wrapping a method of a prototype Object in javascript e coffeescript

wrap.coffee

class A
  say_something: -> alert "hello"

class B extends A

#create new instance of B and call say_something
new B().say_something()


#wrapping method say_something of prototype A
A::say_something= do ->
  fn=A::say_something
  return ->
    alert "before say"
    fn.apply @
    alert "after say"


#create new instance of B and call say_something wrapped
new B().say_something()

wrap.js

var A, B;
var __hasProp = Object.prototype.hasOwnProperty, __extends = function(child, parent) {
  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;
};
A = (function() {
  function A() {}
  A.prototype.say_something = function() {
    return alert("hello");
  };
  return A;
})();
B = (function() {
  __extends(B, A);
  function B() {
    B.__super__.constructor.apply(this, arguments);
  }
  return B;
})();
new B().say_something();
A.prototype.say_something = (function() {
  var fn;
  fn = A.prototype.say_something;
  return function() {
    alert("before say");
    fn.apply(this);
    return alert("after say");
  };
})();
new B().say_something();