When do WeakMap and WeakSet beat Map and Set — covering private instance data, DOM node metadata, and memoization caches that must not block garbage collection?

Imagine you have a sticky note that you can put on toys, but when you throw the toy away, the sticky note disappears too. WeakMap and WeakSet are like that—they hold information about things only as long as those things exist, so they don't prevent the garbage collector from cleaning up.