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Validating Form Data With JavaScript
One of the most commonly used applications for JavaScript on the Web is for form
validation. Form validation allows a web programmer to check information entered onto a
form before it is submitted to the server. The benefit to you as a webmaster is that by
validating the form on the browser (client), you will be saving valuable processing resources
on the web server.

JavaScript form validation occurs on a specific event – when the document is ‘submitted’
(when the submit button is pressed). A JavaScript function is called on the submission
action that ‘looks’ at the form and makes sure that information has been correctly entered
into each field. JavaScript can actually look at the form and each field so long as each
element is named. For instance if you have a field named “email_address” in a form named
“feedback_form”, JavaScript would be able to reference that field as
d o c u m e n t . f e e d b a c k _ f o r m . e m a i l _ a d d r e s s . It is then possible to determine whether
information was submitted to the form and if so, if it is ‘correct’.

LoudOffice.com Guide to HTML – Part II Page 34