https://leetcode.com/problems/design-twitter/

designTwitter.js

/**
 * Initialize your data structure here.
 */
var Twitter = function() {
    this.tweets = {}; // 1: [[5,0]]
    this.following = {};
    this.tweetCount = 0;
};

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

/**
 * 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) {
    if(!this.following.hasOwnProperty(userId)) this.following[userId] = new Set();
    if(!this.tweets[userId]) this.tweets[userId] = [];
    
    const followedTweets = [...this.tweets[userId]];
    for(let followee of this.following[userId]){
        followedTweets.push(...(this.tweets[followee] || []));
    }
    
    const seenTweets = new Set();
    
    return followedTweets
    .sort((a, b) => b[1] - a[1]) // desc order of recency
    .filter(([tweetId, _]) => { // remove duplicates
        if(seenTweets.has(tweetId)) return false;
        seenTweets.add(tweetId);
        return true;
    })
    .slice(0, 10) // pick the first 10
    .map(([tweetId, _]) => tweetId); // fetch just the tweets
        
};

/**
 * 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.following.hasOwnProperty(followerId)) this.following[followerId] = new Set();
    this.following[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.following.hasOwnProperty(followerId)) this.following[followerId] = new Set();
    this.following[followerId].delete(followeeId);
};

/** 
 * Your Twitter object will be instantiated and called as such:
 * var obj = new Twitter()
 * obj.postTweet(userId,tweetId)
 * var param_2 = obj.getNewsFeed(userId)
 * obj.follow(followerId,followeeId)
 * obj.unfollow(followerId,followeeId)
 */