What's the difference between ES6 Map and WeakMap?

They both behave differently when a object referenced by their keys/values gets deleted. Lets take the below example code:

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

(function(){
    var a = {x: 12};
    var b = {y: 12};

    map.set(a, 1);
    weakmap.set(b, 2);
})()

The above IIFE is executed there is no way we can reference {x: 12} and {y: 12} anymore. Garbage collector goes ahead and deletes the key b pointer from “WeakMap” and also removes {y: 12} from memory. But in case of “Map”, the garbage collector doesn’t remove a pointer from “Map” and also doesn’t remove {x: 12} from memory.

Summary: WeakMap allows garbage collector to do its task but not Map.

References: http://qnimate.com/difference-between-map-and-weakmap-in-javascript/

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.

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.