Weak map

• It is like a map
• Has no size
• Has no primitive keys.

It will not hold to a key that is not used by any other element. This is useful to prevent unlimited garbage.

eg. when using a Document Object Model (DOM) element as a key in a map, then the DOM element gets deleted, the weak map will delete that key-value as well.

A weak map holds a weak reference to a key. That means that the reference inside of the weak map doesn't prevent garbage collection of that object.