (#14)
  • .then(), when(), and all other methods that accept callback/errback/progress handlers will throw if you pass something that's not a function. (#15)


  • when.js now assimilates thenables that pass the Promises/A duck-type test, but which may not be fully Promises/A compliant, such as jQuery's Deferred and curl's global API (See the API at a glance section)
    • when(), and when.all/some/any/map/reduce/chain() are all now guaranteed to return a fully Promises/A compliant promise, even when their input is not compliant.
    • Any non-compliant thenable returned by a callback or errback will also be assimilated to protect subsequent promises and callbacks in a promise chain, and preserve Promises/A forwarding guarantees.

Full Changelog

Docs & Examples

See the API section below, and the wiki for more detailed docs and examples



Register a handler for a promise or immediate value:

when(promiseOrValue, callback, errback, progressback)

// Always returns a promise, so can be chained:

when(promiseOrValue, callback, errback, progressback).then(anotherCallback, anotherErrback, anotherProgressback)


Create a new Deferred containing separate promise and resolver parts:

var deferred = when.defer();

var promise = deferred.promise;
var resolver = deferred.resolver;

The deferred has the full promise + resolver API:

deferred.then(callback, errback, progressback);

The promise API:

// var promise = deferred.promise;
promise.then(callback, errback, progressback);

The resolver API:

// var resolver = deferred.resolver;


var is = when.isPromise(anything);

Return true if anything is truthy and implements the then() promise API. Note that this will return true for both a deferred (i.e. when.defer()), and a deferred.promise since both implement the promise API.


when.some(promisesOrValues, howMany, callback, errback, progressback)

Return a promise that will resolve when howMany of the supplied promisesOrValues have resolved. The resolution value of the returned promise will be an array of length howMany containing the resolutions values of the triggering promisesOrValues.


when.all(promisesOrValues, callback, errback, progressback)

Return a promise that will resolve only once all the supplied promisesOrValues have resolved. The resolution value of the returned promise will be an array containing the resolution values of each of the promisesOrValues.


when.any(promisesOrValues, callback, errback, progressback)

Return a promise that will resolve when any one of the supplied promisesOrValues has resolved. The resolution value of the returned promise will be the resolution value of the triggering promiseOrValue.


when.chain(promiseOrValue, resolver, optionalValue)

Ensure that resolution of promiseOrValue will complete resolver with the completion value of promiseOrValue, or instead with optionalValue if it is provided.

Returns a new promise that will complete when promiseOrValue is completed, with the completion value of promiseOrValue, or instead with optionalValue if it is provided.

Note: If promiseOrValue is not an immediate value, it can be anything that supports the promise API (i.e. then()), so you can pass a deferred as well. Similarly, resolver can be anything that supports the resolver API (i.e. resolve(), reject()), so a deferred will work there, too., mapFunc)

Traditional map function, similar to, but allows input to contain promises and/or values, and mapFunc may return either a value or a promise.

The map function should have the signature:



  • item is a fully resolved value of a promise or value in promisesOrValues


when.reduce(promisesOrValues, reduceFunc, initialValue)

Traditional reduce function, similar to Array.prototype.reduce(), but input may contain promises and/or values, and reduceFunc may return either a value or a promise, and initialValue may be a promise for the starting value.

The reduce function should have the signature:

reduceFunc(currentValue, nextItem, index, total)


  • currentValue is the current accumulated reduce value
  • nextItem is the fully resolved value of the promise or value at index in promisesOrValues
  • index the basis of nextItem ... practically speaking, this is the array index of the promiseOrValue corresponding to nextItem
  • total is the total number of items in promisesOrValues


function functionThatAcceptsMultipleArgs(array) {
    // ...

var functionThatAcceptsAnArray = apply(functionThatAcceptsMultipleArgs);

Helper that allows using callbacks that take multiple args, instead of an array, with when.all/some/map:

when.all(arrayOfPromisesOrValues, apply(functionThatAcceptsMultipleArgs));

See the wiki for more info and examples.

Running the Unit Tests

  1. npm install -g buster (if you don't have buster.js installed already)
  2. buster test -e node


Much of this code was inspired by @unscriptable's tiny promises, the async innards of wire.js, and some gists here, here, here, and here

Some of the code has been influenced by the great work in Q, Dojo's Deferred, and uber.js.

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