TypeScript / Node.js — Theory

Single-threaded but non-blocking — how

JS on one thread. Blocking ops (I/O, crypto, DNS) → libuv. Network = OS async (epoll/kqueue/IOCP). File + DNS = thread pool. CPU-bound work blocks every request → use worker threads, child processes, or offload.

Event loop trick questions

• setImmediate vs setTimeout(fn, 0): from main script = nondeterministic. From I/O callback = setImmediate always first.
• nextTick vs Promise: nextTick before Promises. Both between phases.
• Poll phase: blocks waiting for I/O if no timers/setImmediate queued.

Memory

• V8: young (scavenge) + old (mark-sweep + compact). Generational GC.
• Default cap ~1.5GB on 64-bit. Raise: --max-old-space-size=4096.
• Leaks: unbounded caches, listener leaks, closures holding refs, timers never cleared.
• Diagnose: --inspect, Chrome DevTools heap snapshot, clinic.js, --heapsnapshot-signal=SIGUSR2.

Concurrency primitives

• Cluster: master forks N workers per CPU; share port via OS. Each worker own V8. Stateless HTTP.
• Worker threads: own V8 isolate, message-passing or SharedArrayBuffer. CPU-bound work.
• Child process: full new Node process. Heaviest.

TypeScript senior questions

• unknown vs any: unknown forces narrow. Use in catch(e: unknown) (TS 4.4+).
• Discriminated union: literal tag kind. Switch narrows. Better than class hierarchy.
• Generic constraint: <T extends { id: string }> enforces shape.
• infer: type Awaited<T> = T extends Promise<infer U> ? Awaited<U> : T.
• Mapped modifiers: +?/-?, +readonly/-readonly, as rename keys.
• Variance: params contravariant (with strictFunctionTypes), return covariant.

Common interview Qs

1. Walk app.get('/x') lifecycle: TCP accept → llhttp → router → middleware → response → flush.
2. Why await in for serializes — parallelize with Promise.all.
3. Debug event loop lag: monitorEventLoopDelay, perf_hooks, clinic doctor.
4. child_process.fork vs worker_threads.Worker — process boundary vs thread/isolate.
5. Promise rejection in Node 15+ — exits by default.
6. Type pick<T, K extends keyof T>(obj: T, keys: K[]): Pick<T, K>.
7. Graceful shutdown: drain in-flight, close DB/Redis, exit.

Pitfalls

• CJS/ESM interop fragility.
• Arrow this lexical vs regular dynamic.
• JSON.stringify on circular ref throws.
• Race on async init — share single init() Promise.
• Pre-Node 15 unhandled rejections silently lost.