server.js

const express = require('express')

const app = express()
const port = 3000
const increment = function () {
  counter ++
  setTimeout(increment, 1000);
}

var maxAge = 4

increment()

var counter = 0

app.get('*', (req, res) => {
  console.log(`Counter: ${counter}, request cache policy: ${req.headers['cache-policy']}`)
  res.set('Cache-Control', `public, max-age=${maxAge}`)
  res.set(`Etag`, `${Math.floor(counter / 2)}`)
  res.send(`Counter: ${counter}`)
})

app.listen(port, ()=> console.log(`listening at http://0.0.0.0:${port}`))