Use of 'prototype' vs. 'this' in J

Question

What's the difference between

var A = function () {
    this.x = function () {
        //do something
    };
};

and

var A = function () { };
A.prototype.x = function () {
    //do something
};

Answer 1

Prototype is the template of the class; which applies to all future instances of it. Whereas this is the particular instance of the object.

Answer 2

As discussed in other answers, it's really a performance consideration because the function in the prototype is shared with all of the instantiations - rather than the function being created for each instantiation.

I put together a jsperf to show this. There is a dramatic difference in the time it takes to instantiate the class, although it is really only relevant if you are making many instances.

http://jsperf.com/functions-in-constructor-vs-prototype