我刚开始用api构建使用express但得到以下错误 .

下面是我的代码 . 请帮忙 .

Server.js代码

const express = require('express');
const mongoose = require('mongoose')

const users = require('./routes/api/users');
const profile = require('./routes/api/profile');
const posts = require('./routes/api/posts');
const app = express();

//DB config

const db = require('./config/keys').mongoURI;

//connet to MongoDB

mongoose
    .connect(db)
    .then(() => console.log('MongoDB connected'))
    .catch(err => console.log(err));

app.get('/', (req, res) => res.send('Hello Ajas Bakran'));

//Use Routes

app.use('/api/users', users);
app.use('/api/profile', profile);
app.use('/api/posts', posts);

const port = process.env.PORT || 5000;

app.listen(port, () => console.log(`Server running on port ${port}`));

users.js代码

const express = require('express');
const router = express.Router();

router.get('/test', (req, res) => res.json({msg:'Hello Users'}));

module.exports = router;

profile.js代码

const express = require('express');
const router = express.Router();

router.get('/test', (req, res) => res.json({msg:'Hello profile'}));

module.exports = router;

posts.js

const express = require('express');
const router = express.Router();

router.get('/test', (req, res) => res.json({msg:'Hello posts'}));

module.exports = router;

我在stackoverflow上提到的答案很少,但是最后的解决方案就是 module.exports = router; 这行 . 但我确实有线存在我仍然得到相同的错误 . 真的不确定出了什么问题