index.js

import debug from 'debug'
require('dotenv').load()
const log = debug('app')

let applicationEnvVars = [ 'APP_ENVIROMENT', 'PORT', 'USE_MONGODB', 'USE_MYSQL' ]
let mysqlEnvVars = [ 'MYSQL_HOSTNAME', 'MYSQL_PORT', 'MYSQL_USERNAME', 'MYSQL_DATABASE', 'MYSQL_PASSWORD' ]
let mongoEnvVars = [ 'MONGO_HOSTNAME', 'MONGO_PORT', 'MONGO_DATABASE' ]

if (process.env.USE_MONGODB === 'true') applicationEnvVars = [ ...applicationEnvVars, ...mongoEnvVars ]
if (process.env.USE_MYSQL === 'true') applicationEnvVars = [ ...applicationEnvVars, ...mysqlEnvVars ]

let unusedEnvVars = applicationEnvVars.filter((i) => !process.env[i])

if (unusedEnvVars.length) {
  log('Required ENV variables are not set: [' + unusedEnvVars.join(', ') + ']')
  process.exit(1)
}

const { app } = require('./server/app.js')

app.listen(process.env.PORT)

export default app