io.emit vs socket.emit

• socket.emit will send back message to sender only,
• io.emit will send message to all the client including sender
• if you want to send message to all but not back to sender then socket.broadcast.emit

Here's a supplementary documentation for reference :

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

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

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

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

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

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

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

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

socket.emit(); //send to all connected clients

socket.broadcast.emit(); //send to all connected clients except the one that sent the message

socket.on(); //event listener, can be called on client to execute on server

io.sockets.socket(); //for emiting to specific clients

io.sockets.emit(); //send to all connected clients (same as socket.emit)

io.sockets.on() ; //initial connection from a client.

Hope this helps!.

Edit: socket.io docs also contain similar cheatsheet.

The io variable represents the group of sockets. The code you have starts on line one with providing a function in the second parameter that gives you a socket variable every time a new connection is made. The socket variable is only for communicating with each individual connection. You may not see it in the code but there will be one socket variable for each connection established