Gotcha: JavaScript WeakRef and FinalizationRegistry caveats

WeakRef and FinalizationRegistry have unpredictable timing and behavior because garbage collection is non-deterministic.

Understand the limitations of weak references:

// WeakRef - holds a reference that doesn't prevent GC:
let obj = { data: 'important' };
const weakRef = new WeakRef(obj);

obj = null; // Eligible for GC

// Later (GC may or may not have run):
const derefed = weakRef.deref();
if (derefed) {
console.log(derefed.data); // May or may not work!
}

// Gotchas:
// 1. deref() may return undefined at ANY time after obj is unreachable
// 2. GC timing is NOT predictable - don't rely on it
// 3. Never use WeakRef for important data flow
// 4. deref() in the same turn is guaranteed to return the object

// FinalizationRegistry - cleanup callback when object is GC'd:
const registry = new FinalizationRegistry((heldValue) => {
console.log(Object with key ${heldValue} was GC'd);
// Clean up external resources
});

registry.register(obj, 'my-key');

// Gotchas:
// 1. Callback may never run (browser tab closed first)
// 2. Callback timing is unpredictable
// 3. Don't use for critical cleanup - use try/finally instead
// 4. Callback runs in a microtask, not immediately

// Valid use case: Cache that doesn't prevent GC
// Invalid use case: Anything that MUST happen (use explicit cleanup)

WeakRef and FinalizationRegistry are for optimization hints only. They must never be the only mechanism for important cleanup.