JavaScript: Prototypal Inheritance, Extend

ctor-proto.js

// Also see http://michaux.ca/articles/class-based-inheritance-in-javascript
function extend(Super, Sub) {
  // By using a dummy constructor, initialization side-effects are eliminated.
  function Dummy() {}
  // Set dummy prototype to Super prototype.
  Dummy.prototype = Super.prototype;
  // Create Sub prototype as a new instance of dummy.
  Sub.prototype = new Dummy();
  // The .constructor propery is really the Super constructor.
  Sub.baseConstructor = Sub.prototype.constructor;
  // Update the .constructor property to point to the Sub constructor.
  Sub.prototype.constructor = Sub;
  // A convenient reference to the super constructor.
  Sub.superConstructor = Super;
  // A convenient reference to the super prototype.
  Sub.superPrototype = Super.prototype;
}

// THE BASE TYPE

function Mammal(name) {
  this.name = name;
  Mammal.population++;
}

Mammal.population = 0;

Mammal.prototype.toString = function() {
  return "Mammal " + this.name;
};

Mammal.prototype.giveBirth = function() {
  return new this.constructor("Baby " + this.name);
};

var cat = new Mammal("Cat");
console.log("Cat #1:     " + cat);
var kitten = cat.giveBirth();
console.log("Kitten #1:  " + kitten);
console.log("Population: " + Mammal.population);
console.log("-----------");

// ONE LEVEL OF INHERITANCE

function Cat(name) {
  Cat.superConstructor.call(this, name);
}
extend(Mammal, Cat);

Cat.prototype.toString = function() {
  return "Cat " + this.name;
};

Cat.prototype.meow = function() {
  return ">> MEOW <<";
};

Cat.prototype.giveBirth = function() {
  var kitten = Cat.superPrototype.giveBirth.call(this);
  console.log(kitten.meow());
  return kitten;
};

var cat = new Cat("Fluffy");
console.log("Cat #2:     " + cat);
var kitten = cat.giveBirth();
console.log("Kitten #2:  " + kitten);
console.log("Population: " + Mammal.population);
console.log("-----------");

// TWO LEVELS OF INHERITANCE

function ShowCat(name, breed) {
  ShowCat.superConstructor.call(this, name);
  this.breed = breed;
}
extend(Cat, ShowCat);

ShowCat.prototype.toString = function() {
  return "Fancy " + ShowCat.superPrototype.toString.call(this);;
};

ShowCat.prototype.meow = function() {
  return ShowCat.superPrototype.meow.call(this).replace(" ", " FANCY ");
};

var cat = new ShowCat("Fluffy");
console.log("Cat #3:     " + cat);
var kitten = cat.giveBirth();
console.log("Kitten #3:  " + kitten);
console.log("Population: " + Mammal.population);