myObject.prototype in ECMAScript 262

This property is read-only for intrinsic JS classes (e.g. the Date object) but may be re-assigned for user-defined classes.

While the prototype property is 'read-only' for intrinsic classes, it may be used to extend their functionality. For example the following code uses the prototype property of the Number object to extend all numbers to support a custom 'truncate' method:

Number.prototype.trunctate=function(){
	return this>0?Math.floor(this):Math.ceil(this);
}
var x=12.37;
var y=-12.37;

x = x.truncate(); // ** 12
y = y.truncate(); // ** -12

The following code sample shows how to use the prototype property to cause one class to inherit from another:

function Mammal(name){ 
        this.name=name;
        this.offspring=[];
} 
Mammal.prototype.haveABaby=function(){ 
        var newBaby=new this.constructor("Baby "+this.name); //Uses Cat for cats, Mammal for Mammals
        this.offspring.push(newBaby);
        return newBaby;
} 
Mammal.prototype.toString=function(){ 
        return '[Mammal "'+this.name+'"]';
} 


Cat.prototype = new Mammal;          // Here's where the inheritance occurs 
Cat.prototype.constructor=Cat;       // Otherwise instances of Cat would have a constructor of Mammal 
function Cat(name){ 
        this.name=name;
} 
Cat.prototype.toString=function(){ 
        return '[Cat "'+this.name+'"]';
} 

var msg;

var someAnimal = new Mammal('Mr. Biggles');
msg = 'someAnimal is '+someAnimal;         // ** 'someAnimal is [Mammal "Mr. Biggles"]'

var myPet = new Cat('Felix');
msg = 'myPet is '+myPet;                   // ** 'myPet is [Cat "Felix"]' 

myPet.haveABaby();                         // calls a method inherited from Mammal 
msg = myPet.offspring.length;              // ** 1
msg = "First baby is "+myPet.offspring[0]; // ** 'First baby is [Cat "Baby Felix"]'