code: function toggleValidCss(isValid, validationErrorKey) {
      validationErrorKey = validationErrorKey ? '-' + snake_case(validationErrorKey, '-') : '';
      $element.
        removeClass((isValid ? INVALID_CLASS : VALID_CLASS) + validationErrorKey).
        addClass((isValid ? VALID_CLASS : INVALID_CLASS) + validationErrorKey);
    }
docs: convenience method for easy toggling of classes