function trim(data) {
   if (data == '') return '';
   while (data.substring(0,1) == ' ')  // trim left side
      data = data.substring(1, data.length);

   while (data.substring(data.length - 1, data.length) == ' ')  // trim right side
      data = data.substring(0, data.length - 1);

   return data;
}

function Check_Form() {
   var problems = '';
   var invoice;
   var amount;

   invoice = document.forms[0].invoice.value = trim(document.forms[0].invoice.value);
   amount = document.forms[0].amount.value = trim(document.forms[0].amount.value);

   if (invoice == '') problems += "Invoice number must not be blank.\n";
   if (amount == '' || isNaN(amount) || amount < 0) problems += "Amount is not valid.\n";

   if (problems) {
      alert (problems);
      return false; }
   else return true;
}