https://developers.weixin.qq.com/miniprogram/dev/framework/ability/pre-fetch.html

  wx.onBackgroundFetchData(() => {
      console.log(res.fetchedData) // 缓存数据
      console.log(res.timeStamp) // 客户端拿到缓存数据的时间戳
    })这也不对啊  res 都没声明