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Yet Another Promise/A+ Library


This library defaults to exporting a symbol Y (just like the Q module).

This library implements the promise/A+ specfication and passes the Promise/A+ test suite.

The goals were, in order of priority, to:

  1. for me to understand promises better ;)
  2. implement the Promise/A+ Spec and pass the tests
  3. using the deferred pattern.
  4. defaulting to setImmediate due to Node.js v0.10+ warning about recursive calls to process.nextTick. And I needed to use VERY deep promise chains/sequences..
  5. allow for overriding this nextTick-like behaviour as needed.
  6. speed.
  7. make it as Q-like as possible.

The advatages of this library to you that other libraries may or may not have:

  1. Complete data hiding.
    • There is no way to access a promises' the internal queue of pending functions
    • There are no special/undocumented arguments to .resolve, .reject, .then, or .spread functions.
  2. User settable Y.nextTick for your own optimizations or usage patterns.
  3. Y.nextTick comes with reasonable default.
  4. Additional helper functions are implemented that do not impact performance.

Quick Review of the deferred pattern

A deferred is an object coupled with a promise object. The deferred object is responsible for resolving (also known as fulfilling) and rejecting the promise.

The promise is the object with the then method. (It also has the spread method which is the same as the then method but handles the onFulfilled callback slightly differently.)

The two objects are coupled together by a queue of (onFulfilled, onResolved) tuples. The promise.then and promise.spread methods build up the queue. The deferred.resolve and deferred.reject methods dispatch the queue once and only once.

Here is an example in the form of the V.promisify function:

function promisify(nodeFn, thisObj){
  return function(){
    var args =
      , d = Y.defer()

      if (err) { d.reject(err); return }
      if (arguments.length > 2)
        d.resolve(, 1))

    nodeFn.apply(thisObj, args)

    return d.promise


Load Module

var Y = require("ya-promise")

Load the library.

Create a Deferred & Promise

Q-alike: Q.defer()

deferred = Y.defer()
// or
deferred = Y.deferred()

promise = deferred.promise

Promise then

promise.then(onFulfililled, onRejected)

This library does NOT support onProgress. You can have a function as the third argument to promise.then() but it will never be called.

Promise spread

promise.spread(onFulfilled, onRejected)

When onFulfilled is called, and value is an Array, value will be spread as arguments to the function via onFulfilled.apply(undefined, value) rather than onFulfilled(value).

Resolve a Deferred



  1. all onFulfilled functions to be called with value via Y.nextTick.
  2. the promise to change to a fulfilled state as the Promise/A+ spec requires.
  3. further calls to deferred.resolve() or deferred.reject() to be ignored.

Reject a Deferred

Q-alike: Q.reject()



  1. all onRejected functions to be called with value via Y.nextTick.
  2. the promise to change to a rejected state as the Promise/A+ spec requires.
  3. further calls to deferred.resolve() or deferred.reject() to be ignored.

Convert a value or a foreign Promise (thenable) to a Y Promise

Q-alike: Q()

Q-alike: Q.when()


Returns a ya-promise promise given a straight value or thenable.

If a ya-promise promise is passed in, it is returned unchanged.

If a value is passed in a fulfilled ya-promise promise is returned.

If a foreign thenable is passed in it is wrapped in a deferred and a ya-promise promise is returned.

Create a Promise from an Array of Promises

Q-alike: Q.all()

Y.all([promA, promB, promC]).then( function([resA, resB, resC]){ ... }
                                 , function(reason){ ... } )

When all the promises in the array passed to Y.all(array) are resolved the returned promise is resolved. It value is an array of the results of each of the original promises in the same order.

If ANY of the promises in the array are rejected then the returned promise is immediately rejected.


var Y = require('./')

function timeout(n) {
  var d = Y.defer(), t = n * Math.random()
  setTimeout(function(){ d.resolve(t) }, t*1000 )
  return d.promise

var t0 =

Y.all([ timeout(10, "one")
      , timeout(10, "two")
      , timeout(10, "three")
  a.forEach(function(r, i){
    console.log("%d: %d sec", i, r)
  console.log("now-t0: %d sec", (

Timeout a Promise

promise.timeout(ms).then(onFulfilled, onRejected)

If promise is resolved or rejected in less than ms milliseconds then onFulfilled or onRejected (respectively) will be called with the value or reason given.

