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How do you know when an indefinitely long promise chain has completely finished?

I was trying to use promises to force serialization of a series of Ajax calls. These Ajax calls are made one for each time a user presses a button. I can successfully serialize the operations like this:
// sample async function
// real-world this is an Ajax call
function delay(val) {
log("start: ", val);
return new Promise(function(resolve) {
setTimeout(function() {
log("end: ", val);
resolve();
}, 500);
});
}
// initialize p to a resolved promise
var p = Promise.resolve();
var v = 1;
// each click adds a new task to
// the serially executed queue
$("#run").click(function() {
// How to detect here that there are no other unresolved .then()
// handlers on the current value of p?
p = p.then(function() {
return delay(v++);
});
});
Working demo: http://jsfiddle.net/jfriend00/4hfyahs3/
But, this builds a potentially never ending promise chain since the variable p that stores the last promise is never cleared. Every new operation just chains onto the prior promise. So, I was thinking that for good memory management, I should be able to detect when there are no more .then() handlers left to run on the current value of p and I can then reset the value of p, making sure that any objects that the previous chain of promise handlers might have held in closures will be eligible for garbage collection.
So, I was wondering how I would know in a given .then() handler that there are no more .then() handlers to be called in this chain and thus, I can just do p = Promise.resolve() to reset p and release the previous promise chain rather than just continually adding onto it.

I'm being told that a "good" promise implementation would not cause accumulating memory from an indefinitely growing promise chain. But, there is apparently no standard that requires or describes this (other than good programming practices) and we have lots of newbie Promise implementations out there so I have not yet decided if it's wise to rely on this good behavior.
My years of coding experience suggest that when implementations are new, facts are lacking that all implementations behave a certain way and there's no specification that says they should behave that way, then it might be wise to write your code in as "safe" a way as possible. In fact, it's often less work to just code around an uncertain behavior than it is to go test all relevant implementations to find out how they behave.
In that vein, here's an implementation of my code that seems to be "safe" in this regard. It just saves a local copy of the global last promise variable for each .then() handler and when that .then() handler runs, if the global promise variable still has the same value, then my code has not chained any more items onto it so this must be the currently last .then() handler. It seems to work in this jsFiddle:
// sample async function
// real-world this is an Ajax call
function delay(val) {
log("start: ", val);
return new Promise(function(resolve) {
setTimeout(function() {
log("end: ", val);
resolve();
}, 500);
});
}
// initialize p to a resolved promise
var p = Promise.resolve();
var v = 1;
// each click adds a new task to
// the serially executed queue
$("#run").click(function() {
var origP = p = p.then(function() {
return delay(v++);
}).then(function() {
if (p === origP) {
// no more are chained by my code
log("no more chained - resetting promise head");
// set fresh promise head so no chance of GC leaks
// on prior promises
p = Promise.resolve();
v = 1;
}
// clear promise reference in case this closure is leaked
origP = null;
}, function() {
origP = null;
});
});

… so that I can then reset the value of p, making sure that any objects that the previous chain of promise handlers might have held in closures will be eligible for garbage collection.
No. A promise handler that has been executed (when the promise has settled) is no more needed and implicitly eligible for garbage collection. A resolved promise does not hold onto anything but the resolution value.
You don't need to do "good memory management" for promises (asynchronous values), your promise library does take care of that itself. It has to "release the previous promise chain" automatically, if it doesn't then that's a bug. Your pattern works totally fine as is.
How do you know when the promise chain has completely finished?
I would take a pure, recursive approach for this:
function extendedChain(p, stream, action) {
// chains a new action to p on every stream event
// until the chain ends before the next event comes
// resolves with the result of the chain and the advanced stream
return Promise.race([
p.then(res => ({res}) ), // wrap in object to distinguish from event
stream // a promise that resolves with a .next promise
]).then(({next, res}) =>
next
? extendedChain(p.then(action), next, action) // a stream event happened first
: {res, next:stream}; // the chain fulfilled first
);
}
function rec(stream, action, partDone) {
return stream.then(({next}) =>
extendedChain(action(), next, action).then(({res, next}) => {
partDone(res);
return rec(next, action, partDone);
});
);
}
var v = 1;
rec(getEvents($("#run"), "click"), () => delay(v++), res => {
console.log("all current done, none waiting");
console.log("last result", res);
}); // forever
with a helper function for event streams like
function getEvents(emitter, name) {
var next;
function get() {
return new Promise((res) => {
next = res;
});
}
emitter.on(name, function() {
next({next: get()});
});
return get();
}
(Demo at jsfiddle.net)

