To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.

mongoose报错：DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
其实我们阅读报的警告就可以解决:在mongoose连接的时候在第二个参数的对象中加入

  useUnifiedTopology: true   

即为：

mongoose.connect(dbConfig.dbs, {
  useNewUrlParser: true,
  useUnifiedTopology: true     //这个即是报的警告
}).then(res => {
  console.log('数据库连接成功')
})