Use cases for WeakMap

Brendan Eich brendan at
Mon May 16 00:25:08 PDT 2011

On May 16, 2011, at 12:18 AM, Erik Corry wrote:

> 2011/5/16 Brendan Eich <brendan at>:
>> On May 16, 2011, at 12:01 AM, Brendan Eich wrote:
>>> On May 15, 2011, at 11:55 PM, Erik Corry wrote:
>>>> 2011/5/16 Brendan Eich <brendan at>:
>>>>> Not if the object is frozen.
>>>> That shouldn't prevent you adding private names.  See earlier message
>>>> in this thread.
>>> Frozen means [[Extensible]] is false, so you can't add any properties, ...
>> I'll go further: frozen means the implementation should be free to move the object and anything associated with its shallow-frozen key/value state into read-only memory, so the full force of 50-year-plus OS and hardware MMU protection can ensure [[Extensible]] really is false.
> I think this would also preclude the most efficient implementation of
> Weak Maps.  In fact I have a hard time seeing how you would GC weak
> maps at all under these conditions.

Why, because GC metadata has to go in the same page as the frozen object that might be a key in a weak map?

Weak maps are in Firefox nightlies. We're playing with page protection too (not for freezing, yet). This seems like a dare, but it also seems to be dodging my point in replying again: that private names cannot be used to extend frozen objects in the "[[Extensible]] = true" sense of the spec.


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