socket.io中的socket变量是什么

Question

I'm new to Node.js and socket.io.

I go official site of socket.io, and try a tutorial. http://socket.io/get-started/chat/

It work correctly on my computer. But I can't understand essence of its code at all.

QUESTION:what is "socket variable" in below code. And Where is it from?

io.on('connection', function(socket){
  socket.on('chat message', function(msg){
    io.emit('chat message', msg);
  });
});

Maybe "msg variable" is String type variable. And it is from client side, right?

Answer 1

The socket variable is basically a socket to a client. The io.on('conection') will be called each time a new client connects and give a socket to subscribe to.

socket.on('chat message' will subscribe on events sent by client associated with that socket instance.

Here is the different parts of your program explained:

io.on('connection', function(socket){ // Waiting for new clients to connect, then return a socket instance
// msg can be any javascript object like a string or array ect.
  socket.on('chat message', function(msg){ // Subscribe on event 'chat message' will be called when client do io.emit
    io.emit('chat message', msg); // Server send to clients
  });
});

Answer 2

Type of the variable msg is not exactly String but more like whatever emitted in the 'chat message' event. You don't have to emit only strings, you can emit objects too. In the context of the example, yes. It is from the client side.

The variable socket though, socket is the connection between client and the server. If the server receives a 'chat message' event from that connection, or rather 'socket', it emits a 'chat message' event on all the sockets that it has. The content of the 'chat message' event that server emits to all sockets that are connected to it is the same as the content of the 'chat message' event that client, which sent the chat message, emitted to the server in the first place.