WeakMaps are similar to Maps, with the following differences:

• They can be used to attach data to objects, without preventing those objects from being garbage-collected.
• They are black boxes where a value can only be accessed if you have both the WeakMap and the key.
  The next two sections examine in more detail what that means.

This is how you attach a value to an object via a WeakMap:

  const obj = {};
}

31.1.1. The keys of a WeakMap are weakly held

The keys of a WeakMap are said to be weakly held: Normally, references to an object prevent the object from being garbage-collected. However, WeakMap keys don’t. Additionally, WeakMap entries, whose keys were garbage-collected, are also (eventually) garbage-collected.

Weakly held keys only make sense for objects. Thus, you can only use objects as keys:

It is impossible to inspect what’s inside a WeakMap:

• For example, you can’t iterate or loop over keys, values or entries. And you can’t compute the size.
• Additionally, you can’t clear a WeakMap, either – you have to create a fresh instance.
  These restrictions enable a security property. Quoting Mark Miller: “The mapping from weakmap/key pair value can only be observed or affected by someone who has both the weakmap and the key. With clear(), someone with only the WeakMap would’ve been able to affect the WeakMap-and-key-to-value mapping.”

31.3.1. Caching computed results via WeakMaps

const cache = new WeakMap();
    return [cache.get(obj), 'cached'];
    const count = Object.keys(obj).length;
  }

If we use this function with an object obj, you can see that the result is only computed for the first invocation, while a cached value is used for the second invocation:

31.3.2. Keeping private data via WeakMaps

In the following code, the WeakMaps _counter and _action are used to store the data of virtual properties of instances of Countdown:

let _action = new WeakMap();
class Countdown {
    _counter.set(this, counter);
  }
    let counter = _counter.get(this);
    _counter.set(this, counter);
      _action.get(this)();
  }
assert.deepEqual(
  []);

The constructor and the four methods of WeakMap work the same as :

• new WeakMap<K, V>(entries?: Iterable<[K, V]>) [ES6]
• .delete(key: K) : boolean [ES6]
• .get(key: K) : V [ES6]
• [ES6]
• .set(key: K, value: V) : this [ES6]