I have three files when I try running index.js file DB work well but router does not work and if run server.js DB and router does not work

server

import express from 'express'
import cors from 'cors'
import restaurants from './api/restaurants.route.js'

const app = express()

app.use(cors())
app.use(express.json()) 
app.use("/api/v1/restaurants", restaurants)
app.use("*", (req, res) => res.status(404).json({error: "Not Found"})) // any router not found in our routers

export default app;

index

import app from './server.js'
import mongodb from 'mongodb'
import dotenv from 'dotenv'

dotenv.config()
const mongoClient = mongodb.MongoClient
const port = process.env.PORT || 8000

mongoClient.connect(
    process.env.RESTREVIEWS_DB_URI,
    {
        useNewUrlParser: true,
        useUnifiedTopology: true,
    }
).catch((e) => {
    console.log(e.stack)
    process.exit(1)
}).then(async client => {
    app.listen(port, () => {
        console.log(`app listen in link http://localhost:${port}/api/v1/restaurants`)
    })
})

router

import express from 'express'
const router = express.Router();

router.get('/api/v1/restaurants', (req, res) => {
    res.status(200).send("HelloWorld");
})

export default router