Javascript overriding/adding methods to the prototype

Question

Basically, I want to know how/what's the difference between overriding/adding methods on instances of objects vs methods on the 'Class' instance of JS objects (yes class isn't the right word, not sure what is though).

For example, why doesn't this work?

Date.prototype.now= function (){
    return 23;
}

$('#foo').append(Date.now());    

And yet, this does?

Array.prototype.indexOf = function(){
    return 23;
}
var bar = Array();
bar[0] = "a";
$('#bar').append(bar.indexOf("a"));

http://jsfiddle.net/tad604/fMWKA/2/

Answer 1

now is a static method on Date. It is not an instance method; not defined on Date.prototype to begin with. It is defined directly on the Date object. When you write

Date.prototype.now = function () {
    return 23;
}

you are not actually overwriting the function called by Date.now().

---

On the other hand, indexOf for arrays is an instance method. That is, it's defined on Array.prototype, so when you write

Array.prototype.indexOf = function(){
    return 23;
};

you are in fact changing the indexOf function called by

var bar = [];
bar.indexOf('a');

---

See the difference? http://jsfiddle.net/mattball/7qVQy/

---

I recommend reading about the JavaScript prototype lookup chain, which should clarify some of your confusion.