Remove a realtime listener on firestore

I am kinda new on firebase and I don't quite understand how to detach a realtime listener. I already read the official documentation but still don't understand how to. If anyone could simplify it for me and create a function to start the listener and another one to stop it, it would be really awesome.

To start the listener I need just to call start()

function start(){
  firebase.firestore().doc("users/x").onSnapshot(function(doc) {
   console.log(doc.data());
  });
}

But to stop the listener I tried to call the function with noting in it, however it doesn't work

//Doesn't work
function stop(){
 firebase.firestore().doc("users/x").onSnapshot(function(doc) {
   //Not putting anything in here
  });
 }

The following should do the trick:

var fbListener = null;

function start(){
  fbListener = firebase.firestore().doc("users/x").onSnapshot(function(doc) {
   console.log(doc.data());
  });
}

function stop(){
  fbListener();
}

Have a look at the docs here and here: onSnapshot returns "an unsubscribe function that can be called to cancel the snapshot listener".