HTMLDialogElement.show()

show()

  <!-- Simple pop-up dialog box, containing a form -->
  <dialog id="favDialog">
    <form method="dialog">
      <section>
        <p><label for="favAnimal">Favorite animal:</label>
        <select id="favAnimal" name="favAnimal">
          <option></option>
          <option>Brine shrimp</option>
          <option>Red panda</option>
          <option>Spider monkey</option>
        </select></p>
      </section>
      <menu>
        <button id="cancel" type="reset">Cancel</button>
        <button type="submit">Confirm</button>
      </menu>
    </form>
  </dialog>

  <menu>
    <button id="updateDetails">Update details</button>
  </menu>

  <script>
    (function() {
      var updateButton = document.getElementById('updateDetails');
      var cancelButton = document.getElementById('cancel');
      var dialog = document.getElementById('favDialog');
      dialog.returnValue = 'favAnimal';

      function openCheck(dialog) {
        if(dialog.open) {
          console.log('Dialog open');
        } else {
          console.log('Dialog closed');
        }
      }

      // Update button opens a modeless dialog
      updateButton.addEventListener('click', function() {
        dialog.show();
        openCheck(dialog);
      });

      // Form cancel button closes the dialog box
      cancelButton.addEventListener('click', function() {
        dialog.close('animalNotChosen');
        openCheck(dialog);
      });

    })();
  </script>