config.js

const customEnv = require('custom-env');

const NODE_ENV = process.env.NODE_ENV || 'development';

/**
 * Shares behaviour with CRA for envs
 * https://create-react-app.dev/docs/adding-custom-environment-variables#what-other-env-files-can-be-used
 **/
customEnv.env();
customEnv.env('local');
customEnv.env(NODE_ENV);
customEnv.env(`${NODE_ENV}.local`);

const { MY_ENV_VAR } = process.env;

const defaults = { MY_ENV_VAR: null };

// Alert to fill the necessary environment variables
Object.keys(defaults).forEach(key => {
  if (!process.env[key]) {
    throw new Error(
      `Please enter a custom ${key} in .env.${NODE_ENV} on the root directory`
    );
  }
});

module.exports = {
  NODE_ENV,
  MY_ENV_VAR,
};