动态css类返回顶部

.back-to-top {
  position: fixed;
  bottom: 20px;
  right: 20px;
  width: 40px;
  height: 40px;
  background-color: #000;
  color: #fff;
  border-radius: 50%;
  text-align: center;
  line-height: 40px;
  cursor: pointer;
  opacity: 0;
  transition: opacity 0.3s ease-in-out;
}

.back-to-top.show {
  opacity: 1;
}

<a href="#" class="back-to-top">↑</a>

window.addEventListener('scroll', function() {
  var button = document.querySelector('.back-to-top');
  if (window.pageYOffset > 100) {
    button.classList.add('show');
  } else {
    button.classList.remove('show');
  }
});