JS, ASP: Best way to validate? easy [resolved]

Ok here is the plan:

PageA.asp has a form. On that form I have

txtUser
txtPassword
cmdSubmit

After hitting submit, user gets redirected to PageB.asp where the result (user data) is stored in a database.

Now, what is the best way to validate that the user has filled out all the fields.

Do I do it in PageB.asp or PageA.asp
I was thinking of embeding a simple JS function into

Code:

---

<FORM ACTION="JS FUNCTION HERE">
</FORM>

---

Is that possible? Is that a good way of doing it, or does it have something to do with the actual Submit button.

I'm also pretty sure I could check the values on PageB.asp, to see if they are blank and display an error... would that work?

Thanks a lot,
InVitro

This is what we use:

Code:

---

<script language="JavaScript">
    function validate(obj) {
        if (obj.Field.value == "") {
            alert("Some message goes here");
            obj.focus();
            return false;
        }

        return true;
    };
</script>

<form name="blah" method="post" action="somepage.asp" onsubmit="return validate(this);">
    <input type="text" name="Field">
</form>

---

Using Javascript like axion_sa provided is a good way to do a quick check and give the user timely feedback without having to wait for the form to be sent to the server and back.
But it is good practice to do the checks again on the server side like checks for single quotes (used to always catch me out with peoples names)

Perfect, thanks a lot!