<a id="scroll_top_button"><i class="fas fa-arrow-up"></i></a>
<style>
#scroll_top_button{
  display: inline-block;
  background-color: #FF9800;
  width: 50px;
  height: 50px;
  text-align: center;
  border-radius: 4px;
  position: fixed;
  bottom: 30px;
  right: 30px;
  transition: background-color .3s, 
  opacity .5s, visibility .5s;
  opacity: 0;
  visibility: hidden;
  z-index: 1000;
}
#scroll_top_button:hover {
  cursor: pointer;
  background-color: #333;
}
#scroll_top_button:active {
  background-color: #555;
}
#scroll_top_button.show {
  opacity: 1;
  visibility: visible;
}

#scroll_top_button i{
color: #ffffff;
font-size: 30px;
line-height: 50px;
}
</style>

<script>
var btn = $('#scroll_top_button');

$(window).scroll(function() {
  if ($(window).scrollTop() > 300) {
    btn.addClass('show');
  } else {
    btn.removeClass('show');
  }
});

btn.on('click', function(e) {
  e.preventDefault();
  $('html, body').animate({scrollTop:0}, '300');
});
</script>