How is client side validation carried out?

Generally client side validation uses functions or a validation object and methods to check that a user has entered valid data, and that all required data is entered. Validation for text fields is often done by attaching a function call to the "blur" event of the text element. Validating that all required data is present is normally done by attaching an event to the "submit" event on the form.

In all cases, you have to be aware that client side validation is only useful for user experience. It cannot be relied on, and so even with it, server side validation has to be done on the submitted data. This is true even in cases where AJAX is employed, as all client side code is suspect.