diff --git a/core/includes/common.inc b/core/includes/common.inc
index 03af03b..7bb70a2 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -568,7 +568,7 @@ function drupal_js_defaults($data = NULL) {
  * @param $elements
  *   A renderable array element having a #states property as described above.
  *
- * @see form_example_states_form()
+ * @see \Drupal\form_test\Form\JavascriptStatesForm
  */
 function drupal_process_states(&$elements) {
   $elements['#attached']['library'][] = 'core/drupal.states';
diff --git a/core/modules/system/tests/modules/form_test/form_test.routing.yml b/core/modules/system/tests/modules/form_test/form_test.routing.yml
index d8c5833..22b1221 100644
--- a/core/modules/system/tests/modules/form_test/form_test.routing.yml
+++ b/core/modules/system/tests/modules/form_test/form_test.routing.yml
@@ -480,3 +480,10 @@ form_test.get_form:
     _form: '\Drupal\form_test\Form\FormTestGetForm'
   requirements:
     _access: 'TRUE'
+
+form_test.javascript_states_form:
+  path: '/form-test/javascript-states-form'
+  defaults:
+    _form: '\Drupal\form_test\Form\JavascriptStatesForm'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/form_test/src/Form/JavascriptStatesForm.php b/core/modules/system/tests/modules/form_test/src/Form/JavascriptStatesForm.php
new file mode 100644
index 0000000..5bb45c8
--- /dev/null
+++ b/core/modules/system/tests/modules/form_test/src/Form/JavascriptStatesForm.php
@@ -0,0 +1,273 @@
+<?php
+
+namespace Drupal\form_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Builds a simple form that redirects on submit.
+ *
+ * @see \Drupal\form_test\Plugin\Block\RedirectFormBlock
+ */
+class JavascriptStatesForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'javascript_states_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    // Hide name field when the anonymous checkbox is checked.
+    $form['name'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Name'),
+      '#states' => array(
+        'invisible' => array(
+          ':input[name="anonymous"]' => array('checked' => TRUE),
+        ),
+      ),
+    );
+
+    // Uncheck anonymous field when the name field is filled.
+    $form['anonymous'] = array(
+      '#type' => 'checkbox',
+      '#title' => $this->t('I prefer to remain anonymous'),
+      '#default_value' => '1',
+      '#states' => array(
+        'unchecked' => array(
+          ':input[name="name"]' => array('filled' => TRUE),
+        ),
+      ),
+    );
+
+    $form['student_type'] = array(
+      '#type' => 'radios',
+      '#options' => array(
+        'high_school'   => $this->t('High School'),
+        'undergraduate' => $this->t('Undergraduate'),
+        'graduate'      => $this->t('Graduate'),
+      ),
+      '#title' => $this->t('What type of student are you?'),
+    );
+    $form['high_school'] = array(
+      '#type' => 'fieldset',
+      '#title' => $this->t('High School Information'),
+      // This #states rule says that the "high school" fieldset should only
+      // be shown if the "student_type" form element is set to "High School".
+      '#states' => array(
+        'visible' => array(
+          ':input[name="student_type"]' => array('value' => 'high_school'),
+        ),
+      ),
+    );
+
+    // High school information.
+    $form['high_school']['tests_taken'] = array(
+      '#type' => 'checkboxes',
+      '#options' => array_combine(array('SAT', 'ACT'), array(t('SAT'), t('ACT'))),
+      '#title' => $this->t('What standardized tests did you take?'),
+      // This #states rule says that this checkboxes array will be visible only
+      // when $form['student_type'] is set to t('High School').
+      // It uses the jQuery selector :input[name=student_type] to choose the
+      // element which triggers the behavior, and then defines the "High School"
+      // value as the one that triggers visibility.
+      '#states' => array(
+        // Action to take.
+        'visible' => array(
+          ':input[name="student_type"]' => array('value' => 'high_school'),
+        ),
+      ),
+    );
+
+    $form['high_school']['sat_score'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Your SAT score:'),
+      '#size' => 4,
+
+      // This #states rule limits visibility to when the $form['tests_taken']
+      // 'SAT' checkbox is checked."
+      '#states' => array(
+        // Action to take.
+        'visible' => array(
+          ':input[name="tests_taken[SAT]"]' => array('checked' => TRUE),
+        ),
+      ),
+    );
+    $form['high_school']['act_score'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Your ACT score:'),
+      '#size' => 4,
+
+      // Set this element visible if the ACT checkbox above is checked.
+      '#states' => array(
+        // Action to take.
+        'visible' => array(
+          ':input[name="tests_taken[ACT]"]' => array('checked' => TRUE),
+        ),
+      ),
+    );
+
+    // Undergrad information.
+    $form['undergraduate'] = array(
+      '#type' => 'fieldset',
+      '#title' => $this->t('Undergraduate Information'),
+      // This #states rule says that the "undergraduate" fieldset should only
+      // be shown if the "student_type" form element is set to "Undergraduate".
