How would you validate environment configuration at startup?

Direct answer

Parse process.env with Zod or envalid schema at boot — fail fast with clear missing var message; export typed config object; commit .env.example; never read process.env scattered across modules.

Scenario: API must not start with missing DATABASE_URL or JWT_SECRET — catch misconfiguration before accepting traffic (Q48).

Single config module — apps/api/src/config.ts imported first.
Zod schema — z.string().url(), z.coerce.number(), enums for NODE_ENV.
Fail fast — process.exit(1) with formatted Zod error.
No secrets in logs — log which keys missing, not their values.

scenario-env-validation.mjs

import { z } from "zod";

const envSchema = z.object({
  NODE_ENV: z.enum(["development", "test", "production"]).default("development"),
  PORT: z.coerce.number().default(4000),
  DATABASE_URL: z.string().url(),
  JWT_SECRET: z.string().min(32),
  REDIS_URL: z.string().url().optional(),
});

const parsed = envSchema.safeParse(process.env);

if (!parsed.success) {
  console.error("Invalid environment:", parsed.error.flatten().fieldErrors);
  process.exit(1);
}

export const config = parsed.data;