codice:
  <html>

    <head>
      <script type="text/javascript" language="JavaScript">
      <!--

        function checkData( __hForm )
        {
          if ( __hForm.elements["cliente"].value == "" ) {
            alert( "Cliente non valido." );
            return false;
          }

          if ( !(/^\d+(|\.\d+)$/).test(__hForm.elements["importo"].value) ) {
            alert( "Importo non valido." );
            return false;
          }

          return true;
        }


      //-->
      </script>
    </head>

    <body>

      <form onsubmit="return checkData( this );">
        Cliente:

        <input type="text" name="cliente" />



        Importo:

        <input type="text" name="importo" />



        <input type="submit" />
      </form>

    </body>

  </html>
onsubmit va nel <form>