<script type="text/javascript"><![CDATA[
function gup( name )
{
  name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");
  var regexS = "[\\?&]"+name+"=([^&#]*)";
  var regex = new RegExp( regexS );
  var results = regex.exec( window.location.href );
  if( results == null )
    return "";
  else
    return results[1];
}	]]>
</script>


<script type="text/javascript">
        var cat_corrente = gup("id");
		document.write(cat_corrente);
		</script>

estrarre una variabile dal querystring

url

JavaScript