Looking this and this MDN pages it seems like the only difference between Maps and WeakMaps is a missing "size" property for WeakMaps. But is this true? What's the difference between them?
From the very same page, section "Why Weak Map?":
The experienced JavaScript programmer will notice that this API could be implemented in JavaScript with two arrays (one for keys, one for values) shared by the 4 API methods. Such an implementation would have two main inconveniences. The first one is an O(n) search (n being the number of keys in the map). The second one is a memory leak issue. With manually written maps, the array of keys would keep references to key objects, preventing them from being garbage collected. In native WeakMaps, references to key objects are held "weakly", which means that they do not prevent garbage collection in case there would be no other reference to the object.
Because of references being weak, WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys). If they were, the list would depend on the state of garbage collection, introducing non-determinism.
[And that's why they have no size property as well]
If you want to have a list of keys, you should maintain it yourself. There is also an ECMAScript proposal aiming at introducing simple sets and maps which would not use weak references and would be enumerable.
‐ which would be the "normal" Maps. Not mentioned at MDN, but in the harmony proposal, those also have items, keys and values generator methods and implement the Iterator interface.