Example Promises/A+ implementation. Simple, tiny, fast, fully async
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Avow is a tiny, fast, fully asynchronous Promises/A+ implementation, and passes the Promises/A+ Test Suite. It tracks Promises/A+ and is currently forward compatible to the upcoming revision Promises/A+ (likely to be versioned 1.1.0).

It's around 150 lines of JS (sans comments, module boilerplate, and nextTick sniffing), under 650 bytes when closured+gzipped, supports unhandled rejection hooks for debugging, and is very fast in environments where a fast nextTick is available. It uses process.nextTick or setImmediate if available (you can use NobleJS's setImmediate polyfill), and will fall back to setTimeout (srsly, use the polyfill) otherwise.


I wrote avow as a stripped-down test bed for new ideas for when.js, cujo.js's full-featured promise implementation. I also hope that it serves as a simple example implementation for others who might want to implement promises either as a learning exercise, or as a part of a project/library/framework.

Can I use it?

Yes, but you shouldn't. You should try when.js instead. It is even faster and provides many more features, like dealing with collections of promises, competitive races, and timed promises.

Should I use it?

Again, probably not. I have no plans to support it in any way. I'll probably change the API without warning like I did from 1.0.0 to 2.0.0. You're on your own.

Ok, ok, if you want to try it out

Download it, clone it, or npm install avow


var avow = require('avow');

// Create a promise
var promise = avow(function(resolve, reject) {
	// ... do some work ...

	// Resolve the returned promise with a value, another promise,
	// or any well-behaved thenable.
	// resolve(anotherPromise);
	// resolve(thenable);

	// Or reject it

// Create a fulfilled promise
promise = avow.lift(nonPromiseValue);

// Create a promise whose fate follows another promise
promise = avow.lift(anotherPromise);

// Attempt to assimilate and follow a well-behaved thenable's fate
promise = avow.lift(thenable);

// Create a rejected promise that will use
promise = avow.rejected(reason);

Make your own

You can make your own custom configured instance of avow:

var myAvow = require('avow').construct(options);

Where options is an object that can have any of the following properties:

  • enqueue - specify your own nextTick function
  • unhandled - callback to be notified when a promise becomes rejected, but has no rejection handler.
  • handled - callback to be notified if, at some point, a previously unhandled rejected promise become handled. Since promises are temporal, this can happen if a consumer adds a rejection handler using then() at some point after the promise has been rejected.
  • protect - function that is called on every promise avow creates, to give you a chance to protect it, e.g. by supplying Object.freeze() here.

Running the Promises/A+ Test Suite

  1. npm install
  2. npm test



  • Dodge Mocha process global
  • Fix typo in Promises/A+ test adapter. Test results not affect.


  • New API (I warned you!)
  • Tracking forward compatibility with Promises/A+ 1.1.0



MIT License, Copyright (c) 2012-2013 Brian Cavalier