Weak references

WeakMap

Keys must be objects; held weakly — if key object is collected, entry disappears.

const meta = new WeakMap();

function attachMeta(domNode, data) {
  meta.set(domNode, data);
  // when domNode is GC'd, meta entry goes too
}

Use when: metadata for DOM/object lifecycle tied to that object.

Don’t use when: you need a string-keyed cache (keys aren’t objects).

WeakRef + FinalizationRegistry (advanced)

const registry = new FinalizationRegistry((held) => {
  console.log('cleaned up', held);
});

function track(obj, id) {
  const ref = new WeakRef(obj);
  registry.register(obj, id);
  return ref;
}

Rare in app code — useful for custom pools and native interop.

flowchart TD
  strong[Strong reference] --> alive[Object kept alive]
  weak[WeakMap key] -.-> maybe[Object may be collected]

Weak does not fix strong leaks

If you also store the same object in a global Array, WeakMap won’t help.

Broken

const cache = new Map();
const weak = new WeakMap();
cache.set(id, hugeObject);
weak.set(hugeObject, meta); // hugeObject kept alive by Map key/value

Fixed

const weak = new WeakMap();
// only weak reference — no strong Map entry