网页下拉一屏后显示返回顶部按钮

按钮样式：

 

需要加载jq

 

<script>

(function(a){a.fn.scrollToTop=function(c){var d={speed:800};c&&a.extend(d,{speed:c});return this.each(function(){var b=a(this);a(window).scroll(function(){100<a(this).scrollTop()?b.fadeIn():b.fadeOut()});b.click(function(b){b.preventDefault();a("body, html").animate({scrollTop:0},d.speed)})})}})(jQuery);$(function(){ahtml='<a href="javascript:void(0)" id="toTop" style="display:none;position:fixed;bottom:66px;right:10px;width:45px;height:45px;border-radius:50%;overflow:hidden;background-image:url(\'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABcAAAAWBAMAAADZWBo2AAAALVBMVEUAAAB5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl4rtNiAAAADnRSTlMARHe7Zu7dMxGIIqqZzHSj3DwAAAB/SURBVBjTYwADPgYk8OABgs2HLPUAjBA6+JAk4FJ8UJLqYLKxsQNTXhIDs/GWBoZCPcEFeop6CnyKvhMYGOQYGJIYmBL4BBgfgDjsrxi4nvMJsCSAOCChh3yHjjqAZV4wcDznO6TFANYTwsASwCfAAOFMFRCdAOd0v3vdAOIAANnHHKk0/kXuAAAAAElFTkSuQmCC\');background-repeat:no-repeat;background-position:center;z-index:999;cursor:pointer;border:1px solid #d8d8d8;box-sizing:border-box;opacity:0.9;"></a>';$("body").append(ahtml);$("#toTop").scrollToTop(300);});

</script>