Mastering JavaScript Design Patterns

Observer

The observer pattern is perhaps the most used pattern in the JavaScript world. The pattern is used especially with modern single page applications; it is a big part of the various libraries that provide the Model-View-ViewModel (MVVM) functionality. We'll explore those patterns in some detail in Chapter 7, Model View Patterns.

It is frequently useful to know when the value of an object has changed. In order to do so, you could wrap up the property of interest with a getter and a setter:

var GetterSetter = (function () {
  function GetterSetter() {
  }
  GetterSetter.prototype.GetProperty = function () {
    return this._property;
  };
  GetterSetter.prototype.SetProperty = function (value) {
    this._property = value;
  };
  return GetterSetter;
})();

The setter ...