How do WeakRef and FinalizationRegistry extend the weak-reference capabilities beyond WeakMap/WeakSet?

WeakRef lets you hold a weak reference to ANY object (not just as a map key) and later try to access it via .deref(), which returns undefined once collected. FinalizationRegistry lets you register a callback to run (at some unpredictable future point) after an object is actually garbage collected — useful for cleanup, though never guaranteed to run promptly or at all.

const registry = new FinalizationRegistry((heldValue) => {
  console.log('cleaned up:', heldValue);
});
let obj = { data: 'large' };
registry.register(obj, 'obj-label');
obj = null; // callback MAY run later, after GC