It is impossible to detect when no more handlers are added.
It is in fact an undecidable problem. It is not very hard to show a reduction to the halting (or the Atm problem). I can add a formal reduction if you'd like but in handwavey: Given an input program, put a promise at its first line and chain to it at every return or throw - assuming we have a program that solves the problem you describe in this question - apply it to the input problem - we now know if it runs forever or not solving the halting problem. That is, your problem is at least as hard as the halting problem.
You can detect when a promise is "resolved" and update it on new ones.
This is common in "last" or in "flatMap". A good use case is autocomplete search where you only want the latest results. Here is an [implementation by Domenic
(https://github.com/domenic/last):
function last(operation) {
var latestPromise = null; // keep track of the latest
return function () {
// call the operation
var promiseForResult = operation.apply(this, arguments);
// it is now the latest operation, so set it to that.
latestPromise = promiseForResult;
return promiseForResult.then(
function (value) {
// if we are _still_ the last value when it resovled
if (latestPromise === promiseForResult) {
return value; // the operation is done, you can set it to Promise.resolve here
} else {
return pending; // wait for more time
}
},
function (reason) {
if (latestPromise === promiseForResult) { // same as above
throw reason;
} else {
return pending;
}
}
);
};
};
I adapted Domenic's code and documented it for your problem.
You can safely not optimize this
Sane promise implementations do not keep promises which are "up the chain", so setting it to Promise.resolve() will not save memory. If a promise does not do this it is a memory leak and you should file a bug against it.

I tried to check if we can see the promise's state in code, apprantly that is only possible from console, not from code, so I used a flag to moniter the status, not sure if there is a loophole somewhere:
var p
, v = 1
, promiseFulfilled = true;
function addPromise() {
if(!p || promiseFulfilled){
console.log('reseting promise...');
p = Promise.resolve();
}
p = p.then(function() {
promiseFulfilled = false;
return delay(v++);
}).then(function(){
promiseFulfilled = true;
});
}
fiddle demo

You could push the promises onto an array and use Promise.all:
var p = Promise.resolve,
promiseArray = [],
allFinishedPromise;
function cleanup(promise, resolvedValue) {
// You have to do this funkiness to check if more promises
// were pushed since you registered the callback, though.
var wereMorePromisesPushed = allFinishedPromise !== promise;
if (!wereMorePromisesPushed) {
// do cleanup
promiseArray.splice(0, promiseArray.length);
p = Promise.resolve(); // reset promise
}
}
$("#run").click(function() {
p = p.then(function() {
return delay(v++);
});
promiseArray.push(p)
allFinishedPromise = Promise.all(promiseArray);
allFinishedPromise.then(cleanup.bind(null, allFinishedPromise));
});
Alternatively, since you know they are executed sequentially, you could have each completion callback remove that promise from the array and just reset the promise when the array is empty.
var p = Promise.resolve(),
promiseArray = [];
function onPromiseComplete() {
promiseArray.shift();
if (!promiseArray.length) {
p = Promise.resolve();
}
}
$("#run").click(function() {
p = p.then(function() {
onPromiseComplete();
return delay(v++);
});
promiseArray.push(p);
});
Edit: If the array is likely to get very long, though, you should go with the first option b/c shifting the array is O(N).
Edit: As you noted, there's no reason to keep the array around. A counter will work just fine.
var p = Promise.resolve(),
promiseCounter = 0;
function onPromiseComplete() {
promiseCounter--;
if (!promiseCounter) {
p = Promise.resolve();
}
}
$("#run").click(function() {
p = p.then(function() {
onPromiseComplete();
return delay(v++);
});
promiseCounter++;
});

