JsOnceTestController.php

<?php

namespace Drupal\js_once_test\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller for testing the @drupal/once library integration.
 */
class JsOnceTestController extends ControllerBase {
    
    /**
     * Provides elements for testing @drupal/once.
     *
     * @return array
     *   The render array.
     */
    public function onceTest() {
        $output = [
            '#attached' => [
                'library' => [
                    'core/once',
                ],
            ],
        ];
        foreach (range(1, 5) as $item) {
            $output['item' . $item] = [
                '#type' => 'html_tag',
                '#tag' => 'div',
                '#value' => 'Item ' . $item,
                '#attributes' => [
                    'data-drupal-item' => $item,
                ],
            ];
        }
        return $output;
    }

}