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 under 950 bytes when compiled with Google Closure (w/advanced optimizations) and gzipped.

when.js is derived from the async core of wire.js, and future versions of wire will use when.js directly.

Check the wiki for 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);
deferred.resolve(value);
deferred.reject(reason);
deferred.progress(update);

The promise API:

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

The resolver API:

// var resolver = deferred.resolver;
resolver.resolve(value);
resolver.reject(err);
resolver.progress(update);

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.

Much of this code is based on @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.