KVO and Binding for JavaScript

kvo.js

function initWatchProperty (obj, key) {
  if (obj['$$watchers$$' + key])
    return;

  Object.defineProperty(obj, key, {
    get: function () {
      return this['$$' + key];
    },
    set: function (val) {
      var old = this['$$' + key];
      this['$$' + key] = val;

      this['$$watchers$$' + key].forEach(function (f) {
        f(val, old);
      });
    }
  });
  obj['$$watchers$$' + key] = [];
}

function addListener (obj, key, cb) {
  obj['$$watchers$$' + key].push(cb);
}

function bind (from, fromKey, to, toKey) {
  initWatchProperty(from, fromKey);
  addListener(from, fromKey, function (val) {
    to[toKey] = val;
  });
}

var a = {};
a.abc = 30;
bind(a, 'abc', a, 'bcd');
bind(a, 'abc', a, 'cde');
a.abc = 40;