back-to-top回到顶部

function backTop(back) {
    back.hide();
    $(window).scroll(function () {
        $(window).scrollTop() > 0 ? back.fadeIn(500) : back.fadeOut(500);
    });
    back.click(function () {
        $('body,html').animate({scrollTop: 0}, 500);
        return false;
    });
}

或者class名操作显示隐藏淡入淡出

function backTop(obj,options) {
    var options = options||{};
   var offset = options.offset||300;
   var duration = options.duration||500;
   var $back = obj;
       $(window).scroll(function(){
           ( $(this).scrollTop() > offset ) ? $back.show() : $back.hide();
       });
       //smooth scroll to top
       $back.on('click', function(event){
           event.preventDefault();
           $('body,html').animate({
                   scrollTop: 0 ,
               }, duration
           );
           return false;
       });
   }