Work in progress to add tests on States attribute of form for Drupal 8

form_test.routing.yml

...

form_test.states_simple:
  path: '/form_test/form-states-simple'
  defaults:
    _form: '\Drupal\form_test\Form\FormTestFormStatesSimpleForm'
    _title: 'Form #states attribute simple select test'
  requirements:
    _access: 'TRUE'
...

FormStatesTest.php

<?php

namespace Drupal\FunctionalJavascriptTests\Form;

use Drupal\FunctionalJavascriptTests\JavascriptTestBase;

/**
 * Tests Form #states attribute.
 *
 * @group Form
 */
class FormStatesTest extends JavascriptTestBase {

  /**
   * {@inheritdoc}
   */
  public static $modules = ['form_test'];

  /**
   * Tests Form #states attribute for simple select list.
   */
  public function testFormStatesSimple() {
    // First visit the form.
    $this->drupalGet('form_test/form-states-simple');
    $assert = $this->assertSession();

    $page = $this->getSession()->getPage();
    $this->assertEquals('', $page->getHTML());

    // Check dependee_1 A option is not selected by default.
    $option_field = $assert->optionExists('dependee_1', 'a');
    $this->assertFalse($option_field->hasAttribute('selected'), "Option 'a' for field dependee_1 is not selected.");

    // Check dependent_1 field is not visible.
    $assert->fieldNotExists('dependent_1');

    // Select dependent_1 A option and check dependent field is visible.
    $this->getSession()->getPage()->fillField('dependee_1', 'a');
    $assert->fieldExists('dependent_1');
  }

}

FormTestFormStatesSimpleForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Form to test #states form attribute for simple select list.
 */
class FormTestFormStatesSimpleForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_test_form_states_simple';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // Build form with simple select list and no default option. dependent_1
    // field should not be visible.
    $form = [
      'dependee_1' => [
        '#type' => 'select',
        '#title' => 'Dependee 1',
        '#options' => [
          'a' => 'Option A',
          'b' => 'Option B',
          'c' => 'Option C',
        ],
      ],
      'dependent_1' => [
        '#type' => 'textfield',
        '#title' => 'Dependent 1',
        '#states' => [
          'visible' => [
            'select[name="dependee_1"]' => ['value' => 'a'],
          ],
        ],
      ],
      // Add same tests fields but with default A value. dependent_2 field
      // should be visible.
      'dependee_2' => [
        '#type' => 'select',
        '#title' => 'Dependee 2',
        '#options' => [
          'a' => 'Option A',
          'b' => 'Option B',
          'c' => 'Option C',
        ],
        '#default_value' => 'a',
      ],
      'dependent_2' => [
        '#type' => 'textfield',
        '#title' => 'Dependent 2',
        '#states' => [
          'visible' => [
            'select[name="dependee_2"]' => ['value' => 'a'],
          ],
        ],
      ],
      // Add same tests fields but with default B value. dependent_3 field
      // should not be visible.
      'dependee_3' => [
        '#type' => 'select',
        '#title' => 'Dependee 3',
        '#options' => [
          'a' => 'Option A',
          'b' => 'Option B',
          'c' => 'Option C',
        ],
        '#default_value' => 'b',
      ],
      'dependent_3' => [
        '#type' => 'textfield',
        '#title' => 'Dependent 3',
        '#states' => [
          'visible' => [
            'select[name="dependee_3"]' => ['value' => 'a'],
          ],
        ],
      ],
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $form_state->setResponse(new JsonResponse($form_state->getValues()));
  }

}