Basic Inheritance in JavaScript

//
// Function.extend( base )
// sets up inheritance between two constructor functions
// without needing to instantiate the base constructor
//
Function.prototype.extend = function( base )
{
  // Define a surrogate constructor for base
  var Temp = function(){}
  Temp.prototype = base.prototype

  // instantiate the surrogate
  var t = new Temp()
  t.constructor = this

  // copy subclass prototype into surrogate object
  for( key in this.prototype )
    t[key] = this.prototype[key]

  // set subclass prototype as the surrogate object
  this.prototype = t
}

Example Usage

// Create a superclass called Animal
// Give its prototype a function called speak
function Animal(){}
Animal.prototype = {
  sound : "",
  speak : function(){ return this.sound; }
}

// Create animal types
function Cat(){ this.sound = "Meow" }
function Dog(){ this.sound = "Woof" }

// make Cat and Dog inherit from Animal
Cat.extend( Animal )
Dog.extend( Animal )

Outcome

// Create some instances
var cat = new Cat()
var dog = new Dog()

// Test out the inheritance
cat.speak() // Meow
dog.speak() // Woof
  
dog instanceof Animal // true
cat instanceof Animal // true
dog instanceof Cat    // false