What are WeakMap and WeakSet and when would you use them?

WeakMap and WeakSet

Weak collections hold weak references to their keys/values - meaning they don't prevent garbage collection.

WeakMap

js
const weakMap = new WeakMap();

let user = { name: 'Alice' };
weakMap.set(user, { sessionData: '...' });

weakMap.get(user); // { sessionData: '...' }

user = null; // object can now be garbage collected
// weakMap entry is automatically removed

Constraints:

Keys must be objects (or non-registered symbols since ES2023), not primitives.
Not iterable - no .forEach, no .size.
Only .get, .set, .has, .delete.

WeakSet

js
const visited = new WeakSet();

function process(node) {
  if (visited.has(node)) return; // already processed
  visited.add(node);
  // ...
}

When to use

Storing metadata about objects without preventing GC

js
const cache = new WeakMap();

function process(element) {
  if (cache.has(element)) return cache.get(element);
  const result = heavyComputation(element);
  cache.set(element, result);
  return result;
}
// when element is removed from DOM and dereferenced,
// cache entry is cleaned up automatically

Tracking without memory leaks

js
const listeners = new WeakMap();
// safe to store per-element event data - cleans up with the element

vs regular Map/Set

	`Map`	`WeakMap`
Key types	Any	Objects only
Prevents GC	Yes	No
Iterable	Yes	No
Memory leak risk	Yes	No

WeakMap is ideal for private data associated with DOM nodes or class instances - when the object goes away, the associated data disappears automatically.

updated Nov 8, 2026

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WeakMap and WeakSet#

WeakMap

WeakSet

When to use

vs regular Map/Set

WeakMap and WeakSet