Accessing Overridden Methods

Sometimes we want to augment a method of a base class rather than replace it. This is easily accomplished in Java using the super keyword. For example, if we have a Button widget with a draw method, and we extend that creating a BorderButton widget, we might want to use Button's draw method to draw the button and then draw a border around it. To do this, BorderButton.draw could call super.draw() and the compiler would call the draw method of the closest ancestor which in this case would be Button.draw.

JavaScript does not have a super keyword, but we can use features of the language to accomplish the same goal. Each function is passed an object called arguments which includes all arguments passed to this function. It also includes the function itself as the callee. Since a function is an object in JavaScript, we can attach useful information to it such as a reference to a function we are overriding. To take advantage of this information, we will add a method called inherited to the prototype of XObject.

01: inherited: function(args) {
02:     var f = args.callee._inherited;
03:     if ( f ) {
04:         f.apply(this,args);
05:     }
06:     else {
07:         console.error(this.className +
08:                       " does not inherit the method " + args.callee._name);
09:     }
10: }

Next we will see how the _inherited and _name properties are attached to methods.