Get the current socket id of an emitting socket

I have need to know the current socket ID and index of that socket for a given socket.emit() and corresponding socket.on(), but I could not find a good reference for how to do this in socket.io documentation.

For instance:

Client

socket.emit('myMessage', myVar);

Server

socket.on('myMessage', function(myVar){
  //Need the socket id of the client who just emitted
  //and whether or not they are the first socket, second socket, third socket, etc.
  // e.g. something like:

  currentSocketID = io.socket.connected[socketid]; //?? what is the correct syntax?
  currentSocketIndex = io.socket.connected[index[i]]; //?? what is correct syntax?

});

via Dshiz