WeakMap and WeakSet hold keys weakly so objects can be garbage-collected when nothing else references them. This is useful for caches and private data attached to DOM nodes.
Common Interview Questions
Q: Why can't WeakMap keys be primitives?
Answer: Weak references only make sense for objects. Primitives are not garbage-collected individually — they are values, not heap objects with identity.
Q: What happens when a WeakMap key is collected?
Answer: The corresponding entry disappears automatically. You cannot enumerate WeakMap, so you cannot observe removal directly — you simply won't find the key on next get.
Q: Give a practical WeakMap use case
Answer: Store per-instance private state without polluting the object or closing over closures in every method. When the instance is GC'd, metadata goes with it.
const privateData = new WeakMap();
class User {
constructor(name) {
privateData.set(this, { name });
}
getName() {
return privateData.get(this).name;
}
}
Q: WeakSet vs Set?
Answer: WeakSet only stores objects weakly and is not iterable. Use it to tag objects (e.g., visited nodes in a graph) without preventing GC.
Key Points for Interviews
-
Weak collections do not prevent garbage collection
-
No
size,keys, or iteration on WeakMap/WeakSet -
Ideal for DOM-node metadata and private fields (pre-# syntax)
-
Strong Map cache can cause memory leaks for short-lived objects
-
Modern
#privatefields reduce some WeakMap patterns in classes