Javascript Validation Of Radio, Checkboxes And Text

I use the following code on a form (which works)

function SubmitDocument(){
var frm=document.forms[0];
if(frm.QuestionSeventeen.value=="") {
alert("Please enter your postcode");
window.document.forms[0].QuestionSeventeen.focus();
return false;
}
else
document.forms[0].submit();
}

This is fine, because QuestionSeventeen is a text field. However, I have a number of radio and checkbox values on my form which I need to make sure are not NULL.

I want to make this more efficient and not use LOTS of these nested IFs.

i.e., does anyone have any code that will check firstly specified text fields, then radio buttons, then checkboxes in a more efficient manner?

so.... somthing like this process is what I want to achieve

Function validate()

- make array of text fields, loop through them checking not null THEN
- make array of checkbox fields, loop through them checking not null THEN
- make array of radio button fields, loop through them checking not null THEN

If all OK, save doc otherwise alert that field needs to be filled in then gocus on that field END FUNCTION