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var _initBackTotopBtn = (backToTopBtnId) => {
	jQuery(window).scroll( (e) =>{
		if(window.pageYOffset >= 680) {
			jQuery(backToTopBtnId).show('slow');
		}
		else{
			jQuery(backToTopBtnId).hide('slow');
		}
	});
	jQuery(backToTopBtnId).click( (e) => {
		jQuery("html, body").animate({ scrollTop: 0 }, "slow");
		return false;
	});
}

<div class= "backToTop">
  <i class="material-icons">keyboard_arrow_up</i>
</div>

.backToTop {
  align-items: center;
  background: var(--pink);
  border-radius: 50%;
  bottom: 10px;
  color: #fff;
  display: flex;
  font-size: 50px;
  height: 50px;
  justify-content: center;
  padding: 16px 0;
  position: fixed;
  right: 10px;
  width: 50px;
  transition: all .5s ease;
}

.backToTop:hover {
  background: var(--main-text-color);
  cursor: pointer;
  transform: rotate(360deg);
}