Create separate directory:

mkdir server && cd server

Make server.js file with following content:

const express = require('express')
const app = express()
const port = 3000
app.get('/', (req, res) => {
  res.send('Hello World!')
})
app.listen(port, () => {
  console.log(`Example app listening at http://localhost:${port}`)
})

Then:

npm install express
node server.js

• Debian 11 Bullseye