--- /dev/null
+<?php
+
+namespace Drupal\Tests\config\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests the Config operations through the UI.
+ *
+ * @group config
+ */
+class ConfigEntityTest extends JavascriptTestBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public static $modules = ['config_test'];
+
+ /**
+ * Tests ajax operations through the UI on 'Add' page.
+ */
+ public function testAjaxOnAddPage() {
+ $this->drupalLogin($this->drupalCreateUser(['administer site configuration']));
+
+ $page = $this->getSession()->getPage();
+ $assert_session = $this->assertSession();
+
+ $this->drupalGet('admin/structure/config_test/add');
+ // Test that 'size value' field is not show initially, and it is show after
+ // selecting value in the 'size' field.
+ $this->assertNull($page->findField('size_value'));
+ $page->findField('size')->setValue('custom');
+ $this->assertNotNull($assert_session->waitForField('size_value'));
+ }
+
+}