useCutoff.js

function useCutoff(x, cutoff) {
  // track each value of x between renders
  let xPrev = useRef(x);
  useEffect(() => { xPrev.current = x }, [x]);

  // if the cutoff function returns true, render the previously returned value.
  // else, synchronously schedule render withnew state value
  let [ret, setRet] = useState(x);
  if (!cutoff(xPrev.current, x)) {
    setRet(x)
  }
  return ret;
}

// Example usage:
// maintain current value of x if it is not increasing
let x = useCutoff(x, (prev, cur) => cur - prev <= 0);