JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

Form Validation via Hidden Fields
An even more elegant possibility is to use hidden form fields and (believe it or not) routines that
are even more generic than those we just saw. For example, you might define pairs of fields
like this:
You would define hidden form fields for each entry to validate, so to check that a field called
username is not blank, you might use
To check for an e-mail address, you might use
You would then write a loop to look through forms being submitted for hidden fields and to look
for ones in the form of fieldname_check. When you find one, you could use string routines to
parse out the field name and the check to run on it. If the check fails, you can easily find the
associated error message to show by accessing the field fieldname_errormsg.
Note One of the main reasons the hidden field approach is more elegant is that we can easily
have the server-side of the Web equation look at the hidden values passed and run
similar validation checks. This double-checking may seem a waste of time, but it actually
improves security as it is not possible to truly know if client-side validation in JavaScript
was run.
Regardless of the method you choose, it should be clear that the approach is useful as it allows
you to separate out reused JavaScript validation functions into .js files and reference from just
about any form pages. However, before setting out on the task to roll your own validation
routines, consider the number of people who already have needed to do the same thing. Code
is out on the Web already, so it makes sense to start with a library when making your validation
code. For example, take a look at
http://developer.netscape.com/docs/examples/javascript.html for some sample scripts.
Netscape has provided a form validation collection of code ever since JavaScript 1.0 and also
provides regular expression-oriented checks as well.
onchange Handlers
There is no reason you need to wait for the form to be submitted in order to validate its fields.
You can validate a field immediately after the user has modified it by using an onchange event
handler. For example: