Make post_content and other custom fields required

Question

Don’t rely entirely on JavaScript validations. Use below hook for Server side validation.

function check_if_post_content_set( $maybe_empty, $postarr ) {
// Check if post is already created. IMPORTANT
if($postarr['ID'] && (int)$postarr['ID'] > 0){
    if( !$postarr['post_content'] OR $postarr['post_content'] == '' OR $postarr['post_content'] == NULL ){
        $maybe_empty = true;
    }
}
return $maybe_empty;
}
add_filter( 'wp_insert_post_empty_content', 'check_if_post_content_set', 999999, 2 );