Removal of WeakMap/WeakSet clear

Andrea Giammarchi andrea.giammarchi at
Thu Nov 27 06:22:46 PST 2014

Andreas I think Mark few times already gave for granted WeakMaps are based
on transposed representations and it's also partially pointless from a
polyfill point of view to not implement them as such but clear makes fast
polyfills very pointless with `.clear()` in place ( e.g. [1] )

I've had similar thoughts when I've first realized there was a `delete`
method since I thought: if you can delete, you should be able to iterate
and get keys ... which wasn't even the initial case but that's how my
initial polyfill was implemented, with a real `.clear()` ability that was
making the WeakMap itself not weak anymore [2]

I start thinking for performance and polyfill sake *maybe* it's better for
everyone to keep `.clear()` out of the equation so there won't be broken
polyfill and implementations can decide which implementation is better for
their main use cases.

Who wants to expose clear can always do it via wrappers:

// just for example purpose
function CleanableWeakMap() {
  this.delete = this._wm.delete.bind(this._wm);
  this.get = this._wm.get.bind(this._wm);
  this.has = this._wm.has.bind(this._wm);
  this.set = this._wm.set.bind(this._wm);
CleanableWeakMap.prototype.clean = function () {
  this._wm = new WeakMap;

Above code should make everyone happy except the GC in between a fake
`.clean()` and a garbage call but at least it could be sort of consistent
across ES5 to platforms.

Willing also to drop my non WeakMap and bring in either the Polymer
polyfill which is already missing `.clear()` but polluting the global scope
anyway with this "not really full spec" implementation, or the one in link


[1] changing the slot without being able to clean up a thing:

[2] not a WeakMap, you should delete things ( aka: sort of pointless
WeakMap )

On Thu, Nov 27, 2014 at 9:17 AM, Andreas Rossberg <rossberg at>

> The discussion here still makes various untested assumptions about the
> transposed representation. With respect to .clear in particular, it
> seems to assume that this representation does not normally need the
> ability to iterate (internally) over the keys of a weak map. AFAICT,
> that is incorrect. Both the implementation we discussed in the V8
> team, and from what I heard, the implementation in IE, maintain a list
> of all its keys for each weak map. Otherwise, when a map dies, you
> couldn't generally kill the entries and the values associated with
> them easily and in a timely manner. (And of course, this list means
> almost twice the space usage for a map, because you duplicate part of
> the information.)
> So from that perspective at least, .clear is not an issue.
> I also don't see the security issue, to be honest. In any
> security-relevant (or abstraction-relevant) scenario you would be
> crazy to hand out the (mutable!) weak map to third parties anyway.
> They could run just as much havoc by _adding_ random things to the map
> (or removing things at guess) as they could by clearing it.
> Mark, can you explain the specific scenario you have in mind?
> /Andreas
> On 26 November 2014 at 20:21, Mark S. Miller <erights at> wrote:
> > On Wed, Nov 26, 2014 at 9:33 AM, Katelyn Gadd <kg at> wrote:
> >> Is there a detailed rationale for this somewhere?
> >
> > It is a combination of three issues.
> >
> > 1. The security issue.
> > 2. The implementation issue.
> > 3. The ES process issue.
> >
> > The implementation issue is that the use cases for WeakMaps basically
> > divide into the following cases:
> >
> > a. Those for which we're confident that the map's lifetime outlives
> > its typical key.
> > b. Those for which we're confident that the key's lifetime outlives
> > the typical map it is a key in.
> > c. Those for which we're not confident which typically lives longer.
> >
> > For #a, the transposed representation of WeakMaps is strictly better.
> > The non-transposed implementation would promote ephemeral keys to
> > later GC generations, which is very expensive. (I believe the expense
> > of this promotion has been radically underestimated in most prior
> > discussions.) This is the GC pressure that really matters.
> >
> > For #b, just use a Map rather than a WeakMap.
> >
> > For #c, transposed rep or not is a tie. In the interests of minimizing
> > implementation mechanism, we should just use a transposed rep for this
> > as well.
> >
> > Given the transposed representation, the only implementation
> > techniques for clear are
> >
> > x. Enumerating all memory
> > y. Having the WeakMap encapsulate the token used to lookup the value
> > in the key's hidden map, and have .clear replace this token.
> >
> > #x is not reasonable.
> > #y is equivalent to the replacing of the Map that would be the
> > user-level technique for emulating clear in any case. This is exactly
> > what the use of WeakMaps for membranes do, when the membrane should be
> > revoked. If .clear could be implemented for the transposed
> > representation more efficiently than this, membranes would benefit
> > from .clear as well. I have never expected they could.
> >
> >
> > The process issue is that .clear was not in the original proposal (for
> > all of these reasons), and it never achieved consensus in committee.
> > It would have violated our process to keep it in the spec. The process
> > issue is not "Should it be dropped?" since it was never legitimately
> > added. The only issue is "Should it be added?".
> >
> >
> >
> >> Making typical
> >> applications pay the cost here for a specific security scenario seems
> >> really bizarre to me. Clearing standard library data structures is an
> >> incredibly common operation. If you want to ensure that someone can't
> >> clear the map/set, shouldn't you be handing them an encapsulated
> >> version of the data structure? This seems like a corner case that
> >> shouldn't justify removing an important primitive.
> >>
> >> If you have a clear method, the security problem seems solved by
> >> wrapping it in an object or using a proxy to deny the ability to clear
> >> (you hide the actual map/set, so it can't be cleared - you expose only
> >> the operations you want to expose).
> >>
> >> If you don't have a clear method, anyone wanting to clear the data
> >> structure has to throw it away and allocate a new one. This has
> >> significant disadvantages:
> >> The new structure starts empty at a default size, so repopulating it
> >> will have to grow the buffer multiple times - this is undesirable for
> >> cases where you are reusing a single data structure to store state for
> >> a long-running application.
> >> The allocation adds to GC and memory pressure for a long-running
> >> application that needs to clear data structures frequently. Were it a
> >> lightweight data type this would matter less, but a typical map
> >> instance with data in it can occupy a considerable amount of space in
> >> the heap.
> >> Being able to clear the structure now requires that all consumers have
> >> support for replacing their reference(s) to the old map with the new
> >> one. This makes it harder to maintain encapsulation because you may
> >> have saved a reference to the map in a private property or within a
> >> closure. Now you need to add accessibility points to everything that
> >> might retain the map so that you can update the reference. Or, you
> >> have to encapsulate maps and sets just to recreate the clear operation
> >> that should have been there to begin with.
> >>
> >> In either case, encapsulation or shielding the container behind a
> >> proxy is necessary. I insist that the common case is the one that
> >> shouldn't have to encapsulate, because optimizing for that case will
> >> benefit the vast majority of web applications that use it and the
> >> penalty to security-sensitive cases is small.
> >>
> >> -kg
> >> _______________________________________________
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> >> es-discuss at
> >>
> >
> >
> >
> > --
> >     Cheers,
> >     --MarkM
> > _______________________________________________
> > es-discuss mailing list
> > es-discuss at
> >
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