JS: Object.prototype.extend()

mixin.js

var sys = require("sys");

var extend = function extend (another) {
  var prototype = this.__proto__;
  this.__proto__ = another;
  another.__proto__ = prototype;
  return this;
};

Object.defineProperty(Object.prototype, "extend", {value: extend });

var Model = new Function();
Model.prototype = {
  saySomething: function() {
    sys.puts("Yes, I am a model!");
  }
}

var NotModel = new Function();
NotModel.prototype = {
  saySomething: function() {
    sys.puts("No, I am not a model!");
  }
}

var a = new Model();
var b = new NotModel();

sys.puts("Before extending:");
sys.print("a: "); a.saySomething(); // Yes, I am a model!
sys.print("b: "); b.saySomething(); // No, I am not a model!

var SomeMixin = {};

a.__proto__.extend(SomeMixin);
b.__proto__.extend(SomeMixin);

sys.puts("After extending:");
sys.print("a: "); a.saySomething(); // No, I am not a model!
sys.print("b: "); b.saySomething(); // No, I am not a model!