05 — Async Patterns & Error Handling

Core Concepts

• Callbacks — the original Node async pattern (error-first convention)
• Promises — cleaner chaining, .then() / .catch()
• async/await — syntactic sugar for Promises (recommended)
• Error-first callback — (err, result) => {} convention
• try/catch — catch errors in async/await
• Unhandled rejections — process.on('unhandledRejection') for uncaught Promise errors
• Concurrency — Promise.all, Promise.allSettled, Promise.race

Error Handling Rules

1. Always handle errors — never swallow them silently
2. Use try/catch with async/await
3. Add unhandledRejection handler as a safety net
4. Throw errors (don't return them) — let the caller decide
5. Use custom error classes for different error types