If promise is not resolved or rejected within that time limit, then the promise will be rejected with the reason set to "Timed out after " + ms + " ms".

In node.js the timeoutId returned by setTimeout has a unref method that will prevent this timer from allowing the node.js event-loop to end. If timeoutId has a unref method, it is called.

Delay a Promise

Q-alike: promise.delay()

delayed_promise = promise.delay(ms)

From the time where delayed_promise is created a timer is started for ms milliseconds. If promise is fulfilled or rejected within that timer then delayed_promise will not be resolved/rejected till the timer expires. If the timer has already expired delayed_promise will be resolved/rejected immediately. delayed_promise will always be resolved/rejected with the same value/reason promise was.

Create a promise with only an onRejected

Q-alike: promise.catch()


alias: .catch()

another_promise =

I prefer the version but I included the promise.catch as an alias.

Convert any onRejected or a throw error from a callback into a throw

Q-alike: promise.done()

alias: .throwOnReject()

alias: .throw()

This is really not exactly like Q's promise.done(). Unlike Q's promise.done() it takes NO arguments, but like Q's promise.done() it catches any rejected promise and throws the reason in the nextTick.

Q's promise.done() is just like a promise.then() but the execution is slightly different in that any rejection is thrown as above.

It is meant to be use as such:

.done() //<- this will throw any rejection that falls thru the above thens

It still returns a promise, so more thens can follow it, but any rejection that gets to it will throw an exception on the nextTick.

Create a Promise whos Resolution is delayed

Q-alike: Q.delay()

delayed = Y.delay(ms)

This is a promise-like version of setTimeout() but looks nicer.


Create a Fulfilled or Rejected Promise

Q-alike: Q.reject()

fulfilled_promise = Y.resolved(value)
rejected_promise  = Y.rejected(reason)


Y.reolved(42).then( function(value){ value == 42 }
                  , function(reason){/*never called*/})

Y.rejected("oops").then( function(value){/*never called*/}
                       , function(reason){ reason == "oops" })

Detect if an object ISA ya-promise Deferred or Promise.

Q-alike: Q.isPromise()

var d = Y.defer()
  , p = d.promise
Y.isDeferred( d ) // returns `true`
Y.isPromise( p )  // returns `true`

Convert a node-style async function to a promise-style async function.

Q-alike: Q.denodeify

Q-alike: Q.nfbind

promiseFn = Y.promisify(nodeFn)
promiseFn = Y.nfbind(nodeFn)
promiseFn = Y.denodeify(nodeFn)

A node-style async function looks like this

nodeFn(arg0, arg1, function(err, res0, res1){ ... })

where the return value of nodeFn is usually undefined.

The corresponding promise-style async function look like this

promise = promiseFn(arg0, arg1)
promise.then(function([res0, res1]){ ... }, function(err){ ... })

However, for a node-style async function that returns a single result, Y.promisify(nodeFn) does NOT return an single element array. For example:

nodeFn(arg0, arg1, function(err, res0){ ... })

is converted to:

promise = promiseFn(arg0, arg1)
promise.then(function(res0){ ... }, function(err){ ... })

Notice, res0 is not wrapped in an array.


ya-promise was just tested with the following simple script against a few other Promise/A+ libraries. (My results also included.)

Remember "Lies, Statistics, and Benchmarks".

var Y = require('ya-promise')
  , Q = require('q')
  , Vow = require('vow')
  , P = require('p-promise')
  , promiscuous = require('promiscuous')

Y.nextTick = process.nextTick //force the use of process.nextTick = {
  'ya-promise' : function(done){
    var d = Y.defer()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })
, 'Q' : function(done){
    var d = Q.defer()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })
, 'Vow' : function(done){
    var p = Vow.promise()
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })
, 'p-promise' : function(done){
    var d = P.defer()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })
, 'promiscuous': function(done){
    var d = promiscuous.deferred()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })


My Benchmark Results

{ http_parser: '1.0',
  node: '0.10.4',
  v8: '',
  ares: '1.9.0-DEV',
  uv: '0.10.4',
  zlib: '1.2.3',
  modules: '11',
  openssl: '1.0.1e' }
Scores: (bigger is better)

 > 593.063936063936
 > 597.1928071928072
 > 607.999000999001
 > 604.5444555444556
Average (mean) 600.70004995005

