Emit cheatsheet

Server-side

io.on("connection", (socket) => {

  // basic emit back to sender
  socket.emit(/* ... */);

  // to all clients in the current namespace except the sender
  socket.broadcast.emit(/* ... */);

  // to all clients in room1 except the sender
  socket.to("room1").emit(/* ... */);

  // to all clients in room1 and/or room2 except the sender
  socket.to(["room1", "room2"]).emit(/* ... */);

  // to all clients in room1
  io.in("room1").emit(/* ... */);

  // to all clients in room1 and/or room2 except those in room3
  io.to(["room1", "room2"]).except("room3").emit(/* ... */);

  // to all clients in namespace "myNamespace"
  io.of("myNamespace").emit(/* ... */);

  // to all clients in room1 in namespace "myNamespace"
  io.of("myNamespace").to("room1").emit(/* ... */);

  // to individual socketid (private message)
  io.to(socketId).emit(/* ... */);

  // to all clients on this node (when using multiple nodes)
  io.local.emit(/* ... */);

  // to all connected clients
  io.emit(/* ... */);

  // to all clients, with one acknowledgement per client
  io.timeout(5000).emit("hello", "world", (err, responses) => {
    if (err) {
      // some clients did not acknowledge the event in the given delay
    } else {
      console.log(responses); // one response per client
    }
  });

  // WARNING: `socket.to(socket.id).emit()` will NOT work, as it will send to everyone in the room
  // named `socket.id` but the sender. Please use the classic `socket.emit()` instead.

  // with acknowledgement
  socket.emit("question", (answer) => {
    // ...
  });

  // without compression
  socket.compress(false).emit(/* ... */);

  // a message that might be dropped if the low-level transport is not writable
  socket.volatile.emit(/* ... */);

  // with timeout
  socket.timeout(5000).emit("my-event", (err) => {
    if (err) {
      // the other side did not acknowledge the event in the given delay
    }
  });
});

Client-side

// basic emit
socket.emit(/* ... */);

// with acknowledgement
socket.emit("question", (answer) => {
  // ...
});

// without compression
socket.compress(false).emit(/* ... */);

// a message that might be dropped if the low-level transport is not writable
socket.volatile.emit(/* ... */);

// with timeout
socket.timeout(5000).emit("my-event", (err) => {
  if (err) {
    // the other side did not acknowledge the event in the given delay
  }
});

Reserved events

On each side, the following events are reserved and should not be used as event names by your application:

• connect
• connect_error
• disconnect
• disconnecting
• newListener
• removeListener

// BAD, will throw an error
socket.emit("disconnecting");