What is the usage of “socket” in socket.io connection function in NodeJS?

Question

I am learning how to use socket.io in nodejs server. I see a example listed below and I would like to know what's the socket in connection function for? Is it a new socket object? And where's the 'id' from? I have googled it but still not understand its function. Thanks

Code:

io.sockets.on('connection', function (socket) { //the socket in "function(socket)"
    socket.on('session_id', function (session_id) { socket.session_id = session_id; })
    setInterval(function() {
        socket.emit('date', {'date': new Date()});
    }, 1000);
}

Answer 1

Let's rework your example a little bit to remove a nested function.

function connectionHandler (socket) { 
    socket.on('session_id', function (session_id) { 
        socket.session_id = session_id; 
    })
    setInterval(function() {
        socket.emit('date', {'date': new Date()});
    }, 1000);
 }

 io.sockets.on('connection', connectionHandler)

In your example, io.sockets is your socket.io server. io.sockets.on('connection', connectionHandler) registers a connection event handler function.

Then, anytime a client (probably code running in a user's browser) connects, your connectionHandler gets called with the socket object for the current connection.

socket.on('session_id') sets up an event handler to listen for messages sent from clients with emit('session_id','some message payload') .