JS判断URL资源不可用

<script language="javascript">
	function getURL(url) {
		var xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
		xmlhttp.open("GET", url, false);
		xmlhttp.send();
		if (xmlhttp.readyState == 4) {
			if (xmlhttp.Status != 200)
				alert("不存在");
			return xmlhttp.Status == 200;
		}
		return false;
	}
</script>
<a href="http://www.baitw.com:7080/images/zhenka_r1_c20.jpg"
	onclick="return   getURL(this.href)">csdn</a>