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HeavyLifters Deferred Library

Asynchronous programming is the norm in JavaScript, where callbacks are commonly used to manage async processes. Unfortunately, no consistently-followed convention for organizing flow control with callbacks has emerged. With the rise of Node.js and client-side web app frameworks, programmers with little experience with asynchronous programming are struggling to understand and organize their code.

Why should I use this?

We've seen many developers complain about the difficulty of organizing asynchronous code: for example: calling a function when N async processes finish their work.

This is not a new problem, and solutions have existed for years. Our favourite is Twisted's Deferred class; our library brings it to JavaScript. HeavyLifters provides and uses implementations in Objective-C, Objective-J, JavaScript, and Java.

The biggest benefit you get by using Deferred objects in JavaScript is a consistent pattern for organizing callbacks and easily obtaining future values.

Deferred objects also make it easy to have async processes dependent on each other, including waiting for a list of async processes to finish, with optional notifications for the first completion or the first error. This simplifies flow control, makes your code much easier to reason about, and frees you from manually threading callback parameters through every async function call.


This library is designed to work in modern browsers and with Node.js, and should run in just about any ES3+ environment but no promises.

If it doesn't work in your preferred JavaScript environment, submit a bug report!

Running Unit Tests

Install Node.js and npm, then:

npm install vows
cd deferred-js
make test
# for more detail, make spec

How it works

You can create a Deferred object directly, but you typically request one from a data source. A data source is a function that returns a Deferred object.

The Deferred object provides access to the data source's data by allowing you to attach callbacks and/or errbacks to its callback chain.

When the data source has the result, it calls the resolve(result) method on the Deferred object, or reject(failure) (in the case of failure). This causes the Deferred object's callback chain to be fired - meaning each link in the chain (a callback or errback) is called in turn. The result is the input to the first callback, and its output is the input to the next callback (and so on).

If a callback (or errback) returns a Failure object, the next errback is called; otherwise the next callback is called.


The Most Basic Example

function demonstration() {
    var d = new Deferred()
    console.log('created Deferred object')
    d.then(function(result) {
        console.log('Hello ' + result)
        return result
    console.log('added a callback to the Deferred's callback chain')
    // resolve the Deferred, which fires the callback chain
    console.log('You should see Hello, World above this line in the console')
    // Note: use d.reject('DISASTER!') to indicate failure

Adding callbacks and errbacks

Each link in a callback chain is a pair of functions, representing a callback and an errback. Firing the chain executes the callback OR errback of each link, in sequence. For each link, its callback is executed if its input is a result; the errback is executed if its input is a failure (failures are represented by Failure objects).

Adding (just) a callback

To append a link with a callback to an Deferred object, call the then(cb) method, passing in a callback function. Example:

d.then(function(result) {
    // do something useful with the result
    return result

Deferred adds a link to its chain with your callback and a "passthrough" errback. The passthrough errback simply returns its exception parameter.

Adding (just) an errback

To append a link with an errback to an Deferred object, call the fail(eb) method, passing in an errback function. Example: {
    // optionally do something useful with failure.value()
    return failure

Deferred adds a link to its chain with your errback and a "passthrough" callback. The passthrough callback simply returns its result parameter.

Adding a callback and an errback

To add a link with a callback and errback to an Deferred object, call the -then(cb, eb) method or the both(cb) method.

Use then(cb, eb) when you want different behaviour in the case of success or failure:

d.then(function(result) {
    // do something useful with the result
    return result
}, function(failure) {
    // optionally do something useful with failure.value()
    return failure

Use both(cb) when you intend to do the same thing in either case:

d.both(function(result) {
    // in the case of failure, result is a Failure
    // do something in either case
    return result

Deferred in practice

By convention, names of methods returning an Deferred object are prefixed with "request", such as:

// result is a MyThing object
function requestDistantInformation()

We recommend you should document that information somewhere. This convention helps indicate that your function is asynchronous and returns a Deferred object.

A function that fetches something asynchronously can return a Deferred instead of accepting callback parameters. We call these functions data sources. When a data source has the requested value it resolves the Deferred object, which fires the callback chain. If it encounters an error it will reject the deferred, which fires the errback chain.

Here's an example of reading a file using Node.js:

// Reading a file in Node
fs.readFile('/etc/passwd', function(err, data) {
    if (err) throw err

Here's how you might wrap Node.js's readFile API to use the Deferred pattern:

dfs.requestReadFile = function(name) {
    var d = new Deferred()
    fs.readFile(name, function(err, data) {
        if (err) d.reject(err)
        else d.resolve(data)
    return d

Now you can read a file and give the Deferred object out to any party interested in the contents:

// Reading a file with Deferred (assuming we have a Deferred filesystem module)
var contentsDeferred = dfs.requestReadFile('/etc/passwd')

// anything interested in the contents of that file
// can attach callbacks to contentsDeferred

contentsDeferred.then(function(data) { console.log(data) })

This is not a flattering example. In the simplest of cases Deferred is more verbose, but it more complex scenarios it's easier to reason about what's going on.

Waiting on many asynchronous data sources

You can wait for all the values in a list of Deferred objects, or start a bunch of Deferred operations and run the callback chain when the first one has succeeded or failed.

// Note: dfs.requestReadFile is defined above

var dPasswd = dfs.requestReadFile('/etc/passwd')
  , dShadow = dfs.requestReadFile('/etc/shadow')
  , dGroup = dfs.requestReadFile('/etc/group')

// convert ['a', 'b', 'c'] to 'abc'
function join(things) {
    return things.reduce(function(m, t) { return m + t })

// after all values are received, deferred.all runs the callback chain
deferred.all([dPasswd, dShadow, dGroup]).then(join).thenCall(console.log)

// or, after the first value is received, deferred.all runs the callback chain
deferred.all([dPasswd, dShadow, dGroup], {fireOnFirstResult: true}).thenCall(console.log)

// or, if any error occurs, deferred.all runs the errback chain
var dAll = deferred.all([dPasswd, dShadow, dGroup], {fireOnFirstError: true})

// callbacks and errbacks are supposed to return a value
// the value is the input to the next link in the chain
// thenCall and failCall are conveniences that call the
// supplied function with the input and then return the input
// this way you don't have to wrap functions that don't return
// a value

Composing Deferred objects arbitrarily

The return value of a callback is passed to the next callback. When a callback returns a Deferred object, the original Deferred will transparently wait for the other to receive its value and then run its own callback chain using that value. We call this nesting.

var d = dfs.readFile('/etc/passwd').then(function(passwdData) {
    return dfs.readFile('/etc/group').then(function(groupData) {
        return passwdData + groupData
}).then(function(data) {
    return data

Now you can do something with d, return it, pass it to another function, etc. Subsequent callbacks registered on d will receive the value returned from the innermost callback: passwdData + groupData.

Deferred shines when you have higher level constructs built on top of it, such as work queues and data sources for databases and filesystem access. We have more stuff coming out for Node.js soon!


How to contribute





Copyright 2011 HeavyLifters Network Ltd. Licensed under the terms of the MIT license. See included LICENSE file.


Deferred implementation for JavaScript




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