解决 mongoose DeprecationWarning 错误导致数据库连接不成功

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

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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));