How do I send data from my route handler to my socket.io function?

How do I send data from my route handler to my socket.io function? My code is as follows:

// Game
app.get('/game', (req, res) => {
    const cookies = req.cookies;
    const currentUser = cookies['current_user'];
    const roomName = cookies['room_name'];

    if (roomName) {
        res.render('pages/game', {
            room: roomName
        });
    } else {
        res.redirect('/login');
    }
});

io.of('/game').on('connection', socket => {
    console.log('a user connected');
    socket.on('disconnect', () => {
        console.log('user disconnected');
    });

    socket.join(roomName);

    io.of('/game').in(roomName).emit('join', currentUser);
    (async() => {
        const sockets = await io.of('/game').in(roomName).fetchSockets();
        console.log(sockets.length);
    })();
});

I would like the socket to join the room defined in the cookie in the route handler above, but I'm not sure how to get the room name into the socket.io function. I considered putting the socket.io stuff in a function and then passing the data as a parameter, but that would cause the same problem as just putting the socket.io stuff inside the route handler, which as I have learnt from these answers is not the right way to do things. Global variables also don't sound like a good idea - multiple users joining would interfere with each other's data. Any help would be greatly appreciated.