CODE EXAMPLE FOR JAVASCRIPT

varibale as listeneres javascirpit

x = {
  aInternal: 10,
  aListener: function(val) {},
  set a(val) {
    this.aInternal = val;
    this.aListener(val);
  },
  get a() {
    return this.aInternal;
  },
  registerListener: function(listener) {
    this.aListener = listener;
  }
}

Then you can register a listener using the following:

x.registerListener(function(val) {
  alert("Someone changed the value of x.a to " + val);
});