watchUserConfig () {
    this.watchFiles = [
      '.vuepress/config.js',
      '.vuepress/config.yml',
      '.vuepress/config.toml'
    ].concat(
      (
        this.context.siteConfig.extraWatchFiles || []
      ).map(file => normalizeWatchFilePath(file, this.context.sourceDir))
    )

    logger.debug('watchFiles', this.watchFiles)

    this.configWatcher = chokidar.watch(this.watchFiles, {
      cwd: this.context.sourceDir,
      ignoreInitial: true
    })

    this.configWatcher.on('change', target => this.handleUpdate('change', target))
  }
Watch user's config files and extra files.