socket.io emit only to one client code example

Example 1: socket.io emit to all other clients

// sending to sender-client only
socket.emit('message', "this is a test");

// sending to all clients, include sender
io.emit('message', "this is a test");

// sending to all clients except sender
socket.broadcast.emit('message', "this is a test");

// sending to all clients in 'game' room(channel) except sender
socket.broadcast.to('game').emit('message', 'nice game');

// sending to all clients in 'game' room(channel), include sender
io.in('game').emit('message', 'cool game');

// sending to sender client, only if they are in 'game' room(channel)
socket.to('game').emit('message', 'enjoy the game');

// sending to all clients in namespace 'myNamespace', include sender
io.of('myNamespace').emit('message', 'gg');

// sending to individual socketid
socket.broadcast.to(socketid).emit('message', 'for your eyes only');

// list socketid
for (var socketid in io.sockets.sockets) {}
 OR
Object.keys(io.sockets.sockets).forEach((socketid) => {});

Example 2: socket io emit to specific client

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