useeffect after component mount code example

Example 1: componentdidupdate in hooks

const App = props => {
  const didMountRef = useRef(false)
  useEffect(() => {
    if (didMountRef.current) {
      doStuff()
    } else didMountRef.current = true
  }
}

Example 2: clean up useeffect

useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    // Specify how to clean up after this effect:
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });