<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>