In .env files (NODE_ENV=development or production), in package.json scripts ("dev": "NODE_ENV=development nodemon src/server.js"), or in PM2 ecosystem files (env: { NODE_ENV: "production" }). Always set it many Express and npm packages behave differently based on NODE_ENV.
Use separate .env files per environment (.env.development, .env.staging, .env.production), load the right one based on NODE_ENV with dotenv config path, centralize all config in a config module, and use environment-specific behavior for cookies, CORS, and logging. Always use different secrets per environment.
Development is for local coding (local DB, debug logging, dev secrets). Staging is for pre-production testing (staging DB, info logging, staging secrets, mirrors production). Production is the live app (production DB, warn logging, strong production secrets, real users).
Create a src/config/index.js module that loads dotenv, defines a config object with all variables (with defaults and type casting), validates required variables, and exports the config. Use config.isDev, config.isProd, config.isStaging for environment-specific behavior.
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