JavaScript Weak References & FinalizationRegistry — Quiz

Check your understanding of weak liveness, WeakRef, FinalizationRegistry, unregister tokens, and nondeterministic cleanup.

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Question 1 of 16

What is the primary property of a WeakRef target?

Answer options A It is not kept strongly reachable by the WeakRef itself B It can never be collected C It is copied into the WeakRef D It becomes a global variable

WeakRef is a weak observation and does not provide strong ownership.

Question 2 of 16

What does WeakRef.prototype.deref() return when its target is unavailable?

Answer options A undefined B null C false D The old object snapshot

A cleared WeakRef returns undefined.

Question 3 of 16

Which values are currently suitable WeakRef targets?

Answer options A Objects and non-registered Symbols B All primitives C Only strings D Only numbers

The current specification permits objects and non-registered Symbols as weak targets.

Question 4 of 16

Why can two synchronous deref() calls observe the same target after the first succeeds?

Answer options A The target is kept alive for the current synchronous execution B The WeakRef becomes strong forever C Garbage collection is disabled globally D The target is cloned

The specification uses a kept-alive list to preserve synchronous consistency.

Question 5 of 16

What is the safest interpretation of WeakRef for a cache?

Answer options A A cache hit is optional and must have a miss/recompute path B It is permanent storage C It guarantees the value survives D It replaces Map

Weakly held cached values may disappear, so correctness needs a fallback.

Question 6 of 16

What does FinalizationRegistry.register() associate with a target?

Answer options A A held value and optionally an unregister token B A timer that must run C A strong reference to the target D A Promise settlement

Question 7 of 16

Why is register(target, target) rejected?

Answer options A The held value would retain the target strongly B Targets must be strings C FinalizationRegistry accepts no objects D The cleanup callback cannot run

Using the target as held value would defeat weak retention.

Question 8 of 16

What does unregister(token) do?

Answer options A Removes registrations matching that token B Forces garbage collection C Calls the cleanup callback immediately D Makes the target strong

unregister removes matching registration cells; it does not control garbage collection.

Question 9 of 16

What does unregister() return when it removed a matching registration?

Answer options A true B false C undefined D The target

The method returns true if at least one matching registration was removed.

Question 10 of 16

What controls when FinalizationRegistry cleanup may occur?

Answer options A The host environment and implementation's nondeterministic liveness processing B The JavaScript author exactly C A required fixed timeout D The next synchronous statement

The specification leaves cleanup scheduling to the host and provides no deterministic timing guarantee.

Question 11 of 16

Can a FinalizationRegistry callback interrupt synchronous ECMAScript execution?

Answer options A No B Yes, always C Only for objects D Only in strict mode

Cleanup processing does not interrupt currently executing synchronous ECMAScript.

Question 12 of 16

What should deterministic resource release use?

Answer options A Explicit lifecycle control or the appropriate resource-management mechanism B FinalizationRegistry timing C Repeated WeakRef.deref() calls D Garbage collection as a destructor

Required resource release must be deterministic rather than GC-driven.

Question 13 of 16

Why can cleanup callbacks be omitted?

Answer options A The specification does not guarantee garbage collection or callback delivery, including when a registry is no longer relevant B Callbacks are syntax errors C Registries only run in browsers D Held values cannot be objects

Cleanup is optional and host-scheduled; it is not a guaranteed completion mechanism.

Question 14 of 16

What is the role of an unregister token?

Answer options A A weakly holdable cancellation key for registrations B The cleanup callback argument C The target's clone D A forced-GC switch

The token lets code remove matching registrations explicitly.

Question 15 of 16

Which statement about held values is correct?

Answer options A They are supplied to the cleanup callback and must not accidentally retain the target B They must always equal the target C They force the target to stay alive D They are automatically disposed immediately

Held values are cleanup metadata; retaining the target through them defeats the design.

Question 16 of 16

Which ownership boundary is correct?

Answer options A WeakRef/FinalizationRegistry own weak-liveness semantics; WeakMap/WeakSet operations remain in the collections lesson B WeakRef owns all collection APIs C FinalizationRegistry owns browser event loops D WeakMap owns FinalizationRegistry callbacks

The lessons share the liveness model but keep distinct API ownership boundaries.

JavaScript Weak References & FinalizationRegistry — Quiz