Besides creating an instance, how to tell if ClassA descends from ClassB in ActionScript?

Question

I need to determine if one class descends (directly or indirectly) from another.

I can do

var testInstance : Object = new ClassA();
if (testInstance is ClassB)
    ...

but I hate creating an instance just to test ancestry. I hoped that if (ClassA is ClassB) would work, but it does not seem to.

AS help states

isPrototypeOf(theClass:Object):Boolean
Indicates whether an instance of the Object class is in the prototype chain of the object specified as the
parameter.

I don't actually understand ActionScript prototypes (I think it might drive me insane), but I hope that Class objects have some way of tapping into their inheritance information.

Thanks

Answer 1

Prototypes are instances, either of the class (for class inheritance) or the parent class (for prototype inheritance). Try:

Object.prototype.extends=function (theClass:Object):boolean {
    return this.prototype instanceof theClass;
}

ClassA.extends(ClassB);

ActionScript 3 shoehorns prototype inheritance into class based inheritance. You don't use prototypes much anymore in Actionscript, but it's not a complex concept and has its uses, so you'd do well to study it. Read "History of ActionScript OOP support" and "The prototype object" from "Advanced topics"