WeakMap & WeakSet

The plain-language idea

A regular Map holds a strong reference to every key — as long as the Map exists, none of its keys can be garbage-collected, even if nothing else in your program points to them anymore. A WeakMap holds its keys weakly: if an object used as a key has no other references left, the garbage collector can reclaim it, and the WeakMap's entry silently disappears too.

let user = { name: "Ada" };

const strongCache = new Map();
strongCache.set(user, "metadata");
user = null;
// user's object is STILL alive — strongCache is holding a reference to it

let user2 = { name: "Grace" };
const weakCache = new WeakMap();
weakCache.set(user2, "metadata");
user2 = null;
// user2's object CAN now be garbage-collected — weakCache doesn't keep it alive

WeakSet is the same idea for a set of objects instead of key-value pairs.

A Map holds a STRONG reference to its keys — the object stays alive as long as the Map does.

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What people usually miss

Watch out

• This directly connects to the garbage-collection lesson: a WeakMap is the language's built-in answer to "I want to attach data to an object without accidentally causing a memory leak."
• You can't iterate a WeakMap/WeakSet — no .keys(), no .size, no for...of. This isn't a missing feature; it's deliberate, because the contents can change at any moment as the GC runs, so a stable list of entries can't be guaranteed to exist.
• Real use case: private data / DOM node metadata. Attaching a cache entry keyed by a DOM node in a WeakMap means the entry vanishes automatically once that node is removed and garbage-collected — no manual cleanup needed.

Why this matters in interviews

"Why would you use a WeakMap instead of a Map?" checks whether you understand reachability and garbage collection as a real mechanism, not just "it's like Map but weak" — the why is the actual answer.