At last weeks TC39 meeting we had consensus that the value of the @@unscopable
property should be a Set
https://github.com/rwaldron/tc39-notes/blob/master/es6/2013-09/sept-17.md#53-unscopeable
As I begin to look at implementing this (and the other @@unscopable changes
from the meeting) I'm not so sure that Set is such a good idea. My basic
concern is that @@unscopable operates at a very low level of the ES name
binding resolution mechanism. Set exists at a much higher conceptual level of
the ES library and (until now) there was nothing in the fundamental language
semantics of ES that depends upon the existence of a library Set object. Now
that I have thought about this, it seems fundamentally wrong to unnecessarily
create such an up-dependency.
I think we will have a cleaner semantics if we continue to treat an
@@unscopable value as an array-like object for the purpose of accessing its
property blacklist. Implementations, if they wish, can us2 caching scheme to
obtain sub-linear time access to an @@unscopable blacklist.
In light of this consideration, does anybody still want to argue that we should
require an @@unscopable value to be a Set instance?
Allen
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