JavaScript is a prototype-based OOP language in which behaviour reuse (inheritance) is achieved using existing objects which serve as prototypes. Whenever you assign a object as prototype of other object, properties and methods will be available to that object. You can override method or property in child object.

“Everything” in JavaScript is an Object. JavaScript allows you to define your own objects. Objects are just data, with added properties and methods. This looks something similar to ruby for me. Also root level prototype of a object is “Object” and prototype of “Object” is null.

In this type of OOP languages there is no existences of classes like java. c#, ruby etc. Here you create objects and inheritance you can achieve using prototype assignment. Since only one prototype you can assign to a object so Javascript dosen’t provide multiple inheritence. Using prototype assignment chain to nexted level, it will provide multi-level inheritence. eg

# A super class animal
function Animal(){
  this.legs = 0,
  this.word = '',
  this.walk = function(){
    var i = 0, s = '';
    for(; i < this.legs; i++){
      s += ' step '
    }
    console.log(s);
  },
  this.speak = function(){
    console.log(this.word);
  }
};

a = new Animal();
a.walk()               => 
a.speak()              => 

# A child class human
function Human(){
  this.legs = 2,
  this.word = 'Hi'
};

Human.prototype = new Animal();

h = new Human();
h.walk()                => step step 
h.speak()               => Hi