React -> Use Previous State Values

usePrevious.js

// From https://reactjs.org/docs/hooks-faq.html#how-to-get-the-previous-props-or-state
function Counter() {
  const [count, setCount] = useState(0);

  const prevCountRef = useRef();
  useEffect(() => {
    prevCountRef.current = count;
  });
  const prevCount = prevCountRef.current;

  return <h1>Now: {count}, before: {prevCount}</h1>;
}

// Alternatively, extract to a custom hook:
function Counter() {
  const [count, setCount] = useState(0);
  const prevCount = usePrevious(count);
  return <h1>Now: {count}, before: {prevCount}</h1>;
}

function usePrevious(value) {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
}

This is kind of unnecessary now since there's a usePrevious hook in @fluentui/react-hooks