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// HTML
<button class="top" title="Наверх"><i class="far fa-angle-double-up"></i></button>

// JS
function btnUp() {

      let btnScroll = $('.top');

      $(window).scroll( () =>  {
         if ( $(this).scrollTop() > $(window).height() ) {
            btnScroll.addClass('active');
         } else {
            btnScroll.removeClass('active');
         }
      });

      btnScroll.on('click', () => {
         $('html, body').stop().animate({scrollTop:0}, 'slow', 'swing');
      });

   } btnUp();
   
// SASS
.top
  position: fixed
  bottom: 25px
  right: -100px
  background-color: #fafafa
  z-index: 12
  border-radius: 50%
  color: #666666
  font-size: 26px
  width: 50px
  height: 50px
  text-align: center
  cursor: pointer
  transition: all .3s ease
  opacity: .5
  display: flex
  justify-content: center
  align-items: center
  &:hover
    color: #fff
    background-color: $accent
    opacity: 1
  &.active
    right: 25px