mediator-fn.js

// user module
const receive = (_user, sender, message) => {
  console.log(`${new Date().toLocaleString()} [${sender}]: ${message}`);
};

// chat room module
const createChatRoom = (users = []) => ({
  users
});

const addUser = (chatroom, user) => ({
  ...chatroom,
  users: [...chatroom.users, user]
});

const send = (chatroom, user, message) => {
  chatroom.users
    .filter(u => u !== user)
    .forEach(u => receive(u, user.name, message));
};

// usage
const user1 = { name: "John Doe" };
const user2 = { name: "Jane Doe" };

const chatroom = [user1, user2].reduce(
  (chatroom, user) => addUser(chatroom, user),
  createChatRoom()
);

send(chatroom, user1, "Hi there!");
send(chatroom, user2, "Hey!");