报错信息：

POST /api/login 500 25.733 ms - 1139
(node:26228) DeprecationWarning: current URL string parser is deprecated, and will be removed in a future version. To use the new parser, pass option { useNewUrlParser: true } to MongoClient.connect.

解决：根据提示，修改 db.js，添加 { useNewUrlParser: true }

// 连接数据库，没有则自动创建
function _connectDB(callback) {
  let url = settings.dbUrl
  MongoClient.connect(url, { useNewUrlParser: true }, function (err, client) { })
}

重启服务！