how to get AJAX return something

/**
 * adds the ability to use the following url to get something
 *
 * https://www.domainname.com/wp-admin/admin-ajax.php?action=all_questions
 *
 * notice that we add it twice: the first is for logged in users, the second one for non-logged in users, so without privileges
 *
 */
add_action( 'wp_ajax_all_questions' , 'get_all_questions' );
add_action( 'wp_ajax_nopriv_all_questions' , 'get_all_questions' );


/**
 * Get all posts from a CPT called 'questions'
 * echo the array as JSON for a AJAX request to parse
 * @return [type] [description]
 */
function get_all_questions() {

    $questions = [];

    $all_questions = get_posts( [
        'post_type' => 'questions',
        'numberposts' => -1,

    ]  );

    if ( sizeof( $all_questions ) > 0 ) {
        foreach( $all_questions as $item ) {
            // get the terms for the grades taxonomy on this question
            $terms = get_the_terms( $item->ID , 'grades' );
            // just return the name instead of the entire object
            $terms = array_map( function($v) { return $v->name; } , $terms );

            $questions[] = [
                                'id' => $item->ID,
                                'title' => $item->post_title,
                                'grades' => $terms,
                            ];
        }
    }

    echo json_encode( [ 'data' => $questions ] );

    // DONT FORGET THAT WE NEED TO USE wp_die(),
    // otherwise the return will be messed up with the rest of a regular WP-page.
    wp_die();
}