On Mar 13, 2009, at 03:13 , Brendan Eich wrote:
Your proposal does not completely "fix" it, though, since the
[[Class]] of Parent.prototype is "Object". It was not constructed by
Parent. Maybe this is ok.
That actually didn't bother me that much (I was thinking in classical
rather than prototypal inheritance terms). Now that I've got my head
set straight again I do see how this is an issue.
It breaks the equality between the instance's and the prototype's
"[[Class]]" so that:
Object.getClassName(Parent.prototype) !== Object.getClassName(new
Parent());
Turns out that there is a precedent (still in Firebug):
>>> Object.prototype.toString.call(document.createElement('div'))
"[object HTMLDivElement]"
>>> Object.prototype.toString.call(HTMLDivElement.prototype)
"[object XPC_WN_ModsAllowed_NoCall_Proto_JSClass]"
And yes, I know this is a host object.
Outside of built-in classes, [[Class]] is used only for
Object.prototype.toString. In TC39 we've argued about what [[Class]]
means, and I've maintained it's a token corresponding in concrete
implementation terms (vtable or equivalent) to a pile of code
implementing a specific set of internal methods (like Array's custom
[[Put]]).
So changing [[Class]] just to get a differnt
Object.prototype.toString.call(obj) result is arguably wrong.
Another option here is to add a different internal property,
[[ClassName]].
That felt dirty. Now I know why.
From the point of view of implementors, setting the "vtable" for an
instance is not going to be done based on a string value in a
property of F. In either case (step 2 or 3) the same implementation
code for generic Object behavior, appropriate to [[Class]] "Object"
in ES1-3, will be chosen.
Understood. Same issue as above.
The only difference that matters (correct me if I'm wrong) is the
output of Object.prototype.toString.call(obj).
That's correct. However your earlier comment about the broken equality
between the "[[Class]]" of Parent.prototype and a Parent "instance"
opened up my eyes. Is there a way around that ? Or is it a huge can of
worms ?
There's probably a lot of issues I haven't foreseen here (on top of
the yet unspecified name property of functions), but it hopefully
helps to clarify what I was thinking about. It also avoids touching
the behaviour of the constructor property of the prototype object.
Given that spec change, I suspect Object.getClassName(o) could be
desugared to Object.prototype.toString.call(object).slice(8, -1).
No, since outside of strict mode, Object is a writable, deletable
property of the global object, and toString is writable/deletable in
Object.prototype, and call can be shadowed, etc. etc.
Right. The fact that I wouldn't touch Object.prototype doesn't imply
someone else couldn't. :) And thank you for the precise definition of
desugaring!
Tobie
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