js控制的回到页面顶端goTop的代码实现

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(document.compatMode && document.compatMode == \'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;