weak-value.js

class WeakValue extends Map {
  #registry = new FinalizationRegistry(key => {
    if (this.has(key) && !this.get(key).deref())
      this.delete(key);
  });
  get(key) {
    const value = super.get(key);
    return value && value.deref();
  }
  set(key, value) {
    this.#registry.register(value, key);
    return super.set(key, new WeakRef(value));
  }
}