Avoiding duplicate asynchronous service initialisations in Angular.js

I have an Angular.js service which delivers its results asynchronously, after looking around for a while the main pattern for doing this seems to be using $q promises like this
angular.module('fooApp').factory('foo', function ($q) {
var result;
function build() {
var d = $q.defer();
longAsyncInit(function(data) {
result = data;
d.resolve(result);
});
return d.promise;
};
return {
get: function () {
if (result) {
return $q.when(result);
} else {
return build();
}
}
}
});
The problem is that I have a number of services which have this service as a dependency and get is called multiple times before the first longAsyncInit ends (which means that longAsyncInit gets called multiple times, each time creating a new promise). In my case this is unacceptable, I really need longAsyncInit to be called once, no more. I'm currently addressing this issue like this
angular.module('fooApp').factory('foo', function ($q) {
var result
var d;
function build() {
d = $q.defer();
longAsyncInit(function(data) {
result = data;
d.resolve(result);
});
return d.promise;
};
return {
get: function () {
if (result) {
return $q.when(result);
} else if (d) {
return d.promise;
} else {
return build();
}
}
}
});
This means if longAsyncInit is already ongoing when a get() call is made, it returns the current promise, instead of creating a new one and calling longAsyncInit again. This seems to work but feels inelegant and fragile, is there a better way of doing this?

You are looking for debounce method to solve problem.
From Underscore library documentation what _.debounce() do
Creates and returns a new debounced version of the passed function
which will postpone its execution until after wait milliseconds have
elapsed since the last time it was invoked. Useful for implementing
behavior that should only happen after the input has stopped arriving.
For example: rendering a preview of a Markdown comment, recalculating
a layout after the window has stopped being resized, and so on.
Some more explanations
Can someone explain the "debounce" function in Javascript
Some content to read:
http://davidwalsh.name/javascript-debounce-function

flickrapi (js) multiple async calls in a loop

I allmost banged my head into the wall because I can't get the following code too work. I'm trying to code a photo gallery with the flickrApi and have problems with multiple async calls. But perhaps there is a cleaner solution to code this.
openPhotoset() is called when clicking the link of a photoset. Unfortunately getting the description of a photo I need to use a different method, which means another async call. I'm looping through the data, but because I make the call in a loop (that's when I have the photo-id available) the deferred of openPhotoset() doesn't resolve after looping but before. I read and have seen examples of $.when() used in a loop, filling an array with deferreds and checking with $.when but I seem to fail horribly at it. Is this the solution I need or is there another road to salvation? ;)
I want to execute different functions after all calls within openPhotoset() has completed.
function openPhotoset(photosetId) {
var currentPhotoset = [],
deferred = $.Deferred();
_requestPhotosOfSet(photosetId).done(function(data){
$(data.photoset.photo).each(function(i, photo){
var objPhoto = {};
objPhoto.id = photo.id;
objPhoto.title = photo.title;
objPhoto.farm = photo.farm;
objPhoto.server = photo.server;
objPhoto.secret = photo.secret;
// get photo description
requestPhotoInfo(photo.id).done(function(data) {
objPhoto.description = data.photo.description._content;
currentPhotoset.push(objPhoto);
}).then(function() {
// TODO: renders with each iteration, shouldnt!
var template = $('#li-gallery').html(),
result = Mustache.render(template, {currentPhotoset:currentPhotoset});
showGallery();
_$fyGallery.find('.gallery-list').html(result);
deferred.resolve();
});
});
});
return deferred;
}

You can do this by changing .done() for .then() in a couple of places, and rearranging things a bit - well quite
a lot.
I think you've probably been searching for something like this :
function openPhotoset(photosetId) {
return _requestPhotosOfSet(photosetId).then(function(data) {
var promises = $(data.photoset.photo).map(function(photo) {
return requestPhotoInfo(photo.id).then(function(data) {
return {
id: photo.id,
title: photo.title,
farm: photo.farm,
server: photo.server,
secret: photo.secret,
description: data.photo.description._content
};
});
}).get();//.get() is necessary to convert a jQuery object to a regular js array.
return $.when.apply(null, promises).then(function() {
var template = $('#li-gallery').html(),
result = Mustache.render(template, {
currentPhotoset: Array.prototype.slice.apply(arguments)
});
showGallery();
_$fyGallery.find('.gallery-list').html(result);
});
});
}
The main difference here is the creation of an array of promises as opposed to an array of photo objects, and allowing the promises to convey the data. This allows $.when() to fire off a callback when all the promises are fulfilled - ie when data objects have been composed for all photos in the set.
Note the use of .map() instead of .each(), thus simplifying the creation of promises.
And finally, the overall promise returned by openPhotoset() allows whatever action to be taken on completion of the whole process. Just chain .then().
openPhotoset(...).then(function() {
// here, do whatever
});
EDIT
The overall pattern is probably easier to understand if the inner workings are pulled out and rephrased as named promise-returning functions - getPhotoInfoObject() and renderData().
function openPhotoset(photosetId) {
function getPhotoInfoObject(photo) {
return requestPhotoInfo(photo.id).then(function(data) {
//$.extend() is much less verbose than copying `photo`'s properties into a new object longhand.
return $.extend(photo, {description: data.photo.description._content});
});
}
function renderData() {
var template = $('#li-gallery').html(),
currentPhotoset = Array.prototype.slice.apply(arguments),
result = Mustache.render(template, {
currentPhotoset: currentPhotoset
});
showGallery();
_$fyGallery.find('.gallery-list').html(result);
}
// With the inner workings pulled out as getPhotoInfoObject() and renderData(),
// the residual pattern is very concise and easier to understand.
return _requestPhotosOfSet(photosetId).then(function(data) {
var promises = $(data.photoset.photo).map(getPhotoInfoObject).get();
return $.when.apply(null, promises).then(renderData);
});
}

