How to get current menu item “CSS Classes” value?

Question

Do not know if it’s exactly what you’re looking for but this short snippet adds custom classes to nav menu item :

add_filter('nav_menu_css_class' , 'wpse_143574_nav_class' , 10 , 2);
function wpse_143574_nav_class($classes, $item){

         $classes[] = "my-class";

     }
     return $classes;
}

This could be modified with some conditional tags too.

EDIT: $item is often useful for example if ( $item->title == 'Contact')