use DrupalCoreFormFormStateInterface;

function mymodule_form_alter(array &$form, FormStateInterface $form_state, $form_id) {

  if ($form_id == 'node_page_form' || $form_id == 'node_page_edit_form') {
    return;
  }

  if (isset($form['field_test2'])) {
    $states_when_category_is_design = array(
      'visible' => array(
        ':input[name="field_test2"]' => array('value' => '15'),
      ),
    );

    if (isset($form['field_test3'])) {
      $form['field_test3']['#states'] = $states_when_category_is_design;
    };
  }
}

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'node_article_edit_form') {
    $form['field_other']['#states'] = [
      'visible' => [
          'select[name="field_reason"]' => ['value' => 'other']
      ]
    ];
   }
 }

/**
 * Implements hook_form_alter().
 */
function MYMODULE_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'node_CONTENT_TYPE_form' || $form_id == 'node_CONTENT_TYPE_edit_form') {
    conditional_field_select(
      $form,
      'field_target',
      'field_controller',
      ['value_a', 'value_b', 'value_c'],
      'visible'
    );
  }
}

function conditional_field_select(array &$form, $targetField, $controlledBy, array $values, $state = 'invisible', $cond = 'or') {
  if (isset($form[$targetField]) && isset($form[$controlledBy])) {
    $form[$targetField]['#states'][$state] = [];
    foreach ($values as $value) {
      array_push($form[$targetField]['#states'][$state], ['select[name=' . $controlledBy . ']' => ['value' => $value]]);
      if (end($values) !== $value) {
        array_push($form[$targetField]['#states'][$state], $cond);
      }
    }
  }
}

array_push($form[$targetField]['#states'][$state], [':input[name=' . $controlledBy . ']' => ['value' => $value]]);