Yes, this is a very user-friendly feature. It saves the user the need to submit, read the error message, and go back to the form to correct the information, as well as saving server resources.
There are at least two ways to do it. One is an alert or print when the submit button is clicked. Most sites also put a red asterisk or a message like
*Required information by whichever input needs to be changed/filled.
Another is to use the onblur event to do the validation and warning as soon as the user leaves a field that is incorrectly filled out. This can be tricky to style.
Unfortunately, you still need to code sticky fields for
js-disabled users.
If you need any hints about the javascript, you'd probably be better off asking in that thread. There are plenty of good free scripts.
As I read the original post,
djeyewater intends to validate with
PHP. He just wants to know if he should add a second javascript validation.