滚动后,将元素放置在屏幕中间

<div id="myElement">我是要放置在屏幕中间的元素</div>

#myElement {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

window.addEventListener('scroll', function() {
  var element = document.getElementById('myElement');
  element.style.top = (window.pageYOffset + window.innerHeight / 2) + 'px';
  element.style.left = (window.pageXOffset + window.innerWidth / 2) + 'px';
});