[Coding Test] LeetCode - 355. Design Twitter

Example:

Twitter twitter = new Twitter();

// User 1 posts a new tweet (id = 5).
twitter.postTweet(1, 5);

// User 1's news feed should return a list with 1 tweet id -> [5].
twitter.getNewsFeed(1);

// User 1 follows user 2.
twitter.follow(1, 2);

// User 2 posts a new tweet (id = 6).
twitter.postTweet(2, 6);

// User 1's news feed should return a list with 2 tweet ids -> [6, 5].
// Tweet id 6 should precede tweet id 5 because it is posted after tweet id 5.
twitter.getNewsFeed(1);

// User 1 unfollows user 2.
twitter.unfollow(1, 2);

// User 1's news feed should return a list with 1 tweet id -> [5],
// since user 1 is no longer following user 2.
twitter.getNewsFeed(1);

// @lc code=start
/**
 * Initialize your data structure here.
 */
var Twitter = function() {
  this.tweets = {};
  this.followers  = {};
  this.time = 0;
};

/**
 * Compose a new tweet. 
 * @param {number} userId 
 * @param {number} tweetId
 * @return {void}
 */
Twitter.prototype.postTweet = function(userId, tweetId) {
  if(!this.tweets[userId]) this.tweets[userId] = [];
  this.tweets[userId].push({
    tweetId,
    dateTime: ++this.time,
  });
};

/**
 * Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent. 
 * @param {number} userId
 * @return {number[]}
 */
Twitter.prototype.getNewsFeed = function(userId) {
  const userFeed = this.tweets[userId] ? this.tweets[userId] : [];
  const followersFeed = this.followers[userId] ? [...this.followers[userId]].filter((followerId) => {
    return followerId !== userId;
  }).reduce((feed, followerId) => {
    const followerFeed = this.tweets[followerId] || []; 
    return [...feed, ...followerFeed];
  }, []) : [];
  
  let feed = [...userFeed, ...followersFeed].sort((a, b) => {
    return a.dateTime - b.dateTime;
  }).reverse().slice(0, 10).map((feed) => feed.tweetId);

  return feed;
};

/**
 * Follower follows a followee. If the operation is invalid, it should be a no-op. 
 * @param {number} followerId 
 * @param {number} followeeId
 * @return {void}
 */
Twitter.prototype.follow = function(followerId, followeeId) {
  if(!this.followers[followerId]) this.followers[followerId] = new Set();
  this.followers[followerId].add(followeeId)
};

/**
 * Follower unfollows a followee. If the operation is invalid, it should be a no-op. 
 * @param {number} followerId 
 * @param {number} followeeId
 * @return {void}
 */
Twitter.prototype.unfollow = function(followerId, followeeId) {
  if(!this.followers[followerId]) return;
  else this.followers[followerId].delete(followeeId);
};

// 23/23 cases passed (156 ms)
// Your runtime beats 53.52 % of javascript submissions
// Your memory usage beats 72.97 % of javascript submissions (45.6 MB)