JavaScript Map, Set, WeakMap, and WeakSet Quiz

Review

1. Which value can be a Map key?

   Map keys are arbitrary ECMAScript values.
2. What happens when Map.set() receives an existing key?

   A Map has at most one entry per distinct key.
3. Why are two separately created but identical-looking objects different Map keys?

   Object keys are compared by identity.
4. What does Set do with a value already present?

   Set stores each distinct value at most once.
5. Which Set method keeps only shared values?

   intersection keeps members present in both collections.
6. Why does WeakMap not provide keys() or entries()?

   WeakMap deliberately omits enumeration.
7. Which value can be held weakly by WeakMap/WeakSet in the current ECMAScript model?

   Weak collections accept values that can be held weakly, including objects and symbols.
8. What happens to Map insertion order when an existing key is updated?

   Updating an existing key changes its value without creating a new entry.
9. Which collection directly models “has this identifier been seen?”

   Set models membership directly.
10. Which statement about WeakMap and garbage collection is correct?

    Weak collection timing is not an observable scheduling mechanism.
11. A Map already contains key "mode" with value "dark". What does getOrInsert("mode", "light") return?

    getOrInsert returns the existing value and only inserts the supplied value when the key is absent.
12. When does Map.prototype.getOrInsertComputed call its callback?

    The callback is invoked only after the method determines that the key is absent.
13. What fundamental WeakMap property is unchanged by getOrInsert?

    getOrInsert adds lookup-or-default behavior without changing WeakMap key restrictions, weak liveness, or lack of enumeration.
14. A WeakMap key already exists. What happens to getOrInsertComputed’s callback?

    An existing key returns its current value, so the callback is unnecessary and is not invoked.