dean.edwards.name/weblog/2008/01/improvements/

A while back I published my Base class for JavaScript inheritance. There have been a few more solutions to inheritance since then and most of the major JavaScript libraries contain an implementation of some kind.

The implementation of Base has changed very little in base2 but there are some improvements which I will briefly outline.

• All classes now have a static ancestorOf() method.
• Inheritance is now more rugged. If you attempt to extend an object with a method that is already in the inheritance chain then that method will not be added. This avoids circular references.
• Inheritance between prototype objects is now late bound. That means you can extend a prototype and all of its ancestors will inherit the new method even if it has been overridden later.
• All classes can be used to cast JavaScript objects. The instance properties and methods of the class are added to the target object.
• Class definitions now support dynamic object detection (explained below).

Dynamic Object Detection

base2’s detect() method can be used to detect browser features or for straightforward browser sniffing. A special syntax can be used when extending objects that incorporates the detect() method. If a property name starts with “@” then the remainder of the name is passed to detect(). If the detect() method returns true then the associated interface is applied. Reading that back it sounds horribly complicated. Actually it isn’t and hopefully this simple example will illustrate the idea:

var Browser = Base.extend({
  // Default method:
  info: function() {
    print("I am a happy browser!");
  },

  // User agent detection:
  "@MSIE": {
    info: function() {
      print("I am a sad browser!");
    }
  },

  // Object detection:
  "@!(document.getElementById)": {
    info: function() {
      print("I am a very old browser!");
    }
  }
});

var browser = new Browser();
browser.info(); // => (platform specific)

If you like these changes and would like me to update the original Base class then I’m prepared to do a little bit of work…