private function extractPaths(string $mainJsFile): array
  {
    $command = [$this->nodePath,
                __DIR__.'/../../lib/extract_config.js',
                $mainJsFile];
    $output  = $this->execCommand($command);
    $config  = json_decode(implode(PHP_EOL, $output), true);

    return [$config['baseUrl'], $config['paths']];
  }