Mongoose连接报错：uri参数为undefined，求解决并正常打印端口

报错信息 #

MongooseError: The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string. did not connect

项目代码 #

import express from "express";
import bodyParser from "body-parser";
import mongoose from "mongoose";
import cors from "cors";
import dotenv from "dotenv";
import multer from "multer";
import helmet from "helmet";
import morgan from "morgan";
import path from "path";
import { fileURLToPath } from "url";

/*CONFIGURATIONS*/
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
dotenv.config();
const app = express();
app.use(express.json())
app.use(helmet());
app.use(helmet.crossOriginResourcePolicy({policy:"cross-origin"}));
app.use(morgan("common"));
app.use(bodyParser.json({limit: "30mb", extended : true}));
app.use(bodyParser.urlencoded({limit:"30mb", extended:true}));
app.use(cors())
app.use("/assets", express.static(path.join(__dirname, 'public/assets')));

/*FILE STORAGE */
const storage = multer.diskStorage({
    destination : function(req, file, cb){      
        cb(null, "public/assests");
    },
    filename : function(req, file, cb ){
        cb(null, file.originalname);
    }
})
const upload = multer({storage});

/*MONGOOSE SETUP */
const PORT = process.env.PORT || 6001;
mongoose.connect(process.env.MONGO_URL, {
    useNewUrlParser : true,
    useUnifiedTopology : true,
}).then(() => {
    app.listen(PORT, () => console.log(`Server Port : ${PORT}`));
}).catch((error) => console.log( `${error} did not connect`))

.env文件配置 #

MONGO_URL = 'mongodb+srv://username:password@cluster0.nbhppgf.mongodb.net/?retryWrites=true&w=majority'
PORT = 3001

问题原因 #

报错核心是process.env.MONGO_URL读取失败返回undefined，导致Mongoose连接时传入的uri不是有效字符串，主要原因：

1. .env文件变量格式错误（多余引号、等号前后空格）
2. dotenv未正确定位到.env文件（ES模块下路径解析问题）

解决方案 #

1. 修正.env文件格式 #

移除变量值的单引号，同时确保等号前后无空格：

MONGO_URL=mongodb+srv://username:password@cluster0.nbhppgf.mongodb.net/?retryWrites=true&w=majority
PORT=3001

注：.env文件中变量不需要加引号，空格会被解析为变量值的一部分

2. 确保dotenv加载正确路径 #

在ES模块环境下，显式指定.env文件路径，避免路径解析错误：
修改dotenv.config()为：

dotenv.config({ path: path.join(__dirname, '.env') });

3. 验证环境变量读取状态 #

在连接代码前添加日志，确认变量是否正确读取：

console.log('MONGO_URL:', process.env.MONGO_URL);
console.log('PORT:', process.env.PORT);
const PORT = process.env.PORT || 6001;

4. 简化Mongoose连接配置（可选） #

Mongoose 6.x及以上版本已默认启用useNewUrlParser和useUnifiedTopology，可移除这两个参数：

mongoose.connect(process.env.MONGO_URL)

修改后的关键代码片段 #

/*CONFIGURATIONS*/
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
// 指定.env路径加载
dotenv.config({ path: path.join(__dirname, '.env') });
const app = express();
// ...其他中间件配置

/*MONGOOSE SETUP */
console.log('MONGO_URL:', process.env.MONGO_URL);
console.log('PORT:', process.env.PORT);
const PORT = process.env.PORT || 6001;
mongoose.connect(process.env.MONGO_URL)
.then(() => {
    app.listen(PORT, () => console.log(`Server Port : ${PORT}`));
}).catch((error) => console.log( `${error} did not connect`))

内容的提问来源于stack exchange，提问作者Bharadwaj Srikaa