Difference between a constructor and an Object

Question

I definitely need some light on this.

What\'s the diference between:

var MY_APP = function(){
    this.firstMethod = function(){
       //something
          


        

Answer 1

The first is a function, the second is an object literal. Since Functions in JS are first class objects, a function can have properties on it, just like any other object can.

Typically, if you want to create a "class" that you might be familiar with from classical inheritance languages, you would do something like

function MyClass() {...}

as is documented here http://www.crockford.com/javascript/inheritance.html

To answer the question posed in your edits, you would use them both in different situations. Object literals are used to pass configurations around. A typical usage pattern would be a method that accepts an object literal like so

 something.init({
      length: 10,
      height: 10,
      text: 'some text'
 });

and so on.

You could use something similar to your first example when creating a namespace. Javascript has some interesting language features in that you can have so-called "self-invoking functions" that are of the form:

var myApp = (function(){
    var firstMethod = function() {...}
    ...
})();

the motivations behind doing something like this are detailed here
http://sparecycles.wordpress.com/2008/06/29/advanced-javascript/

You can also investigate the differences via your favorite javascript debugging console. In firebug and chrome, I did the following:

var ol = {}; ol.prototype;

var fn = function(){}; fn.prototype;

the first line prints undefined, the second returns a prototype with a constructor of 'function'