codice:
      <script type="text/javascript">
      <!--

        String.prototype.extractValue = function( __strVal )
        {
          var _start = this.indexOf( __strVal + '=' );

          if ( _start < 0 )
            return null;
          else
            _start += __strVal.length + 1;

          var _end   = this.indexOf( ',', _start);

          if ( _end < 0 )
            _end = this.length;

          return this.substring( _start, _end );
        }

        var valori = "campo1=valore1,campo2=valore2,campoN=valoreN";
        alert( valori.extractValue("campo2") );


      //-->
      </script>