Argument in favor of adding "has" in the WeakMap API

David Bruant david.bruant at
Wed May 11 13:49:39 PDT 2011

False alarm. I have finally (just) received the e-mail.


Le 11/05/2011 21:44, Oliver Hunt a écrit :
> On May 11, 2011, at 12:38 PM, David Bruant wrote:
>> Le 11/05/2011 20:38, felix a écrit :
>>> On Wed, May 11, 2011 at 11:16, David Bruant <david.bruant at> wrote:
>>>> A memoizer could be written to improve f time performance (by the cost of
>>>> memory, of course).
>>>> ----
>>>> function memoizer(f){
>>>>  var cache = WeakMap();
>>>>  return function(o){
>>>>    var res;
>>>>    if(WeakMap.has(o))
>>>>      return cache.get(o);
>>>>    res = f.apply(this, o);
>>>>    cache.set(o, res);
>>>>  };
>>>> }
>>>> ----
>>> I'm not sure I understand your code.
>>> it's not clear to me why you need .has at all, since you can check the
>>> return value of .get instead.  in the case where 'undefined' is a
>>> valid value that you would want to memoize, you can store a unique
>>> object to represent 'undefined'.
>> The ambiguity of "get" returning "undefined" when the key is actually
>> undefined or when it is defined but its value is "undefined" already
>> happened with objects. The solution to that was the "in" operator (or
>> Object.prototype.hasOwnProperty depending on cases). I'm just asking an
>> equivalent for weak maps. I agree its not necessary as proved in
>> harmony:weak_maps#explicit_soft_own_fields . However, a native
>> implementation would be a good addition.
> So you want to do
> if (map.has(bar))
>    wiffle = map.get(bar)
> or some such?
> The problem here is that you can't guarantee that GC won't happen between those two calls, and therefore you could still end up getting undefined in response to the get.
> GC is (from the language's perspective) non-deterministic, so any non-atomic operation with a weak map leads to "non-sensical" behaviour.
> --Oliver

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