function fanOutPost(post, followerCount, followers, feeds, celebrityPosts, threshold = 5) {
  // post = { id, author, timestamp }
  if (followerCount > threshold) {
    if (!celebrityPosts[post.author]) celebrityPosts[post.author] = [];
    celebrityPosts[post.author].push(post);
  } else {
    for (const follower of followers) {
      if (!feeds[follower]) feeds[follower] = [];
      feeds[follower].push(post);
    }
  }
}

function getFeed(userId, feeds, following, celebrityPosts, limit = 10) {
  let entries = [...(feeds[userId] || [])];
  for (const followed of following[userId] || []) {
    entries = entries.concat(celebrityPosts[followed] || []);
  }
  entries.sort((a, b) => b.timestamp - a.timestamp);
  return entries.slice(0, limit).map(p => p.id);
}

const feeds = {};
const celebrityPosts = {};
const following = {
  bob: ['alice', 'star'],
  carol: ['alice'],
  dave: ['alice'],
};

fanOutPost({ id: 'p1', author: 'alice', timestamp: 100 }, 3, ['bob', 'carol', 'dave'], feeds, celebrityPosts, 5);
fanOutPost({ id: 'p2', author: 'star', timestamp: 200 }, 6, ['bob', 'carol', 'dave', 'eve', 'frank', 'grace'], feeds, celebrityPosts, 5);
fanOutPost({ id: 'p3', author: 'alice', timestamp: 300 }, 3, ['bob', 'carol', 'dave'], feeds, celebrityPosts, 5);

console.log("bob's feed:", getFeed('bob', feeds, following, celebrityPosts));
console.log("carol's feed:", getFeed('carol', feeds, following, celebrityPosts));