I was so blinded by the defereds and $.when function that I didn't notice all I needed was to create a counter and count down each time requestPhotoInfo was done and after render the html

Difficulty wrapping a javascript behavior and holding it for later

I am getting into programming with javascript and using Promises, right now using Q.js. I have finally gotten to a point where I understand what I am doing, but am having a difficult time with a specific behavior.
I have one situation where I have reasonably similar code repeated several times. It basically goes like this ...
{
// start
var deferred = Q.defer();
// do something {
deferred.resolve();
}
return deferred.promise;
}
Okay, that's all fine and good, but repeating all of this every time was getting annoying, so I attempted to wrap it up in something. This is just an example, it is not the entire javascript file, since most of the other parts are not relevant.
{
var list = [];
queue = function(f) {
var deferred = Q.defer();
list.push(f(deferred));
return deferred.promise;
}
{
queue(function(deferred){
// do some work
// we want the deferred here so we can resolve it at the correct time
deferred.resolve();
});
}
}
The problem is that I don't want this to run the instant I queue it up. I basically want to build the list, and then run it later. I am running the list using the reduce function in Q.js
{
return list.reduce(function(i, f) {
return i.then(f);
}, Q());
}
But this is kind of counter to my goal, since I really don't intend to run them at the same time they are queued. Is there a way to save the execution for later and still pass the deferred object through the function?
Update
I was asked what I expect the code to do, which is a fair question. I'll try to explain. The purpose of this is to split up the logic because I am using ASP.NET MVC, and so I have _Layout pages, and then normal views - so there is logic that cannot run until other things are completed, but some times that is on a per-page basis. This method was contrived to deal with that.
Essentially it works like this ...
Loader.js
This is, for lack of a better term or current implementation, a global object. I have plans to change that eventually, but one step at a time.
{
var Loader = {};
var list = [];
initialize = function() {
Q().then(step1).then(step2).then(process).then(finalStep);
};
queue = function(f) {
// push the given function to the list
};
process = function() {
return list.reduce(function(i,f){
return i.then(f);
}, Q());
};
step1 = function() { // generic example
// create a promise
return deferred.promise;
}; // other steps are similar to this.
return Loader;
}
_Layout
<head>
#RenderSection("scripts", false)
<script type="text/javascript">
// we have the loader object already
Loader.initialize();
</script>
</head>
Index
#section Scripts {
<script type="text/javascript">
Loader.promise(function(deferred){
// do something here.
deferred.resolve();
}));
</script>
}

You could use a closure.
queue(function(deferred) {
return function() {
// this is the actual function that will be run,
// but it will have access to the deferred variable
deferred.resolve();
};
});

I think you should do something like
var Loader = {
promise: function(construct) {
var deferred = Q.defer();
construct(deferred);
return deferred.promise;
},
queue: function(f) {
this.ready = this.ready.then(f);
},
ready: Q.Promise(function(resolve) {
window.onload = resolve; // or whatever you need to do here
// or assign the resolve function to Loader.initialize and call it later
})
};
Then Loader.queue() functions that return other promises.

How is a promise/defer library implemented? [closed]

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How is a promise/defer library like q implemented? I was trying to read the source code but found it pretty hard to understand, so I thought it'd be great if someone could explain to me, from a high level, what are the techniques used to implement promises in single-thread JS environments like Node and browsers.

