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Covenant is a fully compliant Promises/A+ implementation written in Coffeescript. Covenant, its core class is a bare-bones implementation that passes the Promises/A+ Test Suite, as well as the present draft of version 1.1 of the test suite. Covenant's core is very performant and extremely lightweight.

The Covenant (Core) API

{Covenant, Core} = require('covenant')

# create a new promise, which can be figured using subsequent
# subsequent calls to the fulfill, reject and resolve methods
p = new Core 

# the promise can be configured with a callback function, which
# will be passed a resolution and rejection function, as well as
# a reference to the promise itself. Also, `this` is set to reference
# the promise.

pDelay50ms = new Core (resolve, reject, promise) ->
  setTimeout -> (
    resolve('50 milliseconds have passed'), 50)

# fulfill the promise with a value

# reject the promise with a reason

# Wrap another promise or foreign "thennable" and adopt its state.
# Otherwise, fulfill with the value or reject if unable to adopt.
# Works with Promise/A+-conforming and many non-conforming promises.

# schedule asynchronous handlers, as often as you like, before or after resolution
# the handler may be a value, a function or a promise (an object having a function
# property named "then."
covenant.then onFulfilled, onRejected

# A Covenant object with a single `then` method linked to the promise.
# The promise can be passed to a client authorized to schedule callbacks
# on the Core object, but will not be able to change its state.

The Promise (Extended) API

Promise, more full-featured extension of Covenant's core is included, although it is in pre-alpha form at this time. It provides: a nice collection of promise-generating operations, an aggregation function, some convenience functions and functions for securely sharing promise objects with clients for limited use.

Promise Generation Functions

{Promise} = require ('promise')

# Promise.pending(): construct a pending promise
p = Promise.pending()
  .anything(console.log) # => nothing yet!
  .fulfill("I'm all done") # => I'm all done"

# Promise.fulfilled(value): construct a promise fulfilled with value
  .done(console.log) # => 43

# Promise.rejected(reason): construct a promise rejected for reason
Promise.rejected("naughty you")
  .fail(console.error) # => "naughty you"

# Promise.fromNode(nodeOperation): construct a promise generating function based on node functions
f = Promise.fromNode(fs.readFile)
pReadFile = f('')

# construct a promise that fulfills after ms milliseconds

# construct a promise that rejects for timeout unless resolved before ms milliseconds.
Promise.timeout(p, 100)

Aggregate Promise Functions

# Promise.when(promiseOrValueList): Construct a promise from any number of values or promises, which fulfills with an
# array of corresponding values if all promises are fulfilled, and rejects if ANY

# example when promise is rejected
# with raw values
Promise.when(1, 2, 3)
  .done console.log # => [1, 2, 3]

# with pending promises, ultimately fulfilled
p = Promise.pending()
q = Promise.when p, 2, Promise.fulfilled(3)
q.done console.log # => nothing happens
p.fulfill(1) # => [1,2,3] after a tick or two

# with pending promises, one rejectedl
Promise.when(Promise.pending(), Promise.fulfilled(2), Promise.rejected("Error in 3")
  .fail(console.error) # => Error in 3
p = (Promise.when p1, p2, p3).fail(console.error) # => Error in 3

# Promise.all(valueOrPromiseList): same as Promise.all Promise.when 

Promise Instance Convenience Functions

# p.done(callback): convenience function for p.then onFulfill, undefined

# convenience function for p.then undefined, onReject

# p.always(callback): convenience function for p.then callback, callback

# Note that the promise returned by p.always(callback) can resolve
# differently, even when p has already resolved.


Download it, clone it, or npm install wizardwerdna/covenant

Covenant has no dependencies, but does use process.nextTick, found in modern Browsers. If process.nextTick is not a function, Covenant falls back to setImmediate, and then to setTimeout. If you are using ancient browsers, it is highly recommended that you use a shim to implement (fake) nextTick and/or setImmediate.


I am indebted, in particular, to the work of Brian Cavalier, whose when.js, and avow.js libraries illuminate what can be done both in a full-featured and minimalist implementation.

Running the tests

  1. clone the respository
  2. npm install
  3. npm test
  4. npm run-script browserTest

The browser test will run the tests directly on Safari, Chrome, Firefox and phantomJS. Some configuration of karma.conf.js will be necessary if you haven't configured one or more of the browsers. phantomJS was installed when you ran npm install.


MIT License, Copyright (c) 2013 Andrew C. Greenberg (


A tight and performant Promises/A+ compliant implementation in Coffeescript






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