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WeakRef

A WeakRef object, added in ECMAScript 2021, holds a weak reference to a target object — one that does not, by itself, keep that object alive in memory. Call .deref() to obtain the target object, or undefined if it has already been garbage-collected.

let cache = new Map();

function getData(key, load) {
  let ref = cache.get(key);
  let value = ref && ref.deref();
  if (value !== undefined) return value;
  value = load();
  cache.set(key, new WeakRef(value));
  return value;
}

let cache = new Map();

function getData(key, load) {
  let ref = cache.get(key);
  let value = ref && ref.deref();
  if (value !== undefined) return value;
  value = load();
  cache.set(key, new WeakRef(value));
  return value;
}

Because garbage collection timing is not observable or guaranteed, WeakRef should be reached for only as a last resort, eg. for caches and mapping tables, and never to implement functionality that is otherwise observable from a script (the target's disappearance is inherently non-deterministic, and some engines may never collect it at all within a short-lived program).