client side comments validation

I am looking for a way to validate entries in a comment.

For example, if a user doesn’t include any comments or doesn’t include an email address, wp_die is called (server side) and a standard error message is produced.

Now i know we can hack the functions.php file and redirect the user to a custom error message page but i was hoping to create a client side solution.

For example use some javascript – alert() fuction.

Does anyone know how to do this?

Thanks