What's the difference between 2 JavaScript objects?

Question

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I try do improve my JavaScript skills. I don't understand why (5) works and (2) returns error. Isn't the same?

(1) - B.fn() //OK
(2) - B.fn2() //TypeError: Object # has no method 'fn2';
(3) - var a = new A()
(4) - a.fn() //OK
(5) - a.fn2() //OK

C#

var A = function () {
    this.fn = function () { alert(3); }
}
A.prototype = {
    fn2: function () { alert(4); }
};

var B =
    {
        fn: function () { alert(1); }
    }
B.prototype = {
    fn2: function () { alert(2); }
};

Posted 21-Mar-11 22:33pm

TheAteist

Updated 21-Mar-11 22:34pm

v2

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Jonathan Cardy · Accepted Answer · 2011-03-21T22:48:00

Your B object wasn't initialised using the new keyword. When you initialise an object using the new keyword, the functions on the prototype will be applied to the resulting object. The correct way to define a class in JavaScript is:

A = function() {
    //This is the constructor. 
};

A.prototype = {
    //This prototype object contains a definition of the functions on your class.
    fn: function() {
        alert(1);
    }
};

You then instantiate an object in this way;

var a = new A();
a.fn();

I recommend you read some of Douglas Crockford's tutorials[^].