24. Environment-based configuration

Production Node apps read all configuration — ports, database URLs, RPC endpoints, feature flags — from the environment rather than hard-coded values, so the same built artifact runs unchanged across dev, staging and production.

js
function loadConfig() {
  const required = ["RPC_URL", "DATABASE_URL"];
  for (const key of required) {
    if (!process.env[key]) throw new Error(`missing required env var: ${key}`);
  }
  return {
    port: Number(process.env.PORT ?? 3000),
    rpcUrl: process.env.RPC_URL,
    databaseUrl: process.env.DATABASE_URL,
    isProd: process.env.NODE_ENV === "production",
  };
}

Fail fast at startup if required configuration is missing — discovering a missing `DATABASE_URL` mid-request, after already accepting traffic, is far worse than crashing immediately with a clear error at boot.

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