Why my WP_AJAX returning Zero (0) with my response?

Some of you may still surprise, why the callback function for WP_Ajax returning an additional 0 or something else which is very unwanted at the end of your own response.

How to properly work with AJAX on WordPress (WP_AJAX)

Actually using Ajax directly into your plugin or theme is a little immature way. Cause you'll have to keep track of your files to make it work, which sometime become difficult. That's where WP_Ajax make things comfortable.