Dynamic Environment Variables for Different Stages

Provide an example of dynamically setting environment variables based on different deployment stages (development, staging, production) in Next.js.
module.exports = (phase, { defaultConfig }) => {
  // phase is one of the strings 'development', 'production build', or 'production',
  // which allows you to customize the config per build type

  // Create a map of environments to their custom variables
  const envConfig = {
    development: {
      ENV_VAR: 'development-value',
      ANOTHER_ENV_VAR: 'another-development-value',
    staging: {
      ENV_VAR: 'staging-value',
      ANOTHER_ENV_VAR: 'another-staging-value',
    production: {
      ENV_VAR: 'production-value',
      ANOTHER_ENV_VAR: 'another-production-value',

  // Determine the current stage based on phase
  const currentStage = phase === 'development' ? 'development' : (process.env.STAGE === 'staging' ? 'staging' : 'production');

  // Merge environment variables with the default config
  return {
    env: {
This piece of code is a Next.js configuration file that exports a function receiving the 'phase' and a 'defaultConfig'. Based on the phase and an optional STAGE environment variable, it dynamically sets environment variables for different deployment stages such as development, staging, and production. It defines a mapping of stages to their environment variables and then merges the stage-specific variables into the default configuration.