Add support to Next.js for multiple "app environments" .env files (development, preview, staging, production)

loadAppEnv.js

// loadAppEnv.js
const fs = require('fs')
const dotenvFlow = require('dotenv-flow')

// This module loads .env files from the config/ directory based the "app
// environment" that it is being built for or is running in (e.g. development,
// preview, staging, beta, canary, production, etc..). The "app environment"
// is determined by APP_ENV (if defined) or NODE_ENV. It exports a set of
// environment variables which should be passed to Next.js as the env config.
//
// We do not place .env files in the project root because Next.js loads those
// based on NODE_ENV and is limited to just "development", "production", and
// "test". By using a distinct environment variable we can more easily create
// a "production" build (minified JavaScript, etc...) that uses "development"
// or "staging" application settings. This solution was based on the "proposed
// workaround" described in the following Next.js RFC:
//  - https://github.com/vercel/next.js/discussions/25764

function throwErrorIfConflictingFilesDetected() {
  // we throw an exception if .env files are found in the project root since
  // they can conflict with ours. Someone probably just didn't realize we
  // store .env files in ./config
  fileNames = [
    '.env',
    '.env.local',
    '.env.development',
    '.env.development.local',
    '.env.production',
    '.env.production.local',
    '.env.test',
    '.env.test.local',
  ]
  fileNames.forEach((path) => {
    if (fs.existsSync(path)) {
      throw new Error(
        `env file detected in project root: ${path}. This project requires all .env files to be stored in ./config`
      )
    }
  })
}

function loadAppEnv() {
  throwErrorIfConflictingFilesDetected()
  dotenvFlow.config({
    path: 'config',
    node_env: process.env.APP_ENV || process.env.NODE_ENV || 'development',
  })
}

loadAppEnv()

next.config.js

// next.config.js
require('./loadAppEnv')

module.exports = {
  //...
}

thank you very much! Saved my day

Doesnt this need a return env in line 53?

@asp3 hmmm... I think you're right. However, it seems that code isn't even needed. Maybe it used to be needed but isn't any more. Will update.