Before my re-entry in JavaScript (and related) I've done lots of ActionScript 3 and there they had a Dictionary object that had weak keys just like the upcoming WeakMap; but the AS3 version still was enumerable like a regular generic object while the WeakMap specifically has no .keys() or .values().
The AS3 version allowed us to rig some really interesting and usefull constructs but I feel the JS version is somewhat limited. Why is that?
If the Flash VM could do it then what is keeping browsers from doing same? I read how it would be 'non-deterministic' but that is sort of the point right?
Finally found the real answer: http://tc39wiki.calculist.org/es6/weak-map/
A key property of Weak Maps is the inability to enumerate their keys. This is necessary to prevent attackers observing the internal behavior of other systems in the environment which share weakly-mapped objects. Should the number or names of items in the collection be discoverable from the API, even if the values aren't, WeakMap instances might create a side channel where one was previously not available.
It's a tradeoff. If you introduce object <-> object dictionaries that support enumerability, you have two options with relation to garbage collection:
Consider the key entry a strong reference that prevents garbage collection of the object that's being used as a key.
Make it a weak reference that allows its keys to be garbage collected whenever every other reference is gone.
If you do #1 you will make it extremely easy to to shoot yourself in the foot by leaking large objects into memory all over the place. On the other hand, if you go with option #2, your key dictionary becomes dependent on the state of garbage collection in the application, which will inevitably lead to impossible to track down bugs.
来源：https://stackoverflow.com/questions/20508628/why-will-es6-weakmaps-not-be-enumerable