In the 5th edition of my JavaScript book I made the embarrassing mistake of recommending a constructor and method chaining technique that only works for shallow class hierarchies--it works when class B extends A, for example, but not when C extends B and B extends A.
The technique I recommended was to put a superclass property in the prototype object of a class, and then to chain to a superclass constructor by calling this.superclass(). To see why this fails, imagine that we're creating an instance of class C (which extends B which extends A). The constructor C() chains to B() by calling this.superclass(). The constructor method B() is invoked on the same instance of C, however, so when it attempts to chain to A() by calling this.superclass(), it just ends up invoking itself. This incorrect chaining technique is discussed in sections 9.5.1 and 9.5.2, and is also used in example 9-10 and 9-11 at the very end of the chapter. I blogged about this mistake and a possible workaround almost two years ago.
Now, however, O'Reilly is preparing to do a reprint of the book, and I have an opportunity to fix this mistake. Below is a revised code from examples 9-10 and 9-11. I've renamed the defineClass() method to Class() and have modified it so that it automatically does constructor chaining (I was inspired by dojo for this change). More importantly, I've simplified method chaining by defining a global method named chain() for method chaining. If a method overrides a method defined by a superclass (or a "mixin" class) it can invoke that overridden class by invoking chain() like this: chain(this,arguments). (The second argument must be the arguments array of the overriding method, and the first argument must be the object on which that method was invoked.)
The code is below the fold. I think this is interesting JavaScript, and I'd love to have it checked for errors before it goes into print again... Please leave a comment if you think it could be improved! Update: comments are now closed; spammers have struck.
