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What is and how it works RequireJS

By Eduardo García ● CTO | June 1st, 2014

RequireJS is a JavaScript library that allows us to isolate the components of our JS application as modules and resolve their dependencies.

RequireJS implements the design pattern of software AMD (Asynchronous Module Definition) which you could say is very similar to Dependency injection implemented by frameworks like Symfony.

Let’s check how we can use in our JS application in simple steps.

  1. Define entry point for RequireJS.

We must download the latest version of RequireJS library available on the website http://requirejs.org, after download will put in a directory called libs just for purposes of this example (tested with version 2.1.11).

Now we need to define an HTML file and could be named index.html where RequireJS is loading as shown below.

<html>
<head>
</head>
<body>
  <script data-main="libs/main" src="libs/require.js"></script>
</body>
</html>

As you can see the inclusión is done with the script tag at the end of the HTML tag body, but it could also be done within the head tag.

It’s also possible to include RequireJS remotely using the URL http://requirejs.org/docs/release/2.1.11/minified/require.js or similar, but it is a desition of the reader.

As can be seen RequireJS use the meta-data property data-main(, with this property, we can define the entry point for RequireJS configuration, which is a relative path for the file we choose to load the RequireJS configuration (is necessary exclude the js extension).

  1. Configure RequireJS

In the file libs/main.js we define the RequireJS options. The first thing we should define is which JS libraries will use in our application as shown below.

require.config({
    baseUrl: 'libs',
    paths : {
      backbone : 'backbone',
      underscore : 'underscore',
      jquery : 'jquery',
      marionette : 'backbone.marionette',
      wreqr : 'backbone.wreqr',
      eventbinder : 'backbone.eventbinder',
      babysitter : 'backbone.babysitter'
  }
});

All files are sought in the directory defined using the baseUrl property, and the paths property allow define the location of the libraries required with a key to refer to the library later. The js extension is not required and will be added by RequireJS later.

As its name suggests the pattern AMD makes an asynchronous load of packages. But because not all the libraries implement the AMD pattern, is likely to a library A try to use a function defined in library B not yet loaded causing a fatal error.

To resolve this issue we use the property Shim part of RequireJS configuration, as we see below.

require.config({,
  shim : {
    jquery : {
      exports : 'jQuery'
    },
    underscore : {
      exports : '_'
    },
    backbone : {
      deps : ['jquery', 'underscore'],
      exports : 'Backbone'
    },
    wreqr: {
      deps : ['backbone']
    },
    eventbinder : {
      deps : ['backbone']
    },
    babysitter : {
      deps: ['backbone']
    },
    marionette : {
      deps: ['wreqr', 'eventbinder', 'babysitter'],
      exports : 'Marionette'
    }
  }
});

Using Shim we can define dependencies between libraries and allow to export global variables that can be used within other libraries such as jQuery library which export the variable jQuery.

The configuration with the Shim is a replacement for Order plugin used before version 2.0 of RequireJS.

The exports variable can also be an anonymous function to return the call noConflict function if the library supports it.

Using the deps property, you can define multiple dependencies of libraries should be loaded before including the library that is defining, such as backbone that depends on jquery and underscore to be used.

Of course, if we want to use the library with RequireJS and supports AMD configuration pattern Shim isn’t required.

  1. How to execute code required.

Once defined libraries and its dependencies if necessary, we can now rewrite the code we need as shown below.

// Loading dependences and execute Marionette App
require( ["marionette"], function (Marionette) {
    // set up the app instance
    var MyApp = new Marionette.Application();

    MyApp.on("initialize:after", function(){
      alert("Application has started!");
    });

    MyApp.start();
});

The above code checks that all the libraries needed to run Marionette are loaded before executing the code and get the exported global variable as a parameter.

In the anonymous function will create a Marionette Application and start it, As a result, we will get an alert with the message “Application has started”.

The require function has the same effect of an anonymous function, meaning that run without problems but can not be used later.

  1. How to create named modules.

If we want encapsulate our App we must create named modules using the function define; The difference with require function is an extra parameter that is used to name the module and returns an object or variable that will be used when the module is instantiated.

Then consider the previous example as a named module.

define( 'MyApp', ["marionette"], function (Marionette) {

    // set up the app instance
    var MyApp = new Marionette.Application();

    MyApp.on("initialize:after", function(){
      alert("Application has started!");
    });

    // export the app from this module
    return MyApp;
});

// Fetch and execute Marionette App
require( ["MyApp"], function (MyApp) {
    // Execute App
    MyApp.start();
});

As you see we register a named module MyApp and returns an object defined in Marionette.

Then with a require function, the module is loaded, it receives as parameters the object returns by the module and can thus start our application.

It might seem a waste of time doing it this way as it is in the same file, but the idea is to create a module for file and using the configuration of RequireJS load all the libraries that define modules and then use these modules in our application.

You can download the Marionette App Skeleton repository which implements the concepts discussed here.

I hope you have been to his liking.