WeakMap Usage

weakmap.js

const getSomeUserFromAPI = async () => {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve({
        id: 1,
        name: 'Juan',
        state: 'ACTIVE'
      });
    }, 100);
  });
};

const map = new WeakMap();
// map.set(1, 1); // Triggers an error. Keys must be objects

(async () => {
  const user = await getSomeUserFromAPI();
  map.set(user, user.state);

  console.log(map); // If you run it using node: WeakMap { <items unknown> }

  /**
   * If you run it in the browser: WeakMap {{…} => "ACTIVE"}.
   * The console uses the debugging API of the JS engine,
   * which allows access to the internals of objects
   * (also to promise states, wrapped primitives, etc.) and many more.
   *
   * https://stackoverflow.com/a/32543187/2599811
   */
  console.log(map);

  // for (let i of map) {} // Uncaught TypeError: map is not iterable
  console.log(map.get(user)); // ACTIVE
})();