js效果之回到顶部

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
    </head>
    <style>
        .up {
            display: none; width: 100px; height: 100px; background: red;
            position: fixed; right: 10px; bottom: 20px;
        }
    </style>
    <script>
        var num = 0;
        window.onscroll = function(){
            var up = document.getElementById("top");
            //当前页面滚动的距离??
            var _top = document.body.scrollTop || document.documentElement.scrollTop;
            if(_top >= 1000){
                up.style.display = "block";
            } else {
                up.style.display = "none";
            }
        }
        function goTop(){
            document.body.scrollTop = document.documentElement.scrollTop = 0;
        }
    </script>
    <body>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <p>aaaaaaaaaaaaaaaaaaaaaaa</p>
        <div id="top" class="up" onclick="goTop()" >回到顶部</div>
    </body>
</html>

  

 
 
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