Perhaps @@unscopable shouldn't be a Set...

Erik Arvidsson erik.arvidsson at
Wed Sep 25 13:34:50 PDT 2013

Conceptually it is a set but I don't think anyone cares enough to
oppose this being an array given that it is a cleaner layering of the

On Wed, Sep 25, 2013 at 1:07 PM, Allen Wirfs-Brock
<allen at> wrote:
> At last weeks TC39 meeting we had consensus that the value of the
> @@unscopable property should be a Set
> 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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