如何使用javascript(没有jquery)动画滚动到页面顶部?

// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {
  scrollFunction()
};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    document.getElementById("myBtn").style.display = "block";
  } else {
    document.getElementById("myBtn").style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}

body {
  font-family: Arial, Helvetica, sans-serif;
  font-size: 20px;
  background: #ffffff;
}

#myBtn {
  display: none;
  position: fixed;
  bottom: 20px;
  right: 30px;
  z-index: 99;
  background-color: red;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

#myBtn:hover {
  background-color: #555;
}

<button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>
<div style="padding:30px">Scroll Down</div>
<div style="padding:30px 30px 700px">This example demonstrates how to create a "scroll to top" button that becomes visible when the user starts to scroll the page.</div>