javascript判断文件是否存在

<script language="javascript">

function FileExist()

{

　　var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

　　xmlhttp.open("GET",FileURL,false);

        xmlhttp.send();

        if(xmlhttp.readyState==4)

　　{  

         　　if(xmlhttp.status==200) alert("Exist");  

          　  else if(xmlhttp.status==404) alert("Doesn't exist");

            　else alert("Don't know"); 

     　}

 }

</script>