<button onclick="topFunction()" id="scrollToTopBtn" title="Go to top"></button>
<script>
// Show or hide Scrolltotop button
function ToggleScrollToTop() {
  if (document.body.scrollTop > 1000 || document.documentElement.scrollTop > 1000) {
    document.getElementById("scrollToTopBtn").style.display = "block";
  } else {
    document.getElementById("scrollToTopBtn").style.display = "none";
  }
}

// Check every defined value millisecond 
setInterval( ToggleScrollToTop, 250 );
// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0; // For Safari
  document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
}
</script>

#scrollToTopBtn{
  background:url('https://ec.europa.eu/digital-building-blocks/sites/download/attachments/82773348/BackToTop.png?api=v2') no-repeat left center;
  display: none;
  position: fixed;
  bottom: 20px;
  right: 20px;
  z-index: 99;
  border: none;
  outline: none;
  background-color: #bbb;
  color: white;
  cursor: pointer;
  width: 50px;
  height: 50px;
  border-radius: 10px;
  font-size: 18px;
}

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