fetch-user.ts

interface User {
  id: number;
}

const storageCache = new Map<number, Promise<User>>();

function fetchUser(id: number): Promise<User> {
  console.log('id', id)
  return new Promise((res) => {
    setTimeout(() => res({ id }), 2000);
  });
}

function fetchUserCached(id: number): Promise<User> {
  if (!storageCache.has(id)) {
    try {
      const user = fetchUser(id);
      storageCache.set(id, user);
    } catch (e) {
      throw e;
    }
  }

  return storageCache.get(id)!;
}

fetchUserCached(1);
fetchUserCached(1);
fetchUserCached(1);