setenv.js

'use strict'
const fs = require('fs')
const crypto = require('crypto')
const path = require('path')

const envFileRegEx = /env.([a-z0-9]*.)?js/g
const buildDir = './build'

// Generate the configuration script from environment variables
function generateConfigScript() {
  const prefixRegexp = /^REACT_APP_/
  const env = process.env
  const config = Object.keys(env)
    .filter(key => prefixRegexp.test(key))
    .reduce((c, key) => Object.assign({}, c, { [key]: env[key] }), {})

  console.log(config)
  return `window.env=${JSON.stringify(config)};`
}

// Save the configuration script to disk
function saveConfigScript(scriptContents, scriptName) {
  const scriptPath = path.join(buildDir, scriptName)
  console.log('Saving file to: ' + scriptPath)
  fs.writeFile(scriptPath, scriptContents, 'utf8', function (err) {
    if (err) throw err

    console.log('The file was saved!')
  })
}

// Update configuration script name in index.html
function updateIndex(scriptName) {
  const indexPath = path.join(buildDir, 'index.html')

  fs.readFile(indexPath, 'utf8', function (err, data) {
    if (err) throw err

    let result = data.replace(envFileRegEx, scriptName)

    fs.writeFile(indexPath, result, 'utf8', function (err) {
      if (err) throw err

      var indexHash = crypto.createHash('md5').update(result).digest('hex')
      updateIndexInServiceWorker(indexHash)
    })
  })
}

// Update index hash in service-worker.js
function updateIndexInServiceWorker(indexHash) {
  const swPath = path.join(buildDir, 'service-worker.js')

  fs.readFile(swPath, 'utf8', function (err, data) {
    if (err) throw err

    let result = data.replace(/"\/index.html","(.*?)"/, `"/index.html","${indexHash}"`)

    fs.writeFile(swPath, result, 'utf8', function (err) {
      if (err) throw err
    })
  })
}

// Delete old configuration scripts from disk
function deleteOldScript(currentScriptName) {
  const files = fs.readdirSync(buildDir).filter(fn => fn != currentScriptName && fn.match(envFileRegEx))
  files.forEach(fn => {
    let filePath = path.join(buildDir, fn)
    fs.unlink(filePath, err => {
      if (err) throw err
    })
  })
}

// MAIN
console.log('Setting runtime envoronment..')
let scriptContents = generateConfigScript()
let scriptHash = crypto.createHash('md5').update(scriptContents).digest('hex')
let scriptName = `env.${scriptHash}.js`

saveConfigScript(scriptContents, scriptName)
updateIndex(scriptName)
deleteOldScript(scriptName)