socket io emit to specific client

                                    io.on('connection', socket => {  socket.on('disconnecting', () => {    const rooms = Object.keys(socket.rooms);    // the rooms array contains at least the socket ID  });  socket.on('disconnect', () => {    // socket.rooms === {}  });});

                                    io.to(socketid).emit('message', 'for your eyes only');

                                    Let me make it simpler with socket.io rooms. request a server 
with a unique identifier to join a server. here we are using 
an email as a unique identifier.

Client Socket.io
socket.on('connect', function () {
  socket.emit('join', {email: [email protected]});
});
When the user joined a server, create a room for that user

Server Socket.io
io.on('connection', function (socket) {
   socket.on('join', function (data) {    
    socket.join(data.email);
  });
});
Now we are all set with joining. let emit something to from the 
server to room, so that user can listen.

Server Socket.io
io.to('[email protected]').emit('message', {msg: 'hello world.'});

Let finalize the topic with listening to message event to the 
client side
socket.on("message", function(data) {
  alert(data.msg);
});

The reference from Socket.io rooms

                                    io.on('connection', socket => {  socket.on('say to someone', (id, msg) => {    socket.to(id).emit('my message', msg);  });});

                                    io.on('connect', onConnect);function onConnect(socket){  // sending to the client  socket.emit('hello', 'can you hear me?', 1, 2, 'abc');  // sending to all clients except sender  socket.broadcast.emit('broadcast', 'hello friends!');  // sending to all clients in 'game' room except sender  socket.to('game').emit('nice game', "let's play a game");  // sending to all clients in 'game1' and/or in 'game2' room, except sender  socket.to('game1').to('game2').emit('nice game', "let's play a game (too)");  // sending to all clients in 'game' room, including sender  io.in('game').emit('big-announcement', 'the game will start soon');  // sending to all clients in namespace 'myNamespace', including sender  io.of('myNamespace').emit('bigger-announcement', 'the tournament will start soon');  // sending to a specific room in a specific namespace, including sender  io.of('myNamespace').to('room').emit('event', 'message');  // sending to individual socketid (private message)  io.to(socketId).emit('hey', 'I just met you');  // 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.  // sending with acknowledgement  socket.emit('question', 'do you think so?', function (answer) {});  // sending without compression  socket.compress(false).emit('uncompressed', "that's rough");  // sending a message that might be dropped if the client is not ready to receive messages  socket.volatile.emit('maybe', 'do you really need it?');  // specifying whether the data to send has binary data  socket.binary(false).emit('what', 'I have no binaries!');  // sending to all clients on this node (when using multiple nodes)  io.local.emit('hi', 'my lovely babies');  // sending to all connected clients  io.emit('an event sent to all connected clients');};