有没有见过很多在页面的右下角有个【回到顶端】的悬浮东东，并且在开始时没有，一移动滚动条就出现，回到了顶端又消失的样子。
像:
控制的js代码如下：
function goTop(){var _btn = document.getElementById("goTop");if (document.documentElement && document.documentElement.scrollTop) {var _con = document.documentElement;} else if (document.body) {var _con = document.body;}window.onscroll = set;_btn.style.left = document.documentElement.scrollWidth/2 + 500 + "px";_btn.onclick = function (){_btn.style.display = "none";window.onscroll = null;this.timer = setInterval(function() {_con.scrollTop -= Math.ceil(_con.scrollTop * 0.3);if (_con.scrollTop == 0) clearInterval(_btn.timer, window.onscroll = set);},10);};function set() {_btn.style.display = _con.scrollTop ? 'block': "none";}};document.write("<div id=\"goTop\" style=\"font-size:12px; width:20px; height:55px; background:#0950A2; color:#fff; line-height:14px; position:fixed; bottom:30px; display:none; cursor:pointer; text-align:center; padding:5px 0; z-index:1; _position:absolute; _top:expression(eval(patMode && patMode == \'CSS1Compat\') ? documentElement.scrollTop + (documentElement.clientHeight - this.clientHeight) - 30 : document.body.scrollTop + (document.body.clientHeight - this.clientHeight) - 30);\">返<br />回<br />顶<br />部</div>");window.onscroll = goTop;
