TestController.php

<?php

declare (strict_types=1);
namespace Drupal\jswebassert_test\Controller;

use Drupal\Core\Render\Markup;

/**
 * Provides a test page for JavaScript assertions.
 */
class TestController {
  public function page() {
    $markup = <<<JS
    <script>
    var timesRun = 0;
    var interval = setInterval(function() {
      timesRun += 1;
      // Clear the interval after ~1500ms seconds as this is shorter than the 10
      // seconds JSWebAssert::waitForElementVisible() waits for.
      if (timesRun === 3) {
        clearInterval(interval);
      }
      var p = document.createElement("p");
      var txt = document.createTextNode("New Text!! ".concat(timesRun));
      p.setAttribute("id", "test_text");
      p.appendChild(txt);
    
      var div = document.getElementById("test_container");
      div.replaceChild(p, div.childNodes[0]);
    }, 500);
    </script>
    <div id="test_container">
      <p id="test_text"></p>
    </div>
    JS;
    return [
      // Javascript should not be injected into a page this way unless in test
      // code.
'#markup' => Markup::create($markup),
    ];
  }

}