What's the difference between ES6 Map and WeakMap?

Question

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 differ

Answer 1

Maybe the next explanation will be more clear for someone.

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.