// Scroll to top (Event Listenter)
document.querySelector('#to-top').onclick = () => {
  scrollTo(document.documentElement, 0, 500);
}

// Scroll to function
const scrollTo = (element, to, duration) => {
  let start = element.scrollTop,
      change = to - start,
      currentTime = 0,
      increment = 20;

  let animateScroll = () => {
    currentTime += increment;
    let val = Math.easeInOutQuad(currentTime, start, change, duration);
    element.scrollTop = val;
    if (currentTime < duration) {
      setTimeout(animateScroll, increment);
    }
  };

  animateScroll();
}

// Ease function
Math.easeInOutQuad = (t, b, c, d) => {
  t /= d / 2;
  if (t < 1) return c / 2 * t * t + b;
  t--;
  return -c / 2 * (t * (t - 2) - 1) + b;
};