lunr-8.x-1.0/modules/lunr_facet_example/lunr_facet_example.module

<?php

/**
 * @file
 * Contains hook implementations for the Lunr Facet Example module.
 */

use Drupal\lunr\LunrSearchInterface;

/**
 * Implements hook_lunr_search_page_alter().
 */
function lunr_facet_example_lunr_search_page_alter(&$build, LunrSearchInterface $lunr_search) {
  // Add a custom input to the search page.
  if ($lunr_search->id() === 'facet_example') {
    $build['form']['type'] = [
      '#type' => 'select',
      '#title' => t('Type'),
      '#options' => [
        '' => t('Any'),
      ],
      '#attributes' => [
        'data-lunr-search-field' => 'type',
      ],
      '#cache' => [
        'tags' => \Drupal::entityTypeManager()->getDefinition('node_type')->getListCacheTags(),
      ],
    ];
    foreach (\Drupal::entityTypeManager()->getStorage('node_type')->loadMultiple() as $type) {
      $build['form']['type']['#options'][$type->id()] = $type->label();
    }
  }
}