mongoose 版本低提示错误为：
DeprecationWarning: collection.ensureIndex is deprecated. Use createIndexes instead.
通常升级 mongoose 就可以解决：
npm install mongoose
连接数据库参数缺失提示错误：
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.
通常根据错误信息提示，将 useNewUrlparser 添加到 mongoose.connect 即可：
// db const db = "mongodb://localhost:27017/blog"; mongoose.Promise = global.Promise mongoose.connect(db, { useNewUrlParser: true }).then(() => console.log(‘Database Successful！‘)).catch((err) => console.log(err));
解决 mongoose DeprecationWarning 错误导致数据库连接不成功
原文：https://www.cnblogs.com/baiyygynui/p/12742408.html