drupal/core/modules/big_pipe/big_pipe.module

<?php

/**
 * @file
 * Adds BigPipe no-JS detection.
 */

use Drupal\big_pipe\Render\Placeholder\BigPipeStrategy;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Url;

/**
 * Implements hook_help().
 */
function big_pipe_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.big_pipe':
      $output = '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The BigPipe module sends pages with dynamic content in a way that allows browsers to show them much faster. For more information, see the <a href=":big_pipe-documentation">online documentation for the BigPipe module</a>.', [':big_pipe-documentation' => 'https://www.drupal.org/documentation/modules/big_pipe']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Speeding up your site') . '</dt>';
      $output .= '<dd>' . t('The module requires no configuration. Every part of the page contains metadata that allows BigPipe to figure this out on its own.') . '</dd>';
      $output .= '</dl>';

      return $output;
  }
}

/**
 * Implements hook_page_attachments().
 *
 * @see \Drupal\big_pipe\Controller\BigPipeController::setNoJsCookie()
 */
function big_pipe_page_attachments(array &$page) {
  // Routes that don't use BigPipe also don't need no-JS detection.
  if (\Drupal::routeMatch()->getRouteObject()->getOption('_no_big_pipe')) {
    return;
  }

  $request = \Drupal::request();
  // BigPipe is only used when there is an actual session, so only add the no-JS
  // detection when there actually is a session.
  // @see \Drupal\big_pipe\Render\Placeholder\BigPipeStrategy.
  $session_exists = \Drupal::service('session_configuration')->hasSession($request);
  $page['#cache']['contexts'][] = 'session.exists';
  // Only do the no-JS detection while we don't know if there's no JS support:
  // avoid endless redirect loops.
  $has_big_pipe_nojs_cookie = $request->cookies->has(BigPipeStrategy::NOJS_COOKIE);
  $page['#cache']['contexts'][] = 'cookies:' . BigPipeStrategy::NOJS_COOKIE;
  if ($session_exists) {
    if (!$has_big_pipe_nojs_cookie) {
      // Let server set the BigPipe no-JS cookie.
      $page['#attached']['html_head'][] = [
        [
          // Redirect through a 'Refresh' meta tag if JavaScript is disabled.
          '#tag' => 'meta',
          '#noscript' => TRUE,
          '#attributes' => [
            'http-equiv' => 'Refresh',
            'content' => '0; URL=' . Url::fromRoute('big_pipe.nojs', [], ['query' => \Drupal::service('redirect.destination')->getAsArray()])->toString(),
          ],
        ],
        'big_pipe_detect_nojs',
      ];
    }
    else {
      // Let client delete the BigPipe no-JS cookie.
      $page['#attached']['html_head'][] = [
        [
          '#tag' => 'script',
          '#value' => 'document.cookie = "' . BigPipeStrategy::NOJS_COOKIE . '=1; path=/; expires=Thu, 01 Jan 1970 00:00:00 GMT"',
        ],
        'big_pipe_detect_js',
      ];
    }
  }
}