wp_nav_menu([
                'menu'              => 'Primary Menu',
                'theme_location'    => 'primary',
                'depth'             => 3,
                'container'         => 'div',
                'container_class'   => 'a',
                'container_id'      => 'b',
                'menu_class'        => 'nav navbar-nav',
                'fallback_cb'       => 'wp_bootstrap_navwalker::fallback',
                'walker'            => new wp_bootstrap_navwalker()
        ]);

function get_menu_params(){

    $defaults = array(
        'menu'              => 'Primary Menu',
        'theme_location'    => 'primary',
        'depth'             => 3,
        'container'         => 'div',
        'container_class'   => 'a',
        'container_id'      => 'b',
        'menu_class'        => 'nav navbar-nav',
        'fallback_cb'       => 'wp_bootstrap_navwalker::fallback',
        'walker'            => new wp_bootstrap_navwalker()
    );
    return $defaults;
}

wp_nav_menu( get_menu_params( ) );