Model.js

function Model(object) {
  this.initialize(object);
}

Model.prototype.initialize = function (object) {
  this.listeners = {};
  this.listeners._global = [];
  for (var key in object) {
    if (object.hasOwnProperty(key)) {
      this.set(key, object[key]);
    }
  }
};

Model.prototype.set = function (key, value) {
  var old_value = this.key;
  this[key] = value;
  if (value !== old_value) {
    this._broadcast(key);
  }
  return this;
};

Model.prototype.on_change = function (key, callback) {
  if (typeof key === 'function') {
    this.listeners._global.push(key);
    return;
  }

  if (typeof this.listeners[key] === 'undefined') {
    this.listeners[key] = [];
  }
  this.listeners[key].push(callback);

  return this;
};

Model.prototype._broadcast = function (key) {
  var x;
  if (typeof this.listeners[key] !== 'undefined') {
    for (x = 0; x < this.listeners[key].length; x = x + 1) {
      this.listeners[key][x].apply(this);
    }
  }
  for (x = 0; x < this.listeners._global.length; x = x + 1) {
    this.listeners._global[x].apply(this);
  }
};

Model.prototype.extend = function () {
  var _this = this;
  function fn() {
    _this.initialize.apply(this, arguments);
  }
  fn.prototype = Model.prototype;
  return fn;
};