public function renderString($input, array $vars = [], $filename = null)
    {
        $fallback = function () use ($input, $vars, $filename) {
            return $this->renderWithPhp($input, $vars, $filename);
        };

        if ($this->getDefaultOption('pugjs')) {
            return $this->renderWithJs($input, $filename, $vars, $fallback);
        }

        return call_user_func($fallback);
    }