WeakMap.prototype.clear performance

Herby Vojčík herby at mailbox.sk
Wed Jan 23 07:15:33 PST 2013

Allen Wirfs-Brock wrote:
> On Jan 22, 2013, at 4:00 PM, Mark S. Miller wrote:
>> Should Symbols (whether unique or private) also have a .clear
>> method, in order to delete all properties named by that symbol?
> Of course not.  A Symbol is basically a simple immutable value
> disguised as an object.  The is no interesting abstraction layer that

But there is a case for this. Very similar to your cache flushing, I 
admit. Call it unmarking, for example. Symbols (not only private ones) 
are used to monkey-patch object with some "alternative-layer" of 
properties, which some side-process uses. This side-process may use it 
only for some time, afterwards it may want to just clean after itself. 
This is exactly where I want `symbols.clear()`.

Yes, you can say it used bad tool and should have used WeakMap instead...


> to communicate across. A more appropriate question might be should
> there be an Object.deleteAll(obj) function that tries to delete all
> own properties on an object.  As far as I know there is no usage
> experience that suggests that this is  needed  In contrast, there is
> plenty of experience that suggests that "flushing a cache" is a
> common operations and building caches is certainly a use case for
> WeakMap.
> Allen
>> On Tue, Jan 22, 2013 at 3:36 PM, Allen Wirfs-Brock
>> <allen at wirfs-brock.com>  wrote:
>>> On Jan 22, 2013, at 2:35 PM, David Bruant wrote:
>>>> So, to find out if a weakmap is dead, it has to come from
>>>> another source than the mark-and-sweep algorithm (since it
>>>> losts its precision)... Given the additional prohibitive cost
>>>> weakmaps seem to have on the GC, maybe things that would
>>>> otherwise be considered too costly could make sense to be
>>>> applied specifically to WeakMaps. For instance, would the cost
>>>> of reference-counting only weakmaps be worth the benefit from
>>>> knowing early that the weakmap is dead? (I have no idea how
>>>> much each costs, so it's hard for me to compare the costs) For
>>>> WeakMapWithClear, reference counting would declare the weakmap
>>>> dead as soon as the new weakmap is assigned to the private
>>>> property so that's good. It wouldn't work if some weakmaps are
>>>> part of a cycle of course... but maybe that it's such an edge
>>>> case that it's acceptable to ask users doing that to break
>>>> their weakmaps cycle manually if they don't want the GC not to
>>>> be too mad at them.
>>> You know, as much as Jason and I enjoy talking about garbage
>>> collectors, this probably isn't the place to revisit the last 40
>>> years of a highly developed area of specialized CS technology.
>>> We can understand the value of providing a clear method without
>>> talking about GC at all.  Map and WeakMap are examples of object
>>> abstractions that encapsulate a probably complex implementation
>>> level data structure.  As ES programmers we aren't allowed to
>>> know very much about the specifics of those data structures other
>>> than that the spec. says they must provide average access times
>>> that are sublinear with respect to the number of entries in the
>>> collection.  The implementation is probably some sort of hash
>>> table, but it could be based upon b-trees or some other
>>> appropriate data structure.  An implementation might even
>>> dynamically change data structures and algorithms as elements are
>>> added or removed.
>>> Regardless, as ES programmers that is all forbidden knowledge.
>>> All we  can know and express are things that are exposed through
>>> the abstraction's API. An operation like clear, even those it
>>> might be possible to achieve the same abstract end result using
>>> other operators like delete, allows us to express an intent that
>>> tunnels through the abstraction boundary.  For a large number of
>>> elements, it is quiet likely that the underlying data structure
>>> can be cleared or reset to no entries far more efficiently than
>>> performing an sequence of isolated deletes for the total number
>>> of elements.  But if the clear method does it exist we can't
>>> express that intent and hence the optimization is impossible.
>>> Allen
>> -- Cheers, --MarkM

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