buildForm Example

gistfile1.txt

$form['username'] = array(
  '#type' => 'textfield',
  '#title' => t('Username'),
  '#ajax' => [
    'callback' => '::validateEmailAjax',
    'event' => 'change',
  ],
  '#suffix' => '<span class="username-available"></span>'
);

validateEmailAjax

public function validateEmailAjax(array &$form, FormStateInterface $form_state) { 
  $available = $this->validateUsername($form, $form_state); // returns back boolean.
  $response = new AjaxResponse();
  $css = '';
  if ($available) {
    $css = "<style>.username-available { border: 1px solid green; }</style>";
    $message = t('This username is available.');
  }
  else {
    $css = "<style>.username-available { border: 1px solid red; }</style>";
  }
  $response->addCommand(new CssCommand('#edit-username', $css));
  $response->addcommand(new HtmlCommand('.username-available', $message));
  return $response;
}