Use JQUERY to achieve variable transparency return to top effect

<a href="#0" class="cd-top">Top</a>

.cd-top {
  display: inline-block;
  height: 40px;
  width: 40px;
  position: fixed;
  bottom: 40px;
  right: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.05);
  /* image replacement properties */
  overflow: hidden;
  text-indent: 100%;
  white-space: nowrap;
  background: rgba(232, 98, 86, 0.8) url(../images/cd-top-arrow.svg) no-repeat center 50%;
  visibility: hidden;
  opacity: 0;
  -webkit-transition: all 0.3s;
  -moz-transition: all 0.3s;
  transition: all 0.3s;
}
.cd-top.cd-is-visible {
  /* the button becomes visible */
  visibility: visible;
  opacity: 1;
}
.cd-top.cd-fade-out {
  /* 如果用户继续向下滚动,这个按钮的透明度会变得更低 */
  opacity: .5;
}
.no-touch .cd-top:hover {
  background-color: #e86256;
  opacity: 1;
}
@media only screen and (min-width: 768px) {
  .cd-top {
    right: 20px;
    bottom: 20px;
  }
}
@media only screen and (min-width: 1024px) {
  .cd-top {
    height: 60px;
    width: 60px;
    right: 30px;
    bottom: 30px;
  }
}

<script type="text/javascript" src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>

$(document).ready(function($){
	// browser window scroll (in pixels) after which the "back to top" link is shown
	var offset = 300,
		//browser window scroll (in pixels) after which the "back to top" link opacity is reduced
		offset_opacity = 1200,
		//duration of the top scrolling animation (in ms)
		scroll_top_duration = 700,
		//grab the "back to top" link
		$back_to_top = $('.cd-top');

	//hide or show the "back to top" link
	$(window).scroll(function(){
		( $(this).scrollTop() > offset ) ? $back_to_top.addClass('cd-is-visible') : $back_to_top.removeClass('cd-is-visible cd-fade-out');
		if( $(this).scrollTop() > offset_opacity ) { 
			$back_to_top.addClass('cd-fade-out');
		}
	});
	//www.sucaijiayuan.com
	//smooth scroll to top
	$back_to_top.on('click', function(event){
		event.preventDefault();
		$('body,html').animate({
			scrollTop: 0 ,
		 	}, scroll_top_duration
		);
	});

});