Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time


Promise/A+ compliant promises. See the Promises Spec for more information.

Promises/A+ 1.1 compliant npm version Build Status Dependencies Dev Dependencies


  1. Readable. I like learning from examples.
  2. Debuggable. When there are problems, can I figure out what's going wrong?
  3. Testable. All code should be tested.
  4. Simple. Extra functionality often makes bugs or the code will become brittle.
  5. Avoid blowing up a call stack.
  6. Use it everywhere. Browser, node, with YUI, RequireJS and more.

That last goal just means that everything runs with its own timeout, so one can not possibly break the code by accidentally chaining a lot of promises together. If they are all synchronous and fulfill themselves immediately other libraries may recurse too deeply and cause the code to fail. It's a tiny bit slower, but I'd take reliability over extreme speed.

The module uses FidUmd to let you use this library in the following areas:

  • In the browser, attaching to window.FidPromise
  • With YUI as FidPromise
  • In node.js, AMD, CommonJS, and other methods via the regular module.exports
  • With RequireJS in the browser

Quick Usage

function somethingAsync() {
    promise = new FidPromise();

    // Do something async and that will call promise.resolve(data) on
    // success, promise.reject(error) on failure
    return promise;

somethingAsync().then(function (data) {
    console.log('Success!  :-)');
}, function (error) {

Public API

Here's the functions that are intended to be used by outsiders. You're welcome to dig into the source in order to poke around at the rest, but that's hopefully unnecessary.

For the below, FidPromise refers to the constructor function and promise refers to an instance.

FidPromise() constructor

Returns a new FidPromise.


Waits for every promise in the array to be completed. Once they are, this promise will be rejected or resolved. If rejected, the passed data will be an array of all of the rejections. If resolved properly, the passed data will be an array of all of the resolutions in the order they were passed in. This is similar to when() except that this will reject the promise only after everything is done instead of on the first error.

Returns promise.


Attach a callback to both the list of success and error callbacks. The callback should always be called when the promise is completed. Returns a new FidPromise.

FidPromise.debug and promise.debug

If either are defined and are truthy, then debug messages will start being sent to the console. If FidPromise.debug is truthy then all promises will get debugged and promise.debug will debug just a single promise.

You can also set .debug to a callback that will be passed a single string parameter if you wish to implement your own logging mechanism.


Attach a callback to the list of error callbacks. Returns a new FidPromise.


Complete this promise and call its error callbacks. Returns promise.


Complete this promise and call its success callbacks. Returns promise.


Attach a callback to the list of success callbacks. Returns a new FidPromise.

promise.then(onSuccess, onError)

Attach callbacks to the success and error callback lists.

Both onSuccess and onError may be omitted or null for no callbacks, a function, or an array of functions.

This returns a new FidPromise object.


Waits for every promise in the array to be resolved or until the first rejected promise. If rejected, the promise will be immediately rejected with the data passed from the other rejected promise. If resolved properly, the passed data will be an array of all of the resolutions. This is the same as after() but this version does not wait for all of the promises to resolve if any hit an error condition.

Returns a new FidPromise.


Creates a new promise and calls promise.after() on it, passing in your arguments. Returns the new promise. This saves you from potentially creating another local variable and could produce cleaner looking code.


Creates a new promise and calls promise.when() on it, passing in your arguments. Returns the new promise. This saves you from potentially creating another local variable and could produce cleaner looking code.


Not all minor changes are listed here. Just the important ones that affect how you'd use this object.


  • Updated to newer A+ spec tests.


  • Rewrote large chunks of the library to handle the new Promise/A+ spec.
  • Hidden several functions inside the library; no more exposing things that should not be seen by a client.
  • Moved the repository to the tests-always-included organization on GitHub.


  • Constructor now does not take any parameter. Use FidPromise.when() instead.
  • when() will return an array of the results from the promises that are fulfilled.
  • when() now only accepts an array for a parameter.
  • Added .after() method and FidPromise.after() helper function.


First, clone the repository. Then run npm install to fetch dependencies.


Tests are always included. You can run them with the following command. It runs the test suites for both the Promise/A+ spec and custom unit tests.

npm test


This package is licensed under the MIT License with an additional non-advertising clause.


Easy Promise/A+ compliant promises







No packages published