Must built-in prototypes also be valid instances? (Was: Why DataView.prototype object's [[Class]] is "Object"?)

Rick Waldron waldron.rick at
Mon Oct 1 12:41:08 PDT 2012

On Mon, Oct 1, 2012 at 3:21 PM, Brendan Eich <brendan at> wrote:

> Andreas Rossberg wrote:
>> Er, from my reading that's clearly not what the Wiki says for WeakMap.
>> And it also is not what V8 implements, for either WeakMap or Map.
> Sorry, I was relying on Rick's testimony that the answers were 1) yes, 2)
> no.

Yes, when I first considered the topic, it seemed to make some kind of
sense (based on a desire for consistency), but I also don't believe that
should be the case now—I even noted that I had always subjectively viewed
resulting behaviours as "strange".

I'm not sure I've ever encountered code in the wild that made any
intentional use of built-ins Foo.prototype-as-firstborn - can anyone on the
list point to some compelling use cases?

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