How does socket.to(username).emit('eventName',{}) work?

Question

Can someone please explain how does socket.to(username).emit('eventName',{}) work? Basically I want to know how it identifies the 'username' is logged in or not.

Answer 1

socket.to(room).emit(...) will emit messages to all the users that joined room using: socket.join(room).

By default, each socket joins a room identified by the socket id, that's why, you can also do: socket.to(socketId)

---

Without knowing your logic, username room will be empty if the user isn't logged in, and it will have the logged user if the user is online. Socket.io doesn't really know if the user is online or not, it only knows that there is an user in that room.

io.on('connection', (socket) => {
    console.log('New user connected');

    const username = getUsernameSomehow(); // logged user, ip, cookie, or whatever you like

    // When the user is online, it will join to 
    // a room named after his username
    socket.join(username);

    socket.on('send', (message) => {
        console.log(message);
        // Now you can send a message by only knowing the username
        socket.to(message.target).emit('message', message.text);
    });
});

If you don't join the user to username room, your code will never work.

Answer 2

First of all, it's not a username, it's a socket.id value or a room name that works in:

socket.to(roomname).emit(...)

The way it works is very socket that connects to your server is given a socket.id value and then that value is then added to a data structure the socket.io server keeps. Any times a socket disconnects that socket.id is removed from the data structure.

So, when you do this:

socket.to(roomname).emit(...)

socket.io looks up the roomname you pass in its internal data structure. If it's there, then it can get the socket from that data structure and can then send to it.

Because socket.io also automatically creates a room with the name of the socket.id that every socket is given, you can also do:

socket.to(socketID).emit(...)

And, using the same mechanism above, it will look for a room named with the socketID and because there is matching room for every socket that is connected, it will find that room and then send to the socket with that socketID.

---

socket.io itself does not have any notion of username. You would have to add that level of functionality yourself or get it from a cookie (if there's already a login cookie) when the socket.io connection is first established.