两种方法，其实一样，第一种：

<script>
function getRootPath(){
var strFullPath=window.document.location.href;
var strPath=window.document.location.pathname;
var pos=strFullPath.indexOf(strPath);
var prePath=strFullPath.substring(0,pos);
var postPath=strPath.substring(0,strPath.substr(1).indexOf('/')+1);
return(prePath+postPath);
}

alert (getRootPath());
</script>

第二种：

<script type="text/javascript">
function getRootPath()
{
var pathName = window.location.pathname.substring(1);
var webName = pathName == '' ? '' : pathName.substring(0, pathName.indexOf('/'));
return window.location.protocol + '//' + window.location.host + '/'+ webName + '/';
}
alert (getRootPath());
</script>