I find it harder to explain than to show an example, so here is a very simple implementation of what a defer/promise could be.
Disclaimer: This is not a functional implementation and some parts of the Promise/A specification are missing, This is just to explain the basis of the promises.
tl;dr: Go to the Create classes and example section to see full implementation.
Promise:
First we need to create a promise object with an array of callbacks. I'll start working with objects because it's clearer:
var promise = {
callbacks: []
}
now add callbacks with the method then:
var promise = {
callbacks: [],
then: function (callback) {
callbacks.push(callback);
}
}
And we need the error callbacks too:
var promise = {
okCallbacks: [],
koCallbacks: [],
then: function (okCallback, koCallback) {
okCallbacks.push(okCallback);
if (koCallback) {
koCallbacks.push(koCallback);
}
}
}
Defer:
Now create the defer object that will have a promise:
var defer = {
promise: promise
};
The defer needs to be resolved:
var defer = {
promise: promise,
resolve: function (data) {
this.promise.okCallbacks.forEach(function(callback) {
window.setTimeout(function () {
callback(data)
}, 0);
});
},
};
And needs to reject:
var defer = {
promise: promise,
resolve: function (data) {
this.promise.okCallbacks.forEach(function(callback) {
window.setTimeout(function () {
callback(data)
}, 0);
});
},
reject: function (error) {
this.promise.koCallbacks.forEach(function(callback) {
window.setTimeout(function () {
callback(error)
}, 0);
});
}
};
Note that the callbacks are called in a timeout to allow the code be always asynchronous.
And that's what a basic defer/promise implementation needs.
Create classes and example:
Now lets convert both objects to classes, first the promise:
var Promise = function () {
this.okCallbacks = [];
this.koCallbacks = [];
};
Promise.prototype = {
okCallbacks: null,
koCallbacks: null,
then: function (okCallback, koCallback) {
okCallbacks.push(okCallback);
if (koCallback) {
koCallbacks.push(koCallback);
}
}
};
And now the defer:
var Defer = function () {
this.promise = new Promise();
};
Defer.prototype = {
promise: null,
resolve: function (data) {
this.promise.okCallbacks.forEach(function(callback) {
window.setTimeout(function () {
callback(data)
}, 0);
});
},
reject: function (error) {
this.promise.koCallbacks.forEach(function(callback) {
window.setTimeout(function () {
callback(error)
}, 0);
});
}
};
And here is an example of use:
function test() {
var defer = new Defer();
// an example of an async call
serverCall(function (request) {
if (request.status === 200) {
defer.resolve(request.responseText);
} else {
defer.reject(new Error("Status code was " + request.status));
}
});
return defer.promise;
}
test().then(function (text) {
alert(text);
}, function (error) {
alert(error.message);
});
As you can see the basic parts are simple and small. It will grow when you add other options, for example multiple promise resolution:
Defer.all(promiseA, promiseB, promiseC).then()
or promise chaining:
getUserById(id).then(getFilesByUser).then(deleteFile).then(promptResult);
To read more about the specifications: CommonJS Promise Specification. Note that main libraries (Q, when.js, rsvp.js, node-promise, ...) follow Promises/A specification.
Hope I was clear enough.
Edit:
As asked in the comments, I've added two things in this version:
The possibility to call then of a promise, no matter what status it has.
The possibility to chain promises.
To be able to call the promise when resolved you need to add the status to the promise, and when the then is called check that status. If the status is resolved or rejected just execute the callback with its data or error.
To be able to chain promises you need to generate a new defer for each call to then and, when the promise is resolved/rejected, resolve/reject the new promise with the result of the callback. So when the promise is done, if the callback returns a new promise it is bound to the promise returned with the then(). If not, the promise is resolved with the result of the callback.
Here is the promise:
var Promise = function () {
this.okCallbacks = [];
this.koCallbacks = [];
};
Promise.prototype = {
okCallbacks: null,
koCallbacks: null,
status: 'pending',
error: null,
then: function (okCallback, koCallback) {
var defer = new Defer();
// Add callbacks to the arrays with the defer binded to these callbacks
this.okCallbacks.push({
func: okCallback,
defer: defer
});
if (koCallback) {
this.koCallbacks.push({
func: koCallback,
defer: defer
});
}
// Check if the promise is not pending. If not call the callback
if (this.status === 'resolved') {
this.executeCallback({
func: okCallback,
defer: defer
}, this.data)
} else if(this.status === 'rejected') {
this.executeCallback({
func: koCallback,
defer: defer
}, this.error)
}
return defer.promise;
},
executeCallback: function (callbackData, result) {
window.setTimeout(function () {
var res = callbackData.func(result);
if (res instanceof Promise) {
callbackData.defer.bind(res);
} else {
callbackData.defer.resolve(res);
}
}, 0);
}
};
And the defer:
var Defer = function () {
this.promise = new Promise();
};
Defer.prototype = {
promise: null,
resolve: function (data) {
var promise = this.promise;
promise.data = data;
promise.status = 'resolved';
promise.okCallbacks.forEach(function(callbackData) {
promise.executeCallback(callbackData, data);
});
},
reject: function (error) {
var promise = this.promise;
promise.error = error;
promise.status = 'rejected';
promise.koCallbacks.forEach(function(callbackData) {
promise.executeCallback(callbackData, error);
});
},
// Make this promise behave like another promise:
// When the other promise is resolved/rejected this is also resolved/rejected
// with the same data
bind: function (promise) {
var that = this;
promise.then(function (res) {
that.resolve(res);
}, function (err) {
that.reject(err);
})
}
};
As you can see, it has grown quite a bit.

