Q: Javascript Modular Layout : How to call a function defined in one module from another? Below is an example of a modular layout of a javascript application. I want to start using such a structure for my work. I am struggling to get my head round how it works and need to understand how to call a function that is defined in one module from a different module? Is this definitely the bet way to layout a JavaScript heavy application?
window.MainModule = (function($, win, doc, undefined) {
    var modules = {};

    // -- Create as many modules as you need ...
    modules["alerter"] = (function(){
        var someFunction = function(){ alert('I alert first'); };

        return { 
            init: someFunction
        };
    }());

    modules["alerter2"] = (function(){
        var someFunction = function(){ alert('I alert second'); };

        return { 
            init: someFunction
        };
    }());

    return { 
        init: function(){
            for (var key in modules){
                modules[key].init();
            }
        }
    };
}(jQuery, this, document));

$(window.MainModule.init);


A: Modularity with RequireJS:
module1.js
define( ["dependency"] , function( dep ){

   return {
      speak: function(){
          alert( dep.message );
      }
   }

} );

dependency.js
define( {
    message: "Hello world!"
} );

impl.js
if ( $(".someCssExpression").length ) {
    require( [ "module1" ] , function( mod ){

        mod.speak();

    });
}

index.html
...

<script src="require.js" data-main="impl"></script>

...

Your file structure will be modular.
Your implementation will be modular.
And no clunky namespacing or weird constructs to make it feel organised.
Takes some getting used to, but totally worth it.
Also read:


*

*ScriptJunkie article

A: In order to access anything it needs to be available in the scope from where you are calling. "Module" - or any capsulation method for that matter - in JS always means "function". A module is just an anonymous (unnamed) function. So to access an element defined in another function B(module) from inside function A it either has to be made available in GLOBAL SCOPE (in browsers: the window object), OR it must have obtained access some other way, e.g. by having received a reference through some function call. YUI3 ([http://developer.yahoo.com/yui/3/]) is an interesting example for the latter, there nothing of your application ever is available in global scope (I consider YUI3 one of the by far best JS frameworks for SERIOUS softwarfe development, also definitely DO check out http://developer.yahoo.com/yui/theater/, especially any videos from Douglas Crockford, a Javascript God (and I'm not usually given to giving these kinds of statements).
What you have to keep in mind with Javascript is that part of what in languages such as C is done by the compiler now happens at runtime. For such things like immediate function invocations that return a function (causing encapsulation through usage of closures) you should remember that that code runs exactly ONCE DURING LOADING, but during runtime completely different code is executed - which depends on what the on-load code execution did.
In your example the function after window.MainModule=... is executed on loading of the JS code. Note that window.MainModule does NOT POINT TO THAT FUNCTION!!!
Instead, that function is, as I said, executed on load, and the RESULT is assigned to window.MainModule. What is the result? Well, there is just one return statement, and it returns and object, and the object has just one property "init" which points to an anonymous function.
Before returning that object, though, that function creates a variable "modules" in its local scope, which points to an object. The object has two properties, and those properties are assigned functions the same way that window.MainModule is assigned one, so you have three closures all in all.
So after loading you have one global variable
window.MainModule = {
  init: function(){...}
So after loading you have one global variable
window.MainModule = {
  init: function(){...}
}
In the last line that function is executed.
}
In the last line that function is executed.
The example does not make a lot of sense though, because you don't really encapsulate anything. You make available the private function with double pointers: from init to the local variable someFunction, nothing is hidden. Check out the above URLs (Yahoo Developer Theater) for better examples and very thorough explanations. It is MUST-WATCH even if you never touch YUI3 - especially the videos from D. Crockford are general JS knowhow.

A: Well umm..
It all depands on what you need from your application.
I would suggest sticking to the jQuery plugins for as long as you touch the gui.
You could use the Namespace pattern like yahoo, and just forge a great framework for your 
application.
You probably don't need your modules to run on every page, like you did when instantiating the main module (There is no point to it unless f.e. you have a widget in every page on your website).
After you finish to abstract all the actions you need from your javascript into function and modules, build a module that will load the logic according to each page/action/whatever whenever you need it.
By the way, You can always develop OO style using mootools and this is endless really. It all boils down to your application needs
I would really recommend you to watch some lectures of Douglas Crockford(As been stated before me) and here is a nice article about modules that may help you understand it a bit further. Good luck!
