怎么样使用javascript判断文件是否存在？

<script language="javascript">
function FileExist()
{
var sfso=new ActiveXObject("Scripting.FileSystemObject");
var fPath="[The path of the file]";
if(sfso.FileExists(fPath))
{
   alert("Exist");
}
else
{
alert("Doesn't exist");
  }
}
</script>

<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>