css回到顶部按钮

jQuery(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 = 800,
//grab the "back to top" link
$back_to_top = $(&＃39;.cd-top&＃39;);
//hide or show the "back to top" link
$(window).scroll(function(){
( $(this).scrollTop() > offset ) ? $back_to_top.addClass(&＃39;cd-is-visible&＃39;) : $back_to_top.removeClass(&＃39;cd-is-visible cd-fade-out&＃39;);
if( $(this).scrollTop() > offset_opacity ) {
$back_to_top.addClass(&＃39;cd-fade-out&＃39;);
}
});
//smooth scroll to top
$back_to_top.on(&＃39;click&＃39;, function(event){
event.preventDefault();
$(&＃39;body,html&＃39;).animate({
scrollTop: 0 ,
}, scroll_top_duration
);
});
});

.cd-top {
display: inline-block;
height: 40px;
width: 40px;
position: fixed;
bottom: 20px;
right: 20px;
box-shadow: 0 0 10px rgba(0, 0, 0, 0.05);
overflow: hidden;
text-indent: 100%;
white-space: nowrap;
background: rgba(3, 165, 225, 0.8) url(images/cd-top-arrow.svg) no-repeat center 50%;
visibility: hidden;
opacity: 0;
-webkit-transition: opacity .3s 0s, visibility 0s .3s;
-moz-transition: opacity .3s 0s, visibility 0s .3s;
transition: opacity .3s 0s, visibility 0s .3s;
z-index: 1;
}
.cd-top.cd-is-visible, .cd-top.cd-fade-out, .no-touch .cd-top:hover {
-webkit-transition: opacity .3s 0s, visibility 0s 0s;
-moz-transition: opacity .3s 0s, visibility 0s 0s;
transition: opacity .3s 0s, visibility 0s 0s;
}
.cd-top.cd-is-visible {
visibility: visible;
opacity: 1;
}
.cd-top.cd-fade-out {
}
.cd-top:hover {
opacity: 0.5;
}
.no-touch .cd-top:hover {
background-color: #e86256;
opacity: 1;
}
/* Device specific conditional statements */
@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;
}
}