如何让菜单层固定

js部份：

function toolbar(el) {
  el = typeof el == 'string' ? document.getElementById(el) : el;
  var elTop = el.offsetTop;
  var sTop = 0;
  window.onscroll = function () {
  sTop = document.body.scrollTop || document.documentElement.scrollTop;
  if (sTop > elTop) {
	  el.style.top = "0";
	  el.style.position = "fixed";
	  document.getElementById("zuanti").style.marginTop="50px";
  } else {
	  el.style.top = elTop + 'px';
	  el.style.position = "absolute";
	  document.getElementById("zuanti").style.marginTop="50px";
	  }
  }
  }
toolbar('nav');

html部份：

<div id="nav">
	<ul>
	<li><a href="http://www.xingyunba.com/" title="首页">姓名测试</a></li>
	<li><a href="http://www.xingyunba.com/sm/">免费算命</a></li>
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	<li><a href="http://www.xingyunba.com/astro/">十二星座</a></li>
	<li><a href="http://www.xingyunba.com/shuxiang/">十二生肖</a></li>
	<li><a href="http://www.xingyunba.com/paipan/">在线排盘</a></li>
	<li><a href="http://www.xingyunba.com/fengshui/">风水知识</a></li>
	<li><a href="http://www.xingyunba.com/qinglv/">缘分测试</a></li>
	<li><a href="http://www.xingyunba.com/zgjm/">周公解梦</a></li>
	<li><a href="http://www.xingyunba.com/ask/" title="起名大师提供免费宝宝取名咨询">免费起名</a></li>
	</ul>
</div>