请问如何判断链接是否有效

1<input onclick="aa()" type="button" value="check"/>

 1<script>   
 2function aa()   
 3{   
 4a=new ActiveXObject("Microsoft.XMLHTTP")   
 5a.open("GET","http://expert.csdn.net/",false)   
 6a.send()   
 7if(a.status==200) alert("ok")   
 8else   
 9alert("http://expert.csdn.net/不ok")   
10  
11a=new ActiveXObject("Microsoft.XMLHTTP")   
12a.open("GET","http://expertaaaaaaaa.csdn.net/",false)   
13a.send()   
14if(a.status==200) alert("ok")   
15else   
16alert("http://expertaaaaaaaa.csdn.net/不ok")   
17}   
18</script>