On Feb 10, 2012, at 4:08 AM, Andreas Rossberg wrote:
> On 10 February 2012 12:46, Tom Van Cutsem <[email protected]> wrote:
>> I'm also in favor of exposing __proto__ as an accessor on the grounds that
>> it is least magical. I'm worried that the changes made here to the core
>> internal methods [[Get]], [[Put]], [[DefineOwnProperty]] and [[Delete]] will
>> give proxies a hard time to properly emulate this behavior if they would
>> want to do that. How would a proxy handler be able to access the internal
>> variable UnderscoreProtoEnabled?
>
> I also agree that an accessor is much cleaner (if we really want to
> spec this at all -- which will de facto make it a blessed feature
> forever).
I share you concern.
However, we discussed this at the last TC39 meeting. Because of the current
dominance of the smart mobile device space by browsers that support __proto__
browsers that have traditionally not implemented it in the more diverse desktop
space are under growing pressure to include __proto__ support if they want to
compete in the mobile space.
This is rapidly driving __proto__ towards being a de facto standard. Part of
TC39's mission is to turn de facto standards into de jure standards and in the
process provide a uniform definition for the edge cases where current
implementation differ. That is exactly the sort of technical issue that we are
now discussing.
>
> However, I'm not sure the proxy argument applies. Either way, it
> seems, proxies cannot "simulate" the magic of __proto__ themselves,
> because they have no control over the prototype they disclose anyway.
> It can only work for proxies if the magic accessor (whether implicit
> or explicit) transparently forwards the assignment to proxy targets.
> No trap would be involved, just like there is no trap for getting the
> prototype of a proxy.
Proxies could presumably trap the initial [[Put]]("__proto__") call and from
that point do whatever they want. To change the this objects prototype
(without reentering the trap) they would presumably use reflect.set which does
a non-trapping invocation of the default [[Put]] internal method.
>
> I suppose the real question is: do we _want_ proxies to simulate this
> in full gory glory? That would be blessing this (mis)feature even
> more.
how would we prevent it?
>
> /Andreas
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