I'm currently studying basic javascript features, and I am quite struggling with the arrow function method. So I from what I've learnt, if there is only one parameter, it doesn't matter if we use parenthesis or not like the code below:

(singleParam) => { statements }
singleParam => { statements }

Then when emitting events in socket IO, to simplify the codes

// server-side
io.on("connection", (socket) => {
  socket.emit("hello", "world");
});

// client-side
socket.on("hello", (arg) => {
  console.log(arg); // world
});

could be simplified as

// server-side
io.on("connection", socket => {
  socket.emit("hello", "world");
});

// client-side
socket.on("hello", arg => {
  console.log(arg); // world
});

This is working fine in my project, thus I wonder the necessity of using parenthesis when using arrow method function. Maybe I'm not fully grasping the concepts, can anyone explain this method a little bit easier way? Thanks.