socketMiddleware.js

const attach = (io) => {
  let numUsers = 0;

  io.on('connection', (socket) => {
    let addedUser = false;

    // when the client emits 'new message', this listens and executes
    socket.on('new message', (data) => {
      // we tell the client to execute 'new message'
      socket.broadcast.emit('new message', {
        username: socket.username,
        message: data
      });
    });

    // when the client emits 'add user', this listens and executes
    socket.on('add user', (username) => {
      if (addedUser) return;

      // we store the username in the socket session for this client
      socket.username = username;
      ++numUsers;
      addedUser = true;
      socket.emit('login', {
        numUsers: numUsers
      });
      // echo globally (all clients) that a person has connected
      socket.broadcast.emit('user joined', {
        username: socket.username,
        numUsers: numUsers
      });
    });

    // when the user disconnects.. perform this
    socket.on('disconnect', () => {
      if (addedUser) {
        --numUsers;

        // echo globally that this client has left
        socket.broadcast.emit('user left', {
          username: socket.username,
          numUsers: numUsers
        });
      }
    });
  });
}

module.exports = attach;