I misspoke -- you can polyfill WeakSet, but you cannot polyfill WeakMap, since you do not know when you can shrink the map. (But you still cannot use it to emul

I misspoke -- you can polyfill WeakSet, but you cannot polyfill WeakMap, since you do not know when you can shrink the map. (But you still cannot use it to emulate weak references, since you need a strong reference to get data out of the map.)