+      '#states' => array(
+        'visible' => array(
+          ':input[name="student_type"]' => array('value' => 'undergraduate'),
+        ),
+      ),
+    );
+
+    $form['undergraduate']['how_many_years'] = array(
+      '#type' => 'select',
+      '#title' => $this->t('How many years have you completed?'),
+      // The options here are integers, but since all the action here happens
+      // using the DOM on the client, we will have to use strings to work with
+      // them.
+      '#options' => array(
+        1 => $this->t('One'),
+        2 => $this->t('Two'),
+        3 => $this->t('Three'),
+        4 => $this->t('Four'),
+        5 => $this->t('Lots'),
+      ),
+    );
+
+    $form['undergraduate']['comment'] = array(
+      '#type' => 'item',
+      '#description' => $this->t("Wow, that's a long time."),
+      '#states' => array(
+        'visible' => array(
+          // Note that '5' must be used here instead of the integer 5.
+          // The information is coming from the DOM as a string.
+          ':input[name="how_many_years"]' => array('value' => '5'),
+        ),
+      ),
+    );
+    $form['undergraduate']['school_name'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Your college or university:'),
+    );
+    $form['undergraduate']['school_country'] = array(
+      '#type' => 'select',
+      '#options' => array_combine(array('UK', 'Other'), array(t('UK'), t('Other'))),
+      '#title' => $this->t('In what country is your college or university located?'),
+    );
+    $form['undergraduate']['country_writein'] = array(
+      '#type' => 'textfield',
+      '#size' => 20,
+      '#title' => $this->t('Please enter the name of the country where your college or university is located.'),
+
+      // Only show this field if school_country is set to 'Other'.
+      '#states' => array(
+        // Action to take: Make visible.
+        'visible' => array(
+          ':input[name="school_country"]' => array('value' => $this->t('Other')),
+        ),
+      ),
+    );
+
+    $form['undergraduate']['thanks'] = array(
+      '#type' => 'item',
+      '#description' => $this->t('Thanks for providing both your school and your country.'),
+      '#states' => array(
+        // Here visibility requires that two separate conditions be true.
+        'visible' => array(
+          ':input[name="school_country"]' => array('value' => $this->t('Other')),
+          ':input[name="country_writein"]' => array('filled' => TRUE),
+        ),
+      ),
+    );
+    $form['undergraduate']['go_away'] = array(
+      '#type' => 'submit',
+      '#value' => $this->t('Done with form'),
+      '#states' => array(
+        // Here visibility requires that two separate conditions be true.
+        'visible' => array(
+          ':input[name="school_country"]' => array('value' => $this->t('Other')),
+          ':input[name="country_writein"]' => array('filled' => TRUE),
+        ),
+      ),
+    );
+
+    // Graduate student information.
+    $form['graduate'] = array(
+      '#type' => 'fieldset',
+      '#title' => $this->t('Graduate School Information'),
+      // This #states rule says that the "graduate" fieldset should only
+      // be shown if the "student_type" form element is set to "Graduate".
+      '#states' => array(
+        'visible' => array(
+          ':input[name="student_type"]' => array('value' => 'graduate'),
+        ),
+      ),
+    );
+    $form['graduate']['more_info'] = array(
+      '#type' => 'textarea',
+      '#title' => $this->t('Please describe your graduate studies'),
+    );
+
+    $form['graduate']['info_provide'] = array(
+      '#type' => 'checkbox',
+      '#title' => $this->t('Check here if you have provided information above'),
+      '#disabled' => TRUE,
+      '#states' => array(
+        // Mark this checkbox checked if the "more_info" textarea has something
+        // in it, if it's 'filled'.
+        'checked' => array(
+          ':input[name="more_info"]' => array('filled' => TRUE),
+        ),
+      ),
+    );
+
+    $form['average'] = array(
+      '#type' => 'textfield',
+      '#title' => $this->t('Enter your average'),
+      // To trigger a state when the same controlling element can have more than
+      // one possible value, put all values in a higher-level array.
+      '#states' => array(
+        'visible' => array(
+          ':input[name="student_type"]' => array(
+            array('value' => 'high_school'),
+            array('value' => 'undergraduate'),
+          ),
+        ),
+      ),
+    );
+
+    $form['expand_more_info'] = array(
+      '#type' => 'checkbox',
+      '#title' => $this->t('Check here if you want to add more information.'),
+    );
+    $form['more_info'] = array(
+      '#type' => 'details',
+      '#title' => $this->t('Additional Information'),
+      // Expand the expand_more_info fieldset if the box is checked.
+      '#states' => array(
+        'expanded' => array(
+          ':input[name="expand_more_info"]' => array('checked' => TRUE),
+        ),
+      ),
+    );
+    $form['more_info']['feedback'] = array(
+      '#type' => 'textarea',
+      '#title' => $this->t('What do you have to say?'),
+    );
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Core/Form/JavascriptStatesTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Core/Form/JavascriptStatesTest.php
new file mode 100644
index 0000000..beee43b
--- /dev/null
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Core/Form/JavascriptStatesTest.php
@@ -0,0 +1,105 @@
+<?php
+
+namespace Drupal\FunctionalJavascriptTests\Core\Form;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests the state of an element based on another element.
