prototype behavior in javascript

I am confused with the following prototype behavior in javascript.

function A(){
};

A.prototype.toString = function(){
            console.log('first');
               }

var a = new A(), b;

A.prototype = {
    toString:function(){
        console.log('second');
    }
}

b = new A();

a.toString();
//outputs :  first

b.toString();
//outputs :  second

Why does a.toString still prints "frist" when compared to b.toString which prints "second". can anyone please explain what i am missing here.

The prototype link has nothing to do with the constructor that constructed the object, it is stored on objects themselves.

When you call new A() this happens:

var a = {};
a.__proto__ = A.prototype;
A.call(a);

Note that the above is not standard syntax but does work in chrome and firefox.

So when you overwrite A.prototype, a.__proto__ is still linking to the old A.prototype as you would expect with similar code:

var A = 10, a, b;

a = A;
A = 7; //a is still 10
b = A; 

I don't recommend reassigning the prototype, because then you need to re-establish the constructor property and it requires additional level of indentation.

If you want to type less, just store reference to the prototype:

function A() {

}
var fn = A.prototype;

fn.toString = function() {

};

fn.valueOf = function() {

};

fn.toJSON = function() {

};