Create a Map or a Set, and using "on connection" event set to it each connected socket, in reverse "once disconnect" event delete that socket from the Map we created earlier
import * as Server from 'socket.io';
const io = Server();
io.listen(3000);
const connections = new Set();
io.on('connection', function (s) {
connections.add(s);
s.once('disconnect', function () {
connections.delete(s);
});
});