Configure instance express only in development or production - Express + Testing Routes + address already in use :::3000

App.js

import { env, port } from './config/vars'

import app from './config/express'

const development = env === 'development'

if (process.env.NODE_ENV !== 'test')
  app.listen(
    port,
    () => development && console.log(`Server started on port ${port} (${env})`),
  )

module.exports = app

package.json

{
  "name": "example",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "NODE_ENV=test jest"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}