how to stop the loadFriendList when setWorker Value is"0"?

const [worker, setWorker] = useState('1');
 
function start() {
 setInterval(loadFriendList,5000);
}
useEffect(start, []);
 
function n() {
  setWorker("0");
  Alert.alert('check', worker);
}

Don't use strings as number it's an unnecessary hassle .

use useRef hook to store intervalID .

This useEffect hook fires whenever worker is change.

see inside the [] we've added worker variable. It's called dependancy array. Basically you're saying to useEffect look on these variables and fire if anything changes

useEffect(() => {
  if(worker == 0) intID.current && clearInterval(intId.current)
},[worker])

The useRef hook is holding on to the Interval you set up when page loading, to uniquely identify and run methods like clearInterval() in future

Finally It Should Look Like

const [worker, setWorker] = useState(1);
const intId = useRef(null)
 
useEffect(() =>{
  intId.current = setInterval(loadFriendList,5000);
  return () => clearInterval(intId.current)
}, []);
 
useEffect(() => {
  if(worker == 0) intId.current && clearInterval(intId.current)
},[worker])