js页面导行子菜单的读取

<script type="text/javascript">
         /***控制菜单显示***/
         startList = function () {
             if (document.all && document.getElementById) {
                 navRoot = document.getElementById("nav");//nav 是ul 的id
                 for (i = 0; i < navRoot.childNodes.length; i++) {
                     node = navRoot.childNodes[i];
                     if (node.nodeName == "LI") {
                         node.onmouseover = function () {
                             this.className += " over";
                         }
                         node.onmouseout = function () {
                             this.className = this.className.replace(" over", "");
                         }
                     }
                 }
             }
         }
         window.onload = startList;
    </script>