I think its a 5am brain drain, but I'm having trouble with understanding this.
obj = ['a','b'];
alert( obj.prototype ); //returns "undefined"
Why isn't obj.prototype returning function Array(){ } as the prototype? It does reference Array as the constructor.
Because the instance doesn't have a prototype, the class* does.
Possibly you want obj.constructor.prototype or alternatively obj.constructor==Array
* to be more accurate, the constructor has the prototype, but of course in JS functions = classes = constructors