Simple es6 javascript chatroom fiddle

simple-chatroom.js

class ChatRoom {
  constructor(roomName) {
    if (!roomName)
      throw new Error(
        "ChatRoom name must be provided to create new chat room instance!"
      );
    this.roomName = roomName;
    this.roomMembers = [];
  }

  join(member) {
    if (!member) {
      throw new Error(
        "To join a chatroom you must provide a member object instance!"
      );
    }

    if (!this.roomMembers.includes(member.username)) {
      this.roomMembers.push(member.username);
      console.log(`${member.username} joined #${this.roomName}`);
    }

    return this;
  }

  leave(member) {
    if (!member) {
      throw new Error(
        "To leave a chatroom you must provide a member object instance!"
      );
    }
    if (!this.roomMembers.includes(member.username)) {
      throw new Error(`This member was not found in #${this.roomName}!`);
    }
    this.roomMembers = this.roomMembers.filter(
      (username) => username !== member.username
    );
    console.log(`${member.username} has left #${this.roomName}`);
  }

  send(toUsername, fromUsername, message) {
    if (!toUsername)
      throw new Error(
        "To direct message you must provide the room member username to send message!"
      );
    if (!fromUsername)
      throw new Error(
        "To direct message you must provide the room member username sending message!"
      );
    if (!message)
      throw new Error("To message you must provide a message to send!");
    console.log(`\nFrom: ${fromUsername}\nTo: ${toUsername} \n> ${message}\n`);
  }

  get currentRoomMembers() {
    return this.roomMembers;
  }
}

class Member {
  constructor(options) {
    if (!options.username)
      throw new Error("Options object must contain username property!");

    this.username = options.username;
    this.chatRoom = null;
  }

  joinChatRoom(chatRoomInstance) {
    this.chatRoom = chatRoomInstance.join(this);
  }

  leaveChatRoom() {
    this.chatRoom.leave(this);
  }

  send(toUsername, message) {
    if (!this.chatRoom)
      throw new Error("You must first join a chatroom to send message!");
    this.chatRoom.send(toUsername, this.username, message);
  }
}

const GeneralChat = new ChatRoom("General");

const John = new Member({
  username: "John",
});

const Jane = new Member({
  username: "Jane",
});

John.joinChatRoom(GeneralChat);
Jane.joinChatRoom(GeneralChat);

John.send("Jane", "How is the coffee?");
Jane.send("John", "Very good flavor!");

// console.log(GeneralChat.currentRoomMembers);

John.leaveChatRoom();
Jane.leaveChatRoom();