if (document.getElementById('hello')) {

  document.getElementById('hello').innerHTML = 'Hello World - this was inserted using JavaScript';

var A = function () {

    this.x = function () {

    alert('A');

    };

};

​

var B = function () { };

B.prototype.x = function () {

    alert('B');

};

​

var a = new A();

var b = new B();

a.x(); // displays A

b.x(); // displays B

​

A.x = function() {alert('C');};

a.x(); // displays A, but new instances of A will display C when x is called

var a2 = new A();

a2.x(); //displays A still

A.x(); //displays C when called as a singleton

B.prototype.x = function () {alert('D');};

b.x(); // displays D, because by changing prototype.x we have changed it for all

       // instances

b2 = new B();

b2.x();

}

​