Socket.io and Rooms

January 29, 2016

Using socket.io on the server side, we have the ability to emit an event to a particular socket, even outside of an event-emitter from that socket, if we know the id of the socket we are emitting to. Suppose the socket id is '7s6df58sg9as567d', one might logically conclude that this syntax would work:

var io = require('socket.io').listen(server);
io.to('7s6df58sg9as567d').emit('MESSAGE');

There is a caveat here; this won’t actually work.

For individual sockets, it is necessary to add /# in front of the socket id:

io.to('/#7s6df58sg9as567d').emit('MESSAGE');

Alternatively, if we wanted to emit a message to a room, the /# is not needed, and would take the following form:

io.to('ROOM_NAME').emit('MESSAGE');

Now you know, not to spend far too much time trying to debug this error should you run into it.