const uri=mongodb+srv://${process.env.DB_USER}:${process.env.DB_PASS}@cluster0.x3xvq.mongodb.net/myFirstDatabase?retryWrites=true&w=majority
安装dotenv模块并在使用任何环境变量之前运行config函数例子:从'dotenv'中导入{config}
配置();
console.log (process.env.db_url);