function Animal() {

  // animal stuff
}
 
// All of Animal's methods will go into its prototype
Animal.prototype = {
  saySomething: function() {

    return "I am an animal!";
  },
 
  copy: function() {

    var newcopy;
    if (this.subConstructor) {

      newcopy = new this.subConstructor;
    } else {

      newcopy = new Animal();
    }
 
    // maybe copy over other values

 
    return newcopy;
  }
};

function Cat() {
  this.constructor(); // call animal's constructor

  // cat stuff
}
 
// Save the cat's constructor, replace Cat's prototype and constructor
// Then restore cat's constructor as subConstructor
var con = Cat.prototype.constructor;

Cat.prototype = new Animal();
Cat.prototype.constructor = Animal;

Cat.prototype.subConstructor = con
 
// override the saySomething function
Cat.prototype.saySomething = function() {

  return "Meow!";
};

// some tests and their results in the comment
 
var c = new Cat();

console.log(c instanceof Animal);   // true
console.log(c instanceof Cat);      // true

console.log(c.saySomething());      // "Meow!"
 
var a = new Animal();

console.log(a instanceof Animal);   // true
console.log(a instanceof Cat);      // false

console.log(a.saySomething());      // "I am an animal!"
 
var copyCat = c.copy();

console.log(copyCat instanceof Animal); // true
console.log(copyCat instanceof Cat);    // true

console.log(copyCat.saySomething());    // "Meow!"
 
var copyAnimal = a.copy();

console.log(copyAnimal instanceof Animal); // true
console.log(copyAnimal instanceof Cat);    // false

console.log(copyAnimal.saySomething());    // "I am an animal!"

function derive(base, sub) {

  var con = sub.prototype.constructor;
  sub.prototype = new base();

  sub.prototype.constructor = base;
  sub.prototype.subConstructor = con

}

function Cat() {
  this.constructor(); // call animal's constructor

  // cat stuff
}
 
derive(Animal, Cat);
 
// override the saySomething function

Cat.prototype.saySomething = function() {
  return "Meow!";

};