No description, website, or topics provided.
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.




  • Asynchronous
  • Module
  • Definition

Asynchronous module definition (AMD) is a JavaScript API for defining modules such that the module and its dependencies can be asynchronously loaded.


Asynchronous Module Definition (AMD) is the most widely supported JavaScript module format. It's used by cujo.js, jQuery, dojo, Mootools, and several dozens of other libraries and frameworks. AMD is specifically designed for browser environments, but you can also use it in non-browser environments.

(Authoring AMD modules)

Is AMD worth it?

  • There are many different ways for organizing and loading client-side JavaScript.
  • The AMD approach is only one among many.
  • Read: CommonJS Is The Future... and Why Use Make
  • Rails, in its Asset Pipeline, uses a different approach.

Why do I not like the asset pipeline - 1

It is not truly modular and there is no built-in way of protecting variables from the global name-space.

Does this really matter?


One could correctly argue that there is no big chance of variable names such as backbone or jquery conflicting,

  • but things are more subtle than how they appear...
  • ... have a look at these two examples from our code-base:


//= require_tree .

var roundToDecimals = function(number, places) {
    // more code

var addCommasToNumber = function(number) {
    // more code

( mypolygon-v2 - application.js)

These are global variables, with very generic names.

Still not a big issue? Maybe, but as an application grows, chances of conflicts grow too... and there is something more...

This way of organizing code does not help to keep things tidy... and maintainable. Every single piece of functionality should have its right place inside an application. roundToDecimals, for example, could live in a reusable and generic numbers module. This can be achieved within the Asset Pipeline too, but an AMD approach makes it more obvious.


In our code every coffeScript file is compiled with a top-level function wrapper, that protects the global namespace, but then, in order to escape this wrapper, modules start with something like:

  window.Pica ||= {}

This is not elegant. Why?

  • One should not use window to pass around globals in the first place.
  • One should not be concerned about the existence of window.Pica (why || ?).
  • One is not taking advantage of new ECMASript 5 features such as object.freeze
  • In general, it is a matter of personal taste.

Why do I not like the asset pipeline - 2

In the Asset Pipeline, the order of the files required in a manifest matters. And, unsurprisingly, one discovers quite soon that "//= require_tree ." is a bad practice.

Order also matters for the script tags in an HTML file, so what is the big deal here?

  • JavaScript started off as a small scripting language.
  • But nowadays it's huge and with so many files (modules) around the place, one should not be concerned about the loading order. Less stuff to remember the better!

Why do I not like the asset pipeline - 3

The Asset Pipeline does not make it easy to split the css and JavaSript across different pages of a Web application.

Nevertheless the Asset Pipeline has one big advantage: if using Rails its built for Rails.

How do AMD modules work instead?

There are different implementations of the AMD API:

From the excellently explained Authoring AMD modules:

define(dependencyIds, factoryFunction);

As you can see from the first parameter, dependencyIds, you can pass an array of ids into define. These are the ids of other modules that your module requires to do its work. The second parameter, factoryFunction, is a function that creates your module and will be run exactly once. The factory is called with the dependent modules as parameters. Furthermore, it is guaranteed to run only after all of the dependencies are known to be available. In practice, the factory typically runs just before it's needed.

// module app/mime-client
define(['rest', 'rest/interceptor/mime'], function (rest, mime) {
    var client;

    client = rest.chain(mime);

    return client;

A simple configuration file example from a requirejs implementation

  # The path where your JavaScripts are located
  baseUrl: "static/js/"

  # Specify the paths of vendor libraries
    jquery: "vendor/jquery-1.8.3.min"
    jquery_ui: "vendor/jquery-ui-1.10.0.custom"
    laconic: "vendor/laconic"
    bootstrap: "vendor/bootstrap"
    underscore: "vendor/miso-0.4.0/lib/lodash"
    backbone: "vendor/backbone"
    text: "vendor/text-2.0.3"
    d3: "vendor/d3"

  # Not AMD-capables per default,
  # so we need to use the **AMD** wrapping of RequireJS.
      deps: ["jquery"]
      exports: "jQuery"

      exports: "laconic"

      exports: "bootstrap"

      exports: "_"

      deps: ["jquery", "underscore"]
      exports: "Backbone"

      exports: "d3"

  # Start loading the main app file. Put all of
  # your application logic in there.
  requirejs ['main']

Note: non compatible JavaScript libraries are declared in the shim section.

The fact that AMD modules get loaded asynchronously does not mean that modules can not be optimized.

Usually for production we pass the code through an optimizer, such as:

From the docs:

cram.js concatenates the modules of your application into larger bundles that may be loaded much more efficiently than when loaded as separate modules. cram.js can create AMD- compatible bundles that may be loaded by an AMD loader, such as curl.js or bundles with an integrated loader.

An optimizer needs some configuration too. A simple example from a requirejs implementation:

    mainConfigFile: '../js/app.js',
    baseUrl: '../js', 
    name: 'vendor/almond',
    include: [
        // All other app dependencies 
        // are traced down from the following 2:
    insertRequire: ['main']

Note: the reference to the mainConfigFile and how the order of the elements in the include array is irrelevant.

What are the main advantages of this kind of approach?


The global name-space does not get polluted.


The order in which JavaScript modules are loaded does not matter.


For big JavaScript applications one can optimize the payload and memory management by selectively loading different AMD module bundles for different parts of the app.


For small JavaScript applications or more traditional JavaScript enhanced Websites one can optimize the code by minifying all of it in a single file.


It's the future. Harmony will have modules. Not AMD modules, but one might as well get used to the concept.


It is a good way for approaching programs. It helps to write cleaner, more flexible, testable and better organized code.

From the words of John Hann:

(AMD) is the simplest format I've seen that doesn't require a build step. It was designed for browsers. You can get started with AMD by just downloading an AMD loader and writing some code. Hit F5 or Cmd-R and see your first module load.

(interview link)

AMD and the Asset Pipeline

In the end it all boils down to JavaScript, so you could use AMD modules in the Asset Pipeline by just writing, in application.js, something like:

//= require require

... and take it from there

Or alternatively, use a gem such us requirejs-rails.

Would this approach work well within the wider Rails environment, at all levels, client side testing included?

  • I do not know yet.

References and further reading