When should you use Set, Map, WeakSet and WeakMap?

The weak collections are about memory ownership. If a cache or metadata table should not keep an object alive, a WeakMap or WeakSet lets the garbage collector reclaim the key when nothing else references it.

const seen = new Set();
seen.add("a");
seen.add("a");
console.log(seen.size); // 1

const metadata = new WeakMap();
const element = document.querySelector("#app");

metadata.set(element, { clicks: 0 });
console.log(metadata.get(element)); // { clicks: 0 }

The differences

• Set stores unique values of any type and is iterable
• WeakSet stores objects only, is not iterable, and has no size
• Map stores key-value pairs with any key type and is iterable
• WeakMap accepts objects as keys, is not iterable, and has no size
• Weak collection entries disappear when the key is no longer reachable elsewhere

Weak collections are not enumerable because enumeration would expose the garbage collector's timing and make behaviour non-deterministic. You can only get, set, has and delete on a WeakMap; a WeakSet supports add, has and delete.