check-env.js

const env = process.env.NODE_ENV
const task = process.env.npm_lifecycle_event
const packageJSON = require('../package.json')
const availableEnvironments = Object.keys(packageJSON.scripts)
  .filter(key => key.startsWith(task))
  .map(key => key.split(':')[1])
  .filter(key => key)

function displayMessage (message) {
  console.error(`${'='.repeat(message.length)}\n${message}\n${'='.repeat(message.length)}`)
}

if (!env) {
  const errorMessage = `[ Error ] NODE_ENV is required. Use ${task}:${availableEnvironments.join('/')} scripts instead.`
  displayMessage(errorMessage)
  process.exit(1)
}

if (!availableEnvironments.includes(env)) {
  const errorMessage = `[ Error ] ${env} is not valid NODE_ENV. Use ${task}:${availableEnvironments.join('/')} scripts instead.`
  displayMessage(errorMessage)
  process.exit(1)
}

process.exit(0)