whyWeakMaps.js

var k1 = {a: 1};
var k2 = {b: 2};

var map = new Map();
var wm = new WeakMap();

map.set(k1, 'k1');
wm.set(k2, 'k2');

k1 = null;
map.forEach(function (val, key) {
    console.log(key, val); // k1 {a: 1}
});

k2 = null;
wm.get(k2); // undefined

As you see, after removing k1 key from the memory we can still access it inside the map. At the same time removing k2 key of WeakMap removes it from wm as well by reference.

That's why WeakMap hasn't enumerable methods like forEach, because there is no such thing as list of WeakMap keys, they are just references to another objects.

And keys of WeakMaps are of the type Object only. Primitive data types as keys are not allowed (e.g. a Symbol can't be a WeakMap key).