js跨域

<script type="text/javascript">
var getHost = function(url) {
var host = "null";
if (typeof url == "undefined"
|| null == url)
url = window.location.href;
var regex = /.*\:\/\/([^\/|:]*).*/;
var match = url.match(regex);
if (typeof match != "undefined"
&& null != match) {
host = match[1];
}
if (typeof host != "undefined"
&& null != host) {
var strAry = host.split(".");
if (strAry.length > 1) {
host = strAry[strAry.length - 2] + "." + strAry[strAry.length - 1];
}
}
return host;
}
document.domain=getHost();
</script>