Socket.io

emit

When we want to send message from one party to another, we need to use the emit method

on

When we want to receive message from one party to another, we need to use the on handler

Example:

socket.emit("hello-world", {data: "Happy new year!"})

socket.emit takes the event name as the first argument and the payload or data as the second argument.

Broadcasting

To broacast to all clients except the sender

socket.broadcast.emit('talk-to-client', `Someone else said: ${message}`);

To broadcast to all clients including the sender

io.emit('talk-to-client', `Someone else said: ${message}`);

Emit With Acknowledgement

You can add a callback as the last argument of the emit(), and this callback will be called once the other side acknowledges the event.

Server Side

io.on('connection', (socket) => {
    socket.on('talk-to-server', async (message, callback) => {
        // CLIENT HAS RECEIVED AND PROCESSED THE MESSAGE SUCCESSFULLY
        await io.emitWithAck('talk-to-client', `Someone else said: ${message}`);
        callback("I (server) did the work.")
    });
})

Client Side

socket.on('talk-to-client', (message, callback) => {
    const div = document.createElement('div');
    div.className = "new-message";
    div.innerText = message;
    messageDiv.appendChild(div);
    callback("I (client) did the work.");
});

document.getElementById('send-message').addEventListener('click', async () => {
    // SERVER HAS RECEIVED AND PROCESSED THE MESSAGE SUCCESSFULLY
    const text = document.getElementById('message-input').value;
    await socket.emitWithAck('talk-to-server', text);
})

Rooms

A room is an arbitrary channel that sockets can join and leave. It can be used to broadcast events to a subset of clients.

1. You can call join to subscribe the socket to a given channel:

io.on("connection", (socket) => {
  socket.join("some room");
});

2. You can then use to or in (they are the same) when broadcasting or emitting:

io.to("some room").emit("some event");

3. Or exclude a room:

io.except("some room").emit("some event");

4. You can also emit to several rooms at the same time:

io.to("room1").to("room2").to("room3").emit("some event");

5. You can leave a room:

socket.leave(roomName);