Q is a very complex promise library in terms of implementation because it aims to support pipelining and RPC type scenarios. I have my own very bare bones implementation of the Promises/A+ specification here.
In principle it's quite simple. Before the promise is settled/resolved, you keep a record of any callbacks or errbacks by pushing them into an array. When the promise is settled you call the appropriate callbacks or errbacks and record what result the promise was settled with (and whether it was fulfilled or rejected). After it's settled, you just call the callbacks or errbacks with the stored result.
That gives you aproximately the semantics of done. To build then you just have to return a new promise that is resolved with the result of calling the callbacks/errbacks.
If you're interested in a full explenation of the reasonning behind the development of a full on promise implementation with support for RPC and pipelining like Q, you can read kriskowal's reasonning here. It's a really nice graduated approach that I can't recommend highly enough if you are thinking of implementing promises. It's probably worth a read even if you're just going to be using a promise library.

As Forbes mentions in his answer, I chronicled many of the design decisions involved in making a library like Q, here https://github.com/kriskowal/q/tree/v1/design. Suffice it to say, there are levels of a promise library, and lots of libraries that stop at various levels.
At the first level, captured by the Promises/A+ specification, a promise is a proxy for an eventual result and is suitable for managing “local asynchrony”. That is, it is suitable for ensuring that work occurs in the right order, and for ensuring that it is simple and straight-forward to listen for the result of an operation regardless of whether it already settled, or will occur in the future. It also makes it just as simple for one or many parties to subscribe to an eventual result.
Q, as I have implemented it, provides promises that are proxies for eventual, remote, or eventual+remote results. To that end, it’s design is inverted, with different implementations for promises—deferred promises, fulfilled promises, rejected promises, and promises for remote objects (the last being implemented in Q-Connection). They all share the same interface and work by sending and receiving messages like "then" (which is sufficient for Promises/A+) but also "get" and "invoke". So, Q is about “distributed asynchrony”, and exists on another layer.
However, Q was actually taken down from a higher layer, where promises are used for managing distributed asynchrony among mutually suspicious parties like you, a merchant, a bank, Facebook, the government—not enemies, maybe even friends, but sometimes with conflicts of interest. The Q that I implemented is designed to be API compatible with hardened security promises (which is the reason for separating promise and resolve), with the hope that it would introduce people to promises, train them in using this API, and allow them to take their code with them if they need to use promises in secure mashups in the future.
Of course, there are trade-offs as you move up the layers, usually in speed. So, promises implementations can also be designed to co-exist. This is where the concept of a “thenable” enters. Promise libraries at each layer can be designed to consume promises from any other layer, so multiple implementations can coexist, and users can buy only what they need.
All this said, there is no excuse for being difficult to read. Domenic and I are working on a version of Q that will be more modular and approachable, with some of its distracting dependencies and work-arounds moved into other modules and packages. Thankfully folks like Forbes, Crockford, and others have filled in the educational gap by making simpler libraries.

