JavaScript hashmap equivalent

Question

As made clear in update 3 on this answer, this notation:

var hash = {};
hash[X]

does not actually hash the object X; it actually

Answer 1

In ECMAScript 6 you can use WeakMap.

Example:

var wm1 = new WeakMap(),
    wm2 = new WeakMap(),
    wm3 = 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')

wm3.set(o1, 37);
wm3.get(o1); // 37
wm3.clear();
wm3.get(o1); // Undefined, because wm3 was cleared and there is no value for o1 anymore

wm1.has(o1);   // True
wm1.delete(o1);
wm1.has(o1);   // False

But:

Because of references being weak, WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys).