Challenge 2: Fail Fast on Boot — Possible Solution
====================================================================

import { z } from "zod";

const ConfigSchema = z.object({
  NODE_ENV: z.enum(["development", "production", "test"]),
  PORT: z.coerce.number().int().positive(),
  LOG_LEVEL: z.enum(["debug", "info", "warn", "error"]),
});

type Config = z.infer<typeof ConfigSchema>;

function loadConfig(env: NodeJS.ProcessEnv = process.env): Config {
  const result = ConfigSchema.safeParse(env);

  if (!result.success) {
    console.error("❌ Invalid configuration. Fix these issues and restart:\n");
    for (const issue of result.error.issues) {
      console.error(`  - ${issue.path.join(".")}: ${issue.message}`);
    }
    process.exit(1);
  }

  return result.data;
}

// Called once, at the very top of the app's entry point —
// before setting up routes, connecting to a database, or starting a server.
export const config = loadConfig();

// If NODE_ENV or PORT is missing/invalid, the process exits immediately
// with a clear message, e.g.:
//
//   ❌ Invalid configuration. Fix these issues and restart:
//
//     - PORT: Expected number, received nan
//     - NODE_ENV: Invalid enum value. Expected 'development' | 'production' | 'test', received undefined
//
// instead of crashing three requests into production with a vague
// "Cannot read properties of undefined" deep inside some handler.

WHY THIS WORKS
--------------
- safeParse() (instead of parse()) returns a { success, data } or
  { success, error } result rather than throwing — that lets us print every
  validation issue at once instead of stopping at the first one.
- process.exit(1) ensures a misconfigured app never starts serving traffic
  at all — "fail fast" means failing at boot, in full view of whoever
  deployed it, not failing later against a real user's request.
- Accepting `env` as a parameter (defaulting to process.env) makes
  loadConfig() itself testable — a test can pass a fake env object without
  needing to mutate the real process.env.