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Asynchronous Control Flow with Promises

Introduction to Promises

A Promise is an object that represents the result of an asynchronous function call. Promises are also called futures and deferreds in some communities.

Goal of this Article

The goal of this article is to introduce CommonJS promises as they exist in NodeJS user-land. With jQuery popularizing a variant of CommonJS promises for AJAX operations, promises will likely gain a broader user-base in the browser and on the server.

Brief History of Promises in Node

Promises based on EventEmitters were originally a part of Node. They looked like this:

var promise = fs.stat("foo");
promise
.addListener("success", function (value) {
   
// ok
})
promise
.addListener("error", function (error) {
   
// error
});

The style was ugly and required more allocations than strictly necessary. There were political issues because these "promises" did not fulfill all of the contracts that advocates for promises wished they did. Between aesthetics, politics, and reductionism, Ryan removed promises in the v0.2 era and settled on the present callback style, leaving promises as an exercise for user-land.

fs.stat("foo", function (error, value) {
   
if (error) {
       
// error
   
} else {
       
// ok
   
}
});

Promise Terminology

  • Fulfillment: When a successful promise is fulfilled, all of the pending callbacks are called with the value. If more callbacks are registered in the future, they will be called with the same value. Fulfilment is the asynchronous analog for returning a value.
  • Rejection: When a promise cannot be fulfilled, a promise is 'rejected' which invokes the errbacks that are waiting and remembers the error that was rejected for future errbacks that are attached. Rejection is the asynchronous analog for throwing an exception.
  • Resolution: A promise is resolved when it makes progress toward fulfillment or rejection. A promise can only be resolved once, and it can be resolved with a promise instead of a fulfillment or rejection.
  • Callback: A function executed if a a promise is fulfilled with a value.
  • Errback: A function executed if a promise is rejected, with an exception.
  • Progressback: A function executed to show that progress has been made toward resolution of a promise.

Promise Libraries

Server Side

There are two primary implementations of CommonJS promises for Node:

Q library by Kris Kowal.

Install with NPM: npm install q

var q = require('q');

Promised-IO by Kris Zyp.

Install with NPM: npm install promised-io

var q = require('promised-io/lib/promise'); // <=v2.3

var q = require('promised-io/promise'); // >=v2.4

They are compatible with one anothers promises, so which you use is merely a matter of preference. These libraries do contain different helper functions outside of the basic functions necessary to consume and create promises.

The rest of the server-side examples will assume that a q variable containing a promise module is in scope.

Client Side

jQuery implemented Promises in version 1.5.

$.when($.get(...))
.then(function(value) {
    console
.log('success!')
}, function(error) {
    console
.log('rejection');
});

The Promise Contract

There is ONE Resolution of Rejection

A promise is resolved one time. It will never be fulfilled if it has been rejected or rejected if it has been fulfilled.

Listeners are executed ONE time

An individual callback or errback will be executed once and only once. This follows from the first rule of the contract.

Promises remember their state

A promise that is resolved with a value remembers the fulfillment. If a callback is attached in the future to this promise, it will be executed with the previously resolved value. The same is true of errbacks. If a promise is rejected and an errback is attached after the rejection, it will be executed with the rejected value. Promises behave the same way regardless of whether they are already resolved or resolved in the future.

Thenables

CommonJS promises are sometimes referred to as thenables. This term derives from the then method available on a promise. The then method is the gateway to attaching callbacks, errbacks, and progressbacks.

myPromise.then(function() {
 
// callback, executed on successful promise resolution
}, function() {
 
// errback, executed on rejection
}, function() {
 
// progressback, executed if the promise has progress to report
});

When

The when function is a helper to attach listeners to an object that you are not sure is a promise. This is useful because it is often helpful for methods to be able to return a promise or a direct value. If a direct value is returned, it will not have a then method. Also, if the promise does not implement then properly, the when function makes sure that it behaves properly (making sure your callbacks get called in separate events and never more than once).

when(maybePromise, function() {
 
// callback, executed on successful promise resolution or if maybePromise is not a promise but a value
}, function() {
 
// errback, executed on rejection
}, function() {
 
// progressback, executed if the promise has progress to report
});

Bubbling

The value returned by a callback is bubbled up the chain of promises. The same is true of rejections. This allows for rejections to be handled at a higher level than the function that called the promise-returning function that is rejected. This also allows promise-based APIs to compose responses for rejections and successes by changing the value or error as it is bubbled.

function doSomethingAsync() {
 
return asyncHelper().then(function(val) {
   
// do some extra processing on val
   
return val; <-- becomes the resolution of the promise returned by doSomethingAsync
 
});
}

doSomethingAsync
().then(function(val) {
  console
.log('resolved', val);
}, function(err) {
 
// Will receive rejections from doSomethingAsync or bubbled from asyncHelper
  console
.log('error', err);
});

Creating Promises using the Deferred helper

A Deferred is an object that helps create and manipulate promises. A Deferred has a promise property that is the promise that it is managing. It also has resolve and reject methods that are responsible for resolving the promise.

resolve is a function that may be passed a value or a promise. If it’s passed a value, the promise is fulfilled. If it’s passed a promise, that promise’s resolution will eventually be forwarded to this one.

Your milage will vary from library to library if you call the resolver more than once, but all libraries enforce the rule that a promise can only be resolved once. Kris Kowal’s Q library allows multiple users to “race” to be the first to resolve the promise. Kris Zyp’s PromisedIO throws an exception if you attempt to resolve a promise again.

Reject is a function that accepts a reason for rejection. The rejection may be of any type but is commonly a string. The Deferred rejects its promise with the rejection reason.

function doSomethingAsync() {
 
var deferred = q.defer();
  setTimeout
(function() {
    deferred
.resolve('hello world');
 
}, 500);

 
return deferred.promise;
}

doSomethingAsync
().then(function(val) {
  console
.log('Promise Resolved!', val);
});

Wrapping a function that takes a Node-style callback

The majority of functions that take Node-style callbacks are suitable for wrapping in a Promise. Any Node-style callback function that only calls its callback one time may be wrapped.

The following function is taken from Bogart.

function promisify(nodeAsyncFn, context) {
 
return function() {
   
var defer = q.defer()
     
, args = Array.prototype.slice.call(arguments);

    args
.push(function(err, val) {
     
if (err !== null) {
       
return defer.reject(err);
     
}

     
return defer.resolve(val);
   
});

    nodeAsyncFn
.apply(context || {}, args);

   
return defer.promise;
 
};
};

This function can be used with node functions like fs.readFile.

var readFile = promisify(fs.readFile);
readFile
('test.txt').then(function(data) {
  console
.log(data);
});

Both Q and PromisedIO provide utilities for wrapping or calling Node-style functions.

With Kris Zyp’s PromisedIO:

var readFile = q.convertNodeAsyncFunction(fs.readFile);
readFile
('test.txt')
.then(function (data) { });

// or

q
.execute(fs.readFile, 'test.txt')
.then(function (data) { });

With Kris Kowal’s Q:

var readFile = q.node(fs.readFile);
readFile
('test.txt')
.then(function (data) { });

// or

q
.ncall(fs.readFile, fs, 'test.txt')
.then(function (data) { });

// or

var deferred = q.defer();
fs
.readFile('test.txt', deferred.node());
return deferred.promise;

Conclusions

Promises are a valualbe tool for managing asynchronous control flow. With jQuery adopting Promises, more JavaScript programmers will see them as the preferred method of managing asynchronous state. If you have any questions, please leave them in the comments section.


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