 > 402.68431568431566
 > 398.86013986013984
 > 398.8851148851149
 > 401.8061938061938
Average (mean) 400.55894105894106

 > 399.93806193806194
 > 396.82917082917083
 > 387.72427572427574
 > 396.3046953046953
Average (mean) 395.19905094905096

 > 133.1098901098901
 > 134.56043956043956
 > 134.16683316683316
 > 133.2067932067932
Average (mean) 133.76098901098902

 > 3.3366533864541834
 > 3.3716283716283715
 > 3.3846153846153846
 > 3.3506493506493507
Average (mean) 3.3608866233368224

Winner: Vow
Compared with next highest (promiscuous), it's:
33.32% faster
1.5 times as fast
0.18 order(s) of magnitude faster

Compared with the slowest (Q), it's:
99.44% faster
178.73 times as fast
2.25 order(s) of magnitude faster

This is not fair to p-promise because it uses setImmediate if avalable.

So here is the fair comparison:

var Y = require('ya-promise')
  , P = require('p-promise') = {
  'ya-promise' : function(done){
    var d = Y.defer()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })
, 'p-promise' : function(done){
    var d = P.defer()
      , p = d.promise
    p.then(function(v){ return v+1 })
    p.then(function(v){ done() })

{ http_parser: '1.0',
  node: '0.10.4',
  v8: '',
  ares: '1.9.0-DEV',
  uv: '0.10.4',
  zlib: '1.2.3',
  modules: '11',
  openssl: '1.0.1e' }
Scores: (bigger is better)

 > 133.78121878121877
 > 136.0979020979021
 > 137.86713286713288
 > 139.1988011988012
Average (mean) 136.73626373626374

 > 108.32167832167832
 > 98.51548451548452
 > 106.22477522477523
 > 106.47152847152847
Average (mean) 104.88336663336663

Winner: p-promise
Compared with next highest (ya-promise), it's:
23.3% faster
1.3 times as fast
0.12 order(s) of magnitude faster


Performance Lessons Learned

Constructors do not HAVE to be more expensive then Plain-Ole-Objects

IE new Promise(thenFn) does not have to be more expensive than { then: thenFn }.

then, reject, & resolve are closures not methods

This is total tl;dr. ("To Long Don't Read" for non-internet-hipsters, like me:).

This is a cute fact about the implementation that has a few implications.


var deferred = Y.defer()

deferred.resolve and deferred.reject are closures not methods. That means that you could separate the function foo = deferred.resolve from the deferred object and calling foo(value) will still work.

Basically, deferred is just a plain javascript object {} with three named values promise, resolve, and reject.

For that matter, promise.then is a closure not a method. If you look at it promise only contains a then entry.

This turns out to be a good thing for two reasons, and bad for one reason:

  1. Converting a foreign promise to a ya-promise promise is easy.
function convert(foreign_promise){
  var deferred = Y.defer()
  foreign_promise.then(deferred.resolve, deferred.reject)
  return deferred.promise
  1. There is no way to access to the internals of the deferred or promise mechanisms. They are truely private.

This could be bad when the initial deferred.resolve is called, it replaces deferred.resolve with a new function. So, if you copy the original function to a new variable AND that function gets called twice it will call the previous queued up then functions twice as well. Simple don't do what I did above in 1. do the following instead:

function convert(foreign_promise){
  var deferred = Y.defer()
  foreign_promise.then( function(value) { deferred.resolve(value) }
                      , function(reason){ deferred.reject(reasone) })
  return deferred.promise

Put in terms of code the folowing function returns true:

function compareResolves(){
  var deferred = Y.defer()
    , resolveFnBefore = deferred.resolve

  deferred.resolve("whatever")  //this function call changes `deferred.resolve`

  return deferred.resolve !== resolveFnBefore  //returns `true`

This applys to the promise's then function as well:

function compareThens(){
  var deferred = Y.defer()
   , thenFnBefore = deferred.promise.then


  return deferred.promise.then !== thenFnBefore  //returns `true`

Advice: Screw Nike comercials, "Just DON'T Do It". Don't try to be too clever by half and take advantage of the fact that deferred.resolve, deferred.reject, and promise.then are closures not methods because they "close over" deffered and promise as well.


Promise/A+ Specification Promise/A+ Test Suite p-promise NPM module [promiscuous NPM mdule[promiscuous] Q NPM module bench NPM module Promise/A+ terminology tl;dr definition

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