First make sure you're understanding how Promises are supposed to work. Have a look at the CommonJs Promises proposals and the Promises/A+ specification for that.
There are two basic concepts that can be implemented each in a few simple lines:
A Promise does asynchronously get resolved with the result. Adding callbacks is a transparent action - independent from whether the promise is resolved already or not, they will get called with the result once it is available.
function Deferred() {
var callbacks = [], // list of callbacks
result; // the resolve arguments or undefined until they're available
this.resolve = function() {
if (result) return; // if already settled, abort
result = arguments; // settle the result
for (var c;c=callbacks.shift();) // execute stored callbacks
c.apply(null, result);
});
// create Promise interface with a function to add callbacks:
this.promise = new Promise(function add(c) {
if (result) // when results are available
c.apply(null, result); // call it immediately
else
callbacks.push(c); // put it on the list to be executed later
});
}
// just an interface for inheritance
function Promise(add) {
this.addCallback = add;
}
Promises have a then method that allows chaining them. I takes a callback and returns a new Promise which will get resolved with the result of that callback after it was invoked with the first promise's result. If the callback returns a Promise, it will get assimilated instead of getting nested.
Promise.prototype.then = function(fn) {
var dfd = new Deferred(); // create a new result Deferred
this.addCallback(function() { // when `this` resolves…
// execute the callback with the results
var result = fn.apply(null, arguments);
// check whether it returned a promise
if (result instanceof Promise)
result.addCallback(dfd.resolve); // then hook the resolution on it
else
dfd.resolve(result); // resolve the new promise immediately
});
});
// and return the new Promise
return dfd.promise;
};
Further concepts would be maintaining a separate error state (with an extra callback for it) and catching exceptions in the handlers, or guaranteeing asynchronity for the callbacks. Once you add those, you've got a fully functional Promise implementation.
Here is the error thing written out. It unfortunately is pretty repetitive; you can do better by using extra closures but then it get's really really hard to understand.
function Deferred() {
var callbacks = [], // list of callbacks
errbacks = [], // list of errbacks
value, // the fulfill arguments or undefined until they're available
reason; // the error arguments or undefined until they're available
this.fulfill = function() {
if (reason || value) return false; // can't change state
value = arguments; // settle the result
for (var c;c=callbacks.shift();)
c.apply(null, value);
errbacks.length = 0; // clear stored errbacks
});
this.reject = function() {
if (value || reason) return false; // can't change state
reason = arguments; // settle the errror
for (var c;c=errbacks.shift();)
c.apply(null, reason);
callbacks.length = 0; // clear stored callbacks
});
this.promise = new Promise(function add(c) {
if (reason) return; // nothing to do
if (value)
c.apply(null, value);
else
callbacks.push(c);
}, function add(c) {
if (value) return; // nothing to do
if (reason)
c.apply(null, reason);
else
errbacks.push(c);
});
}
function Promise(addC, addE) {
this.addCallback = addC;
this.addErrback = addE;
}
Promise.prototype.then = function(fn, err) {
var dfd = new Deferred();
this.addCallback(function() { // when `this` is fulfilled…
try {
var result = fn.apply(null, arguments);
if (result instanceof Promise) {
result.addCallback(dfd.fulfill);
result.addErrback(dfd.reject);
} else
dfd.fulfill(result);
} catch(e) { // when an exception was thrown
dfd.reject(e);
}
});
this.addErrback(err ? function() { // when `this` is rejected…
try {
var result = err.apply(null, arguments);
if (result instanceof Promise) {
result.addCallback(dfd.fulfill);
result.addErrback(dfd.reject);
} else
dfd.fulfill(result);
} catch(e) { // when an exception was re-thrown
dfd.reject(e);
}
} : dfd.reject); // when no `err` handler is passed then just propagate
return dfd.promise;
};

