var Animal = (function(){
  var Animal = function() {
    
  };
  
  Animal.prototype.doSomething = function() {
    console.log('do something');
  };
  
  return Animal;
})();

var Cat = (function(Animal){
  var Cat = function() {
  
  };
  
  var parent = Animal;
  Cat.prototype = new parent();
  Cat.prototype.constructor = Cat;
  
  Cat.prototype.doSomething = function() {
    parent.prototype.doSomething.call(this);
    console.log('do something more');
  };
  
  return Cat;
})(Animal);

var cat = new Cat();
cat.doSomething();