0.4.0 • Public • Published


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v0.4.0 (jQuery 1.8.3)

This is a port of jQuery.Deferred as an Underscore mixin, but it can be used without any depencencies. It currently matches the Deferred specifications and implementation from jQuery 1.8.0, with all the associated helpers.

Deferreds are great, let's take them everywhere

jQuery offers a robust, consistent and well documented API; this project aims to make it portable. jQuery added a handful of helper methods to their implementation of the Common.js Promises/A Spec and they're faithfully reproduced without any dependencies.


underscore.deferred works on the server and in the browser.

In the browser, just require it like you would any other file. If you're including Underscore on the page, make sure you include it before underscore.deferred. If you don't have Underscore, the plugin attaches to window._.

On the server, simply install via npm and include it with require:

var _ = require('underscore.deferred');
var dfd = new _.Deferred(); // tada!

Or if you'd like to use it as an Underscore or Lodash mixin:

var _ = require('underscore');
_.mixin( require('underscore.deferred') );

Underscore isn't a strict requirement, and assigning it to another name will always work.

Addionally, underscore.Deferred can be used with Ender.js, if you're still into that sort of thing.

Enhancements / Changes

So far, there are only 2 substantial differences between the jQuery API and underscore.deffered's. This may change in the future as new or divergent functionality is tested out, but rest assured that all differences will be called out here (and will have proper test coverage.)


underscore.deferred will apply an Array if it's the only argument, providing a useful shortcut for using when with an array of promises. Example:

var form = _.Deferred();
var auth = _.Deferred();

var promises = [ form.promise(), auth.promise() ];

  // ...



Inspired by @domenic's Promises/A compliance suite (and the accompanying gist), the 0.4.0 release of underscore.deferred diverges from jQuery's implementation slightly.

First, throwing an error from within a handler in then will reject the deferred object issued by then with the error message. Example:

var dfd = _.Deferred();

  throw new Error("Oops!");
}).fail(function( err ){
  console.log(err.message); // "Oops!"


This behavior alone isn't divergent from jQuery (all tests pass). It's handy for bubbling errors from callbacks without entering callback hell.

Second, when chaining then's or using the new deferred object issued by a call to then the state of the first deferred is not passed to subsequent calls to then. Example:

var dfd = _.Deferred();
dfd.then(null, function( a, b ){
  return a * b;
}).then(function( value ){
  equal(value, 6); // the second deferred is resolved
}, function(){
  // reject handler never called

dfd.reject(2, 3); // the first deferred in the chain is rejected

When paired with the first behavior, it makes chaining deferred's even more useful.

Unfortunately, this second point is divergent from jQuery's implementation, with one failing test. jQuery maintains the state of the first deferred object unless the handler returns a new deferred.

Here's an example of the "old" behavior (what's currently in jQuery 1.8.3):

var dfd = _.Deferred();
dfd.then(null, function( a, b ){
  return a * b;
  // resolve handler never called
}, function( value ){
  equal(value, 6); // the second deferred is also rejected

dfd.reject(2, 3); // the first deferred in the chain is rejected

In the spirit of fidelity to Promises/A, underscored.deferred favors the new behavior.


underscore.deferred is an implementation of the jQuery.Deferred API. Here are some of the methods implemented:

For the complete API documentation, see jQuery's Documentation.

This project wouldn't exist if not for the the hard work and effort put into jQuery by its core team and contributors--thanks for making this possible!

Minified Build

One time setup command:

$ npm install

To build with grunt

$ grunt

To run headless Qunit tests (must have phantomjs in your path)

$ grunt qunit



The goal is to slim the code footprint for robust deferreds as much as possible while maintaining mixin integration with Underscore and faithfullness to the jQuery API.

This is a work in progress, feel free to contribute.

Release Notes

0.4.0 - adding divergent behavior for _.then

0.3.1 - updating to match jQuery 1.8.3

0.3.0 - adding divergent behavior for _.when

0.2.0 - updating to match jQuery 1.8.0

0.1.4 - updating to match jQuery 1.7.2; Please note that as of 0.1.4 underscore.deferred will be ALL LOWERCASE on npm. The camelcasing was a mistake from the outset, please update your package.json appropriately.

MIT License.




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