This function grabs the value from a passed in field. You should pass in the handle to the field, as in "document.forms[0].MyField". The return value is either a string or an array, depending on the field type (a check box would return an array).

function getFieldValue(field)
{
   switch(field.type)
   {
      case "text" :
      case "textarea" :
      case "password" :
      case "hidden" :
         return field.value;

      case "select-one" :
         var i = field.selectedIndex;
         if (i == -1)   return "";
         else   return (field.options[i].value == "") ? field.options[i].text : field.options[i].value;

      case "select-multiple" :
         var allChecked = new Array();
         for(i = 0; i < field.options.length; i++)
            if(field.options[i].selected)
               allChecked[allChecked.length] = (field.options[i].value == "") ? field.options[i].text : field.options[i].value;
         return allChecked;

      case "button" :
      case "reset" :
      case "submit" :
         return "";

      case "radio" :
      case "checkbox" :
         if (field.checked) { return field.value; } else { return ""; }
      default :
         if(field[0].type == "radio")
         {
            for (i = 0; i < field.length; i++)
               if (field[i].checked)
                  return field[i].value;

            return "";
         }
         else if(field[0].type == "checkbox")
         {
            var allChecked = new Array();
            for(i = 0; i < field.length; i++)
               if(field[i].checked)
                  allChecked[allChecked.length] = field[i].value;

            return allChecked;
         }
         else
            var str = "";
            for (x in field) { str += x + "\n"; }
            alert("I couldn't figure out what type this field is...\n\n" + field.name + ": ???\n\n\n" + str + "\n\nlength = " + field.length);
         break;
   }
   
   return "";
}

Any field can be processed by this function. This function is used in the Generic Form Validation JavaScript.