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:
git://github.com/cujojs/when.git
https://cujojs@github.com/cujojs/when.git
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?
1.2.0
promise.otherwise(errback)
as a shortcut for promise.then(null, errback)
. See discussion here and here. Thanks to @jonnyreeves for suggesting the name "otherwise".- when/debug now detects exceptions that typically represent coding errors, such as SyntaxError, ReferenceError, etc. and propagates them to the host environment. In other words, you'll get a very loud stack trace.
1.1.1
- Updated wiki map/reduce examples, and added simple promise forwarding example
- Fix for calling
when.any()
without a callback (#33) - Fix version number in
when.js
source (#36)
1.1.0
when.all/any/some/map/reduce
can all now accept a promise for an array in addition to an actual array as input. This allows composing functions to do interesting things like when.reduce(when.map(...))
when.reject(promiseOrValue)
that returns a new, rejected promise.promise.always(callback)
as a shortcut for promise.then(callback, callback)
- Highly experimental when/debug module: a drop-in replacement for the main
when
module that enables debug logging for promises created or consumed by when.js
Full Changelog
Quick Start
AMD
-
git clone https://github.com/cujojs/when
or git submodule add https://github.com/cujojs/when
-
Configure your loader with a package:
packages: [
{ name: 'when', location: 'path/to/when/', main: 'when' },
]
-
define(['when', ...], function(when, ...) { ... });
or require(['when', ...], function(when, ...) { ... });
Script Tag
git clone https://github.com/cujojs/when
or git submodule add https://github.com/cujojs/when
<script src="path/to/when/when.js"></script>
when
will be available as window.when
Node
npm install git://github.com/cujojs/when
(NOTE: npm seems to require a url that starts with "git" rather than http or https)var when = require('when');
RingoJS
ringo-admin install cujojs/when
var when = require('when');
Docs & Examples
See the API section below, and the wiki for more detailed docs and examples
API
when()
Register a handler for a promise or immediate value:
when(promiseOrValue, callback, errback, progressback)
when(promiseOrValue, callback, errback, progressback).then(anotherCallback, anotherErrback, anotherProgressback)
Getting an already-resolved Promise
You can also use when()
to get an already-resolved promise for a value, similarly to using when.reject()
to get a rejected promise (see below):
var resolved = when(anything);
when.defer()
Create a new Deferred containing separate promise
and resolver
parts:
var deferred = when.defer();
var promise = deferred.promise;
var resolver = deferred.resolver;
Promise API
promise.then(callback, errback, progressback);
Extended Promise API
Convenience methods that are not part of the Promises/A proposal.
promise.always(alwaysback [, progressback]);
promise.otherwise(errback);
Resolver API
resolver.resolve(value);
resolver.reject(err);
resolver.progress(update);
The deferred has the full promise
+ resolver
API:
deferred.then(callback, errback, progressback);
deferred.resolve(value);
deferred.reject(reason);
deferred.progress(update);
when.reject()
var rejected = when.reject(anything);
Return a rejected promise for the supplied promiseOrValue. If promiseOrValue is a value, it will be the rejection value of the returned promise. If promiseOrValue is a promise, its completion value will be the rejected value of the returned promise.
This can be useful in situations where you need to reject a promise without throwing an exception. For example, it allows you to propagate a rejection with the value of another promise.
when(doSomething(),
handleSuccess,
function(error) {
return when.reject(processError(e));
}
);
when.isPromise()
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()
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()
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()
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()
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.
when.map()
when.map(promisesOrValues, mapFunc)
Traditional map function, similar to Array.prototype.map()
, 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:
mapFunc(item)
Where:
item
is a fully resolved value of a promise or value in promisesOrValues
when.reduce()
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)
Where:
currentValue
is the current accumulated reduce valuenextItem
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
when/apply
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
Install buster.js
npm install -g buster
Run unit tests in Node:
buster test -e node
Run unit tests in Browsers (and Node):
buster server
- this will print a url- Point browsers at /capture, e.g.
localhost:1111/capture
buster test
or buster test -e browser
References
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.