TestController.php

<?php

namespace Drupal\js_webassert_test\Controller;

use Drupal\Core\Render\Markup;
class TestController {
  public function page() {
    $markup = <<<JS
    <script>
    var timesRun = 0;
    var interval = setInterval(function() {
      timesRun += 1;
      // Clear the interval after 1.1 seconds as this is longer than the time
      // WebDriverCurlService would retry for if retries are enabled but shorter
      // than the 10 seconds JSWebAssert::waitForElementVisible() waits for.
      if (timesRun === 1100) {
        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]);
    }, 1);
    </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),
    ];
  }

}