WeakMap
WeakMaps basically allow you to have a HashTable with a key that isn't a String.
So you can set the key to be, i.e. [1]
and then can say Map.get([1])
Example from the MDN:
var wm1 = new WeakMap(),
wm2 = new WeakMap();
var o1 = {},
o2 = function(){},
o3 = window;
wm1.set(o1, 37);
wm1.set(o2, "azerty");
wm2.set(o1, o2); // a value can be anything, including an object or a function
wm2.set(o3, undefined);
wm2.set(wm1, wm2); // keys and values can be any objects. Even WeakMaps!
wm1.get(o2); // "azerty"
wm2.get(o2); // undefined, because there is no value for o2 on wm2
wm2.get(o3); // undefined, because that is the set value
wm1.has(o2); // true
wm2.has(o2); // false
wm2.has(o3); // true (even if the value itself is 'undefined')
wm1.has(o1); // true
wm1.delete(o1);
wm1.has(o1); // false
The reason for its existance is:
in order to fix a memory leak present in many uses of weak-key tables.
Apparently emulating weakmaps causes memory leaks. I don't know the details of those memory leaks.