simple object model binding

dog.js

function Dog (name, age) {
  this._f = {};
  this._prop("name", name);
  this._prop("age", age);
}

Dog.prototype = {
  _prop: function (prop, initialValue) {
    var self = this;
    Object.defineProperty(this, prop, {
      get: function () {
        return initialValue;
      },
      set: function (v) {
        initialValue = v;
        self._dispatchChange(prop, v);
      }
    });
  },
  _dispatchChange: function (prop, s) {
    (this._f[prop]||[]).forEach(function (f) {
      f(s);
    });
  },
  onChange: function (prop, f) {
    this._f[prop] = (this._f[prop]||[]).concat([ f ]);
  },
  syncChange: function (prop, f) { // bind on change and call once with the current value
    this.onChange(prop, f);
    f(this[prop]);
  }
};

var dog = new Dog("max", 10);
dog.syncChange("age", function (age) {
  document.body.textContent = age+" years old";
});