User following/followers

mock-user.js

class User {
  id = undefined;
  username = undefined;
  following = [];
  followers = [];

  constructor({ id, username }) {
    this.id = id;
    this.username = username;
  }

  follow(user) {
    if (this === user) throw Error("Cannot follow yourself");
    this.following.push(user);
    user.followers.push(this);
  }

  get following() {
    return this.following;
  }

  get followers() {
    return this.followers;
  }
}

const user1 = new User({ id: 1, username: "foo" });
const user2 = new User({ id: 3, username: "bar" });
const user3 = new User({ id: 4, username: "doo" });
const user4 = new User({ id: 5, username: "wee" });
const user5 = new User({ id: 6, username: "buz" });

user1.follow(user2);
user1.follow(user4);
user1.follow(user5);
user2.follow(user1);
user3.follow(user2);
user3.follow(user5);
user5.follow(user2);