prototypal.js

//Extend function
Function.prototype.extend = function(parent) {
  this.prototype = Object.create(parent.prototype);
	this.prototype.constructor = this;
	return this;
};

//Base Class
var BaseClass = function() {};
BaseClass.prototype.sharedMethod = function() {
    console.log("This method is shared");   
};
BaseClass.prototype.baseMethod = function() {
    console.log("I am the base method.");   
};

//Child Class
var ChildClass = function() {};
ChildClass.extend(BaseClass);
ChildClass.prototype.baseMethod = function() {
     console.log("I am the overridden method.");   
};

//Implementation
var baseClass = new BaseClass();
var childClass = new ChildClass();
baseClass.sharedMethod(); //"This method is shared"
childClass.sharedMethod(); //"This method is shared"

baseClass.baseMethod(); //"I am the base method"
childClass.baseMethod(); //"I am the overridden method"