+ *
+ * @group javascript
+ */
+class JavascriptStatesTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['form_test'];
+
+  /**
+   * #states
+   */
+  public function testJavascriptStates() {
+    $this->drupalGet('form-test/javascript-states-form');
+    $page = $this->getSession()->getPage();
+
+    $this->assertElementNotVisible('#edit-name');
+
+    // This should make the name element visible.
+    $page->uncheckField('I prefer to remain anonymous');
+    $this->assertElementVisible('#edit-name');
+
+    // This should make the name element invisible.
+    $page->checkField('I prefer to remain anonymous');
+    $this->assertElementNotVisible('#edit-name');
+
+    // This should make the name element visible.
+    $page->uncheckField('I prefer to remain anonymous');
+    $this->assertElementVisible('#edit-name');
+
+    // Ensure all the details elements are invisible.
+    $this->assertElementNotVisible('#edit-high-school');
+    $this->assertElementNotVisible('#edit-undergraduate');
+    $this->assertElementNotVisible('#edit-graduate');
+    $this->assertElementNotVisible('#edit-average');
+
+    // High school form testing.
+    $this->click('#edit-student-type-high-school');
+    $this->assertElementVisible('#edit-high-school');
+    $this->assertElementNotVisible('#edit-undergraduate');
+    $this->assertElementNotVisible('#edit-graduate');
+    $this->assertElementVisible('#edit-average');
+
+    $this->assertElementNotVisible('#edit-sat-score');
+    $this->assertElementNotVisible('#edit-act-score');
+    $this->click('#edit-tests-taken-sat');
+    $this->assertElementVisible('#edit-sat-score');
+    $this->assertElementNotVisible('#edit-act-score');
+    $this->click('#edit-tests-taken-act');
+    $this->assertElementVisible('#edit-act-score');
+    $this->assertElementVisible('#edit-sat-score');
+    $this->click('#edit-tests-taken-sat');
+    $this->click('#edit-tests-taken-act');
+    $this->assertElementNotVisible('#edit-sat-score');
+    $this->assertElementNotVisible('#edit-act-score');
+
+    // Undergraduate form testing.
+    $this->click('#edit-student-type-undergraduate');
+    $this->assertElementNotVisible('#edit-high-school');
+    $this->assertElementVisible('#edit-undergraduate');
+    $this->assertElementNotVisible('#edit-graduate');
+    $this->assertElementVisible('#edit-average');
+
+    $this->assertElementNotVisible('#edit-comment--description');
+    $page->selectFieldOption('How many years have you completed?', '5');
+    $this->assertElementVisible('#edit-comment--description');
+    $this->assertElementNotVisible('#edit-country-writein');
+    $page->selectFieldOption('In what country is your college or university located?', 'Other');
+    $this->assertElementVisible('#edit-country-writein');
+    $page->selectFieldOption('In what country is your college or university located?', 'UK');
+    $this->assertElementNotVisible('#edit-country-writein');
+
+    // Graduate form testing.
+    $this->click('#edit-student-type-graduate');
+    $this->assertElementNotVisible('#edit-high-school');
+    $this->assertElementNotVisible('#edit-undergraduate');
+    $this->assertElementVisible('#edit-graduate');
+    $this->assertElementNotVisible('#edit-average');
+
+    $this->assertSession()->checkboxNotChecked('Check here if you have provided information above');
+    $page->fillField('Please describe your graduate studies', 'Some text');
+    $this->assertSession()->checkboxChecked('Check here if you have provided information above');
+    $page->fillField('Please describe your graduate studies', '');
+    // Empty and filled states are triggered on keyup event.
+    $page->findField('Please describe your graduate studies')->keyUp('a');
+    $this->assertSession()->checkboxNotChecked('Check here if you have provided information above');
+
+    // Feedback testing.
+    $this->assertElementNotVisible('#edit-feedback');
+    $page->checkField('Check here if you want to add more information.');
+    $this->assertElementVisible('#edit-feedback');
+    $page->uncheckField('Check here if you want to add more information.');
+    $this->assertElementNotVisible('#edit-feedback');
+  }
+
+}
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
index 41e5b79..2862d4c 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
@@ -43,6 +43,7 @@ protected function initMink() {
    *   Optional message to show alongside the assertion.
    */
   protected function assertElementVisible($css_selector, $message = '') {
+    $message = $message ?: "Element ($css_selector) is visible.";
     $this->assertTrue($this->getSession()->getDriver()->isVisible(CssSelector::toXPath($css_selector)), $message);
   }
 
@@ -55,6 +56,7 @@ protected function assertElementVisible($css_selector, $message = '') {
    *   Optional message to show alongside the assertion.
    */
   protected function assertElementNotVisible($css_selector, $message = '') {
+    $message = $message ?: "Element ($css_selector) is not visible.";
     $this->assertFalse($this->getSession()->getDriver()->isVisible(CssSelector::toXPath($css_selector)), $message);
   }