jquery,返回到顶部按钮

HTML:

<footer>
  <a href="#" class="top">&uarr;</a>
</footer>

CSS:

.top:hover{
    opacity:1;
    transition:1s;
}

JS:

 $(document).ready(function() {
     var offset=250, // At what pixels show Back to Top Button
     scrollDuration=300; // Duration of scrolling to top
     $(window).scroll(function() {
         if ($(this).scrollTop() > offset) {
              $('.top').fadeIn(500); // Time(in Milliseconds) of appearing of the Button when scrolling down.
         } else {
         　　　$('.top').fadeOut(500); // Time(in Milliseconds) of disappearing of Button when scrolling up.
         }
     });

     // Smooth animation when scrolling
     $('.top').click(function(event) {
     　　event.preventDefault();
        $('html, body').animate({
             scrollTop: 0}, scrollDuration);
         })
     });

Link