Whats the advantage of using a constructor function in a JavaScript object?

Question

What's the advantage of using a constructor function like so:

var MyLibrary = new function(){
    var self = this;
    self.MyLibrary = function(){
         // init code
    }

}

Instead of simply writing code inside the object?

var MyLibrary = new function(){
    // init code
}

Answer 1

Well, if you're using prototype inheritance to create new classes, you'll do something like this:

function MyBaseClass() {
    // common stuff here
}

function MySubClass() {
    // subclass-specific stuff here
}

MySubClass.prototype = new MyBaseClass();

That last line is required to establish the inheritance chain. However, it also has the side-effect of executing the body of MyBaseClass, which might cause problems (particularly if the MyBaseClass function is expecting arguments).

If you don't want that to happen, do something like this:

function MyBaseClass() {
    this.init = function() {
        // initialisation stuff here
    }
    // common stuff here
}

function MySubClass() {
    // subclass-specific stuff here
    this.init();
}

MySubClass.prototype = new MyBaseClass();

The initialisation code in init is now only executed when you create an instance of MySubClass.

Answer 2

Neither of those are quite right, although the second one might work, but isn't really an object, more like a singleton(but in a weird way). Here's an example of a class with a constructor:

// Class & Constructor definition
function Rectangle(w,h) {
    this.width = w;
    this.height = h;
}

// Now your class methods go on the prototype object
Rectangle.prototype.area = function() {
    return this.width * this.height;
}

Now to use this class:

var myRect = new Rectangle(3,4);
myRect.area();

You can also define a class by saving the 'constructor' to a var using anonymous functions instead of named functions:

// Class & Constructor definition
var Rectangle = function(w,h) {
    this.width = w;
    this.height = h;
}

// Now your class methods go on the prototype object
Rectangle.prototype.area = function() {
    return this.width * this.height;
}