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