Q:

Express - socket.io - session.  Refer to user id as socket id

I am using express-socket.io-session.  I think I was able to setup the basic config by seeing the tutorials:
//BASIC CONFIG? 

var clients = [];

var session = require("express-session")({
    secret: 'some key',
    resave: true,
    saveUninitialized: true
});
var sharedsession = require("express-socket.io-session");
app.use(session);

io.use(function(socket, next){
   next();
});

io.use(sharedsession(session, {
  autoSave:true
})); 

io.on('connection', function(socket) {

    console.log("CLIENT CONNECTED");
    var session = socket.handshake.session;
    clients.push(socket);

    socket.on('disconnect', function() {
       console.log("CLIENT DISCONNECTED");
    });

});

What I want to be able to do now is to refer to a specific client socket not by the socket but by the session id that should be assigned to that socket.  When a user logins this happens: 
req.session.user_id = user_id; 
//(user_id is equal to DB {0,1,2,3...} ids

I was able to send sockets to specific clients when I did this:
clients[0].emit("to_do",info); // I don't know who is client index 0 compared to the login reference...

I would like to be able to do this or similar:
user_id = 3; // which would have a socket assigned

clients(user_id).emit("to_do",info);

That would mean each client would have a socket assigned to its previously assigned id. How could I do this so I could specify the socket by that id?  I am not experienced at all with all of this so sorry for any big mistakes.  Thanks

A:

Your problem can be solved by each socket joining a group named after it's id:
socket.join(socket.id);

io.sockets.in(socket.id).emit('to_do', info);

//or

io.sockets.in(clients[0].id).emit('to_do', info);

