I'd love to be able to get rid of SHADOWABLE, actually! I just
recently found out we can do it with a little rearranging. Extra
state-space in property attributes is unavoidable complexity for us.
My own preference would be not to standardize it. :)
There's also the [Replaceable] attribute in WebIDL, which arises from
similar compatibility needs. It's implemented as a setter that
redefines the property being set.
This is a bad bunch of options. I think renaming ("hasValue"?) is
probably least-worst, followed by waiting a year and trying again.
-j
On Wed, Oct 1, 2014 at 1:50 PM, Brendan Eich <[email protected]> wrote:
> Claude Pache wrote:
>>
>> The more I consider the issue, the more I think that the most elegant
>> solution is to complete data properties with an additional "overridable"
>> attribute, whose purpose is to refine the meaning of the "writable"
>> attribute in the manner sketched below (in opposition of having an
>> @@unscopables-like hack).
>
>
> A new attribute, per property, is safer in general (backward compatibility)
> than a per-object flag. A per-object list seems an obtuse way to encode an
> attribute, and I am quite sure implementations would prefer an attribute.
>
>> At first glance, I expect that the addition of such an attribute should
>> not trigger much BC issues, but that needs to be examined more closely.
>
>
> If it's opt-in, e.g. we define Array.prototype.contains with [[Overridable]]
> among its attributes, then at least MooTools works.
>
> What could break? Code that did not expect 'contains' to exist on
> Array.prototype, but did expect assigning to someArray.contains would make a
> non-enumerable and/or non-own property. That would seem like nonsense or
> broken code, but it could exist for some name and prototypal relation,
> you're right.
>
> Only way to find out is to implement and test at scale.
>
> It would be good to hear from implementors on the idea of an [[Overridable]]
> attribute. SpiderMonkey has something similar but more restricted (see
> http://dxr.mozilla.org/mozilla-central/source/js/src/vm/Shape.h?from=SHADOWABLE&case=true#1013).
>
> /be
>
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