You might want to check out the blog post on Adehun.
Adehun is an extremely lightweight implementation (about 166 LOC) and very useful for learning how to implement the Promise/A+ spec.
Disclaimer: I wrote the blog post but the blog post does explain all about Adehun.
The Transition function – Gatekeeper for State Transition
Gatekeeper function; ensures that state transitions occur when all required conditions are met.
If conditions are met, this function updates the promise’s state and value. It then triggers the process function for further processing.
The process function carries out the right action based on the transition (e.g. pending to fulfilled) and is explained later.
function transition (state, value) {
if (this.state === state ||
this.state !== validStates.PENDING ||
!isValidState(state)) {
return;
}
this.value = value;
this.state = state;
this.process();
}
The Then function
The then function takes in two optional arguments (onFulfill and onReject handlers) and must return a new promise. Two major requirements:
The base promise (the one on which then is called) needs to create a new promise using the passed in handlers; the base also stores an internal reference to this created promise so it can be invoked once the base promise is fulfilled/rejected.
If the base promise is settled (i.e. fulfilled or rejected), then the appropriate handler should be called immediately. Adehun.js handles this scenario by calling process in the then function.
``
function then(onFulfilled, onRejected) {
var queuedPromise = new Adehun();
if (Utils.isFunction(onFulfilled)) {
queuedPromise.handlers.fulfill = onFulfilled;
}
if (Utils.isFunction(onRejected)) {
queuedPromise.handlers.reject = onRejected;
}
this.queue.push(queuedPromise);
this.process();
return queuedPromise;
}`
The Process function – Processing Transitions
This is called after state transitions or when the then function is invoked. Thus it needs to check for pending promises since it might have been invoked from the then function.
Process runs the Promise Resolution procedure on all internally stored promises (i.e. those that were attached to the base promise through the then function) and enforces the following Promise/A+ requirements:
Invoking the handlers asynchronously using the Utils.runAsync helper (a thin wrapper around setTimeout (setImmediate will also work)).
Creating fallback handlers for the onSuccess and onReject handlers if they are missing.
Selecting the correct handler function based on the promise state e.g. fulfilled or rejected.
Applying the handler to the base promise’s value. The value of this operation is passed to the Resolve function to complete the promise processing cycle.
If an error occurs, then the attached promise is immediately rejected.
function process() {
var that = this,
fulfillFallBack = function(value) {
return value;
},
rejectFallBack = function(reason) {
throw reason;
};
if (this.state === validStates.PENDING) {
return;
}
Utils.runAsync(function() {
while (that.queue.length) {
var queuedP = that.queue.shift(),
handler = null,
value;
if (that.state === validStates.FULFILLED) {
handler = queuedP.handlers.fulfill ||
fulfillFallBack;
}
if (that.state === validStates.REJECTED) {
handler = queuedP.handlers.reject ||
rejectFallBack;
}
try {
value = handler(that.value);
} catch (e) {
queuedP.reject(e);
continue;
}
Resolve(queuedP, value);
}
});
}
The Resolve function – Resolving Promises
This is probably the most important part of the promise implementation since it handles promise resolution. It accepts two parameters – the promise and its resolution value.
While there are lots of checks for various possible resolution values; the interesting resolution scenarios are two – those involving a promise being passed in and a thenable (an object with a then value).
Passing in a Promise value
If the resolution value is another promise, then the promise must adopt this resolution value’s state. Since this resolution value can be pending or settled, the easiest way to do this is to attach a new then handler to the resolution value and handle the original promise therein. Whenever it settles, then the original promise will be resolved or rejected.
Passing in a thenable value
The catch here is that the thenable value’s then function must be invoked only once (a good use for the once wrapper from functional programming). Likewise, if the retrieval of the then function throws an Exception, the promise is to be rejected immediately.
Like before, the then function is invoked with functions that ultimately resolve or reject the promise but the difference here is the called flag which is set on the first call and turns subsequent calls are no ops.
function Resolve(promise, x) {
if (promise === x) {
var msg = "Promise can't be value";
promise.reject(new TypeError(msg));
}
else if (Utils.isPromise(x)) {
if (x.state === validStates.PENDING){
x.then(function (val) {
Resolve(promise, val);
}, function (reason) {
promise.reject(reason);
});
} else {
promise.transition(x.state, x.value);
}
}
else if (Utils.isObject(x) ||
Utils.isFunction(x)) {
var called = false,
thenHandler;
try {
thenHandler = x.then;
if (Utils.isFunction(thenHandler)){
thenHandler.call(x,
function (y) {
if (!called) {
Resolve(promise, y);
called = true;
}
}, function (r) {
if (!called) {
promise.reject(r);
called = true;
}
});
} else {
promise.fulfill(x);
called = true;
}
} catch (e) {
if (!called) {
promise.reject(e);
called = true;
}
}
}
else {
promise.fulfill(x);
}
}
The Promise Constructor
And this is the one that puts it all together. The fulfill and reject functions are syntactic sugar that pass no-op functions to resolve and reject.
var Adehun = function (fn) {
var that = this;
this.value = null;
this.state = validStates.PENDING;
this.queue = [];
this.handlers = {
fulfill : null,
reject : null
};
if (fn) {
fn(function (value) {
Resolve(that, value);
}, function (reason) {
that.reject(reason);
});
}
};
I hope this helped shed more light into the way promises work.