Error-first callbacks and unhandled rejections

Model answer. Older Node APIs take a callback whose first argument is the error (or null), and the result after it:

fs.readFile('config.json', 'utf8', (err, text) => {
  if (err) return console.error('could not read config', err);   // always check first
  start(JSON.parse(text));
});

The convention makes it impossible to forget that an operation can fail. But errors thrown inside the callback are not caught by a try around readFile, and nesting many callbacks gets unreadable ("callback hell").

With promises / async–await, errors become rejections, which you handle with .catch() or try/catch around await. util.promisify converts callback APIs, and most core modules have promise versions (fs/promises).

Process-level failures:

• Unhandled rejection: since Node 15 it crashes the process by default.
• Uncaught exception: also crashes. A process.on('uncaughtException') handler should only log and exit, because the app may be in a broken state.

In production, let the process crash, and have a supervisor (systemd, Docker, PM2, Kubernetes) restart it, with logs and alerts.