求一段js返回顶部的代码

Question

当页面有滚动条并且滚动条不位于顶部时，出现返回顶部的图片或者按钮，返回顶部后自动消失，求大神指点

Answer 1

 <style>#toTop{position:fixed;right:0;bottom:20px;display:none}</style>
<div style="height:2000px"></div>
<a href="#" id="toTop">top↑</a>
<script>
    function winScroll() {
        var st = Math.max(document.body.scrollTop, document.documentElement.scrollTop);
        document.getElementById('toTop').style.display=st>=100?'block':'none'
    }
    var timer
    window.onscroll = function () {
        clearTimeout(timer);
        timer = setTimeout(winScroll, 10);
    }
</script>