Why should you validate forms using javascript?

Question

What is the point in validating your HTML forms using Javascript, if you are always going to need to validate the forms using PHP anyway? I realize that you get a speed boost from this, and its more convenient to the user, but beyond that, is the time spent on it worth it? If anyone has any good evidence on this I would love to hear it.

Thanks for any help!
Metropolis

UPDATE

After receiving numerous answers I would like to change the question a little. We all know that javascript is much more convenient for the user and it gives faster feedback. What I am wondering is: Has anyone ever seen any "evidence" that its worth it? Or do we just do it because it makes things a little better and everyone says we should? The speed difference is not that significant, and as the internet gets faster javascript validation will become even more obsolete I would think.

I am starting to wonder if the time spent validating a page using javascript could be better spent.

Answer 1

It's all about usability. It is much more convenient for the user to read what errors they have made before the page reloads, rather than continuously submit and reload the page. It can also give a nicer look with some AJAX and the likes, rather than a reload of the page and the very ugly looking red error messages, I think. So the advantage? Much more usable than having server side validation alone.

Answer 2

Ideally, you validate through javascript and (in your case) PHP.

Both validation methods will work in-tandem to ensure you get the most robust and user friendly functionality possible for your end user.

You will use client-side validation to ensure that all fields are filled in, email addresses are valid, etc.. this will provide instant feedback and won't burden your servers or the user's internet connection.

you validate server-side for security. You can control everything on the server and nothing on the client machine. It's here that you ensure that all entered data is non-malicious and correct.

Keep this in mind: if you are only going to go with one type of validation, choose server-side validation because it is more secure. You should never rely on client-side code for any kind of security.

Using both types of validation gives you the best of both worlds (responsiveness and security) while having none of the downsides. Of course, this means you have to write more code, but in my opinion, it's worth it.

EDIT: In response to the comments

Yes, you have to write more code this way... As a rule of thumb, if it's harder for the programmer, it's easier on the user. It might not make sense in some budgets to do both types of validation and that's a call you're going to have to make. Just make sure your server side validation is rock-solid regardless.

Yes, time is money, and time invested in improving the user's experience is time well spent. If you can't afford to do it now (deadlines/schedule/budget) then do it when you can.