handler(...data, [ackFn])

Socket event listener handler expects one or more data arguments and an optional ACK callback.

// Client:
socket.on("news": function handler(data, ackCb){
  console.log("received some news", data);
  ackCb("Acknowledged", "thank you");
});

// Server:
server.emit("news", {some: "news here"}, function ackFn(...data){
  console.log("Client acknowledged data receiving")
});