Class inheritance in Javascript

inheritance.js

var __hasProp = Object.prototype.hasOwnProperty;
var __extends = function(child, parent) {
  // copy all properties from parent to child
  for (var key in parent) {
    if (__hasProp.call(parent, key))
      child[key] = parent[key];
  }
  function ctor() {
    this.constructor = child;
  }
  ctor.prototype = parent.prototype;
  child.prototype = new ctor;
  child.__super__ = parent.prototype;
  return child;
};

// Base class

var Animal = (function() {

  Animal.prototype.speak = function() {
    console.log("Hi, I'm a " + this.name);
  };

  function Animal(name) {
    this.name = name;
    this.speak();
  }

  return Animal;
})();

// Subclass of Animal

var Dog = (function() {
  __extends(Dog, Animal);

  // Override the speak method of the base class
  Dog.prototype.speak = function() {
    console.log("I'm a dog. I can't speak.");
  };

  function Dog(name) {
    Dog.__super__.constructor.apply(this, arguments);
  }
  return Dog;
})();

var dog = new Dog('dog');