debounce.js

function throttle(fn, delay) {
  let last;
  let timer;

  return () => {
    const now = +new Date;

    if (last && now < last + delay) {
      clearTimeout(timer);

      timer = setTimeout(() => {
        last = now;
        fn();
      }, delay);
    } else {
      last = now;
      fn();
    }

  };
}

window.addEventListener('scroll', throttle(onScroll, 25));