Subj : Re: instanceof XML and instanceof XMLList?
To   : Martin.Honnen
From : Brendan Eich
Date : Mon Mar 28 2005 05:53 pm

Martin Honnen wrote:

> So far each time I try to figure whether something is an XML object or 
> an XMLList object and turn to the E4X specification I end up in a 
> section saying something alike "This intentionally blurs the distinction 
> between XML objects and XMLLists".


The spec tries to allow for a future unification, but it won't be easy 
to do 100% compatibly.  See 13.4.3.10 [[HasInstance]] (V) defined in the 
XML constructor, which does require that for an XML or XMLList instance 
x, x instanceof XML will be true.  This does not imply x instanceof 
XMLList for an XML instance x, however, and there's no override for 
XMLList's [[HasInstance]].


> In the above test case not even the implementations agree. Which 
> implementation is right in the above test case? Or is Spidermoney doing 
> something different in the prototype chain but both ways are allowed?


SpiderMonkey unifies the prototype object for XML and XMLList, but hides 
methods depending on the type of the |this| parameter.  Since there is 
only one prototype for both constructors, an instance of either is an 
instance of the other, according to the default Function object 
[[HasInstance]] internal method.

I don't plan on fixing this; I'm not sure it's even a bug, given all the 
last-minute attempts before ECMA finalized ECMA-357 to "intentionally 
blur the distinction".  I hope you can live with it.

/be

.