/**
 * Add the search button as the last item of the main navigation
 *
 */
function add_last_nav_item( $items, $args ) {
    $search = '<li class="menu-item menu-search"><a href="#" data-js="search"><i class="fa fa-search" aria-hidden="true"></i></a></li>';
    $items .= $search;

    return $items;
}
add_filter( 'wp_nav_menu_items', __NAMESPACE__ . '\\add_last_nav_item', 10, 2 );