JavaScript and Node.js Experienced

JavaScript & Node.js Interview Questions Experienced – 30+ Advanced Topics

Mastering the toughest JavaScript & Node.js Interview Questions Experienced developers encounter gives you a clear advantage. In the first question alone, we dive deep into asynchronous patterns and event loops. Thus, from core JS internals and memory management to server-side architecture and performance tuning, this guide offers more than 30 expert-level questions designed to sharpen your understanding and prepare you for senior-level interviews.

Furthermore, answering JavaScript & Node.js Interview Questions Experienced with confidence shows recruiters your in-depth knowledge of modern JavaScript, asynchronous design, system patterns, and debugging skills. Ready to level up? Let’s begin!

1. Explain the Event Loop and microtasks vs macrotasks.

The Node.js event loop manages execution phases, with timers, I/O callbacks, and check phases. Microtasks (Promises, nextTick) run before the next phase, while macrotasks (setTimeout, setImmediate) schedule later. Use examples to show how promise resolution always precedes timers.

2. How do callbacks, promises, and async/await differ?

Callbacks are simple but lead to “callback hell.” Promises provide cleaner chaining and error handling. Async/await builds on promises with synchronous syntax. Explain error flows, cancellation challenges, and stack trace clarity.

3. What are memory leaks in Node.js and how do you detect them?

Common leaks include listeners, global caches, large buffers, or closures. Use tools like Chrome DevTools, heap snapshots, and `heapdump` to trace retained memory. Walk through fixing a common closure leak scenario.

4. Explain scoping and hoisting in JavaScript.

var variables hoist to function scope, while let/const hoist to block scope but remain uninitialized (TDZ) until declared. Show practical examples that demonstrate unexpected behavior during access before declaration.

5. Describe prototype-based inheritance and ES6 classes.

Classical vs prototypal: explain how each object points to its prototype. Show ES5 constructor + prototype chain, then transition to ES6 `class` syntax while explaining how it remains syntactic sugar.

6. What is the difference between call, apply, and bind?

`call` and `apply` invoke functions immediately with specified `this`. `apply` takes array arguments. `bind` returns a new function with bound `this`. Detail examples and tell how to borrow methods or implement partial application.

7. Explain the module systems: CommonJS vs ES Modules.

Node.js uses CommonJS (`require`, `module.exports`), which is synchronous and mutable. ES Modules (`import`, `export`) are statically analyzable, support tree-shaking, and can be asynchronous. Discuss interop considerations like `.mjs`, transpiling, and import bindings.

8. How do you manage concurrency in Node.js?

Node.js is single-threaded but supports concurrency via the event loop and libuv thread pool. Use cluster, worker_threads, or child_process to scale CPU-bound workloads. Explain when to choose each approach.

9. What are Streams in Node.js and how do they improve performance?

Streams handle large data in chunks, avoiding full buffering. Describe readable, writable, duplex, and transform streams. Use piping (`stream.pipe()`) to build memory-efficient pipelines for I/O operations.

10. How do you implement rate limiting in a distributed Node API?

Use in-memory for single-instance, but distributed systems require Redis- or Memcached-based tokens or leaky bucket algorithm. Explain using `express-rate-limit` with a Redis store, handling TTL and bursts.

11. Describe debugging techniques in JS and Node.js.

Trace in browser DevTools for client-side, use `–inspect`, `node –inspect-brk`, VSCode breakpoints, logs with `pino`/`winston`, and `async_hooks` for tracking async lifespan. Combine logs and heap profiles to pinpoint errors.

12. Explain event emitter memory leaks and maxListeners.

Adding listeners without removal leads to leaks. Increase `emitter.setMaxListeners()`, remove listeners via `off()`, and use `once()`. Monitor warnings and track outstanding subscription counts.

13. What are TypedArrays and Buffers?

TypedArrays (`Uint8Array`, etc.) let you manipulate binary data in JS. Node Buffers handle raw binary streams. Use typed views for performance, such as parsing binary protocols or images.

14. How do you handle uncaught exceptions and process.exit cleanup?

Attach listeners to `process.on(‘uncaughtException’)` and `unhandledRejection`, then clean up state before exit. Prefer domains or child processes for fault isolation. Explain how async cleanup can delay exit.

15. What is the difference between synchronous and asynchronous filesystem APIs?

Synchronous APIs block the event loop. Always use async versions (`fs.promises`, callbacks, streams) for performance. Use chunked reads to avoid overloading memory.

16. Describe the Job and Task queues in the event loop.

Job queue handles microtasks like promises and `process.nextTick()`, which run immediately after the current stack. Tasks come from timers, I/O, setImmediate, and UI events. Demonstrate ordering effects.

17. Explain cross-site scripting (XSS) prevention in Node.js APIs.

On the client side, escape user input using frameworks or sanitizers. On server, set secure headers, sanitize inputs, and encode outputs. Use HTTPS and CORS properly for layered security.

18. How do you implement clustering in Node.js?

Use `cluster` module to fork child processes. Workers share socket handles for load balancing. Discuss sticky sessions, graceful reloads, and zero-downtime upgrades with `cluster.fork()`.

19. What is memory profiling with Node.js?

Use `–inspect`, `heapdump`, Chrome DevTools, Clinic.js, or `memwatch-next` to collect heap snapshots, analyze memory growth, and identify leaks via object retainers.

20. Explain Type Coercion and the `===` vs `==` operators.

`==` coerces types (e.g., 0 == “0” → true), while `===` checks strict equality. Show common pitfalls like `null == undefined`. Recommend always using strict equality.

21. How do you secure Node.js applications?

Validate and sanitize input, use Helmet to set secure headers, run Express middleware to limit payload size, and update dependencies via `npm audit`. Protect against SQL injection with parameterized queries.

22. What is server-side rendering vs client-side? Use cases?

SSR (Next.js, Nuxt.js, Express View engines) improves SEO and time-to-first-paint. CSR benefits from rich interactivity. Use SSR for content-heavy pages and CSR for dynamic apps.

23. Explain GraphQL vs REST in Node.js context.

REST uses fixed endpoints; GraphQL enables querying multiple resources in one request. Use Apollo Server or express-graphql. GraphQL allows versionless APIs and introspection, but also potential performance pitfalls (N+1 problems).

24. How do you monitor Node.js services in production?

Use Prometheus/Grafana or ELK for metrics, logging, and tracing. Use APMs like New Relic, Datadog. Monitor event loop lag with `event-loop-delay` metrics and memory growth over uptime.

25. Describe build and deployment pipelines in Node.js.

Use Docker and CI/CD (Jenkins, GitHub Actions). Lint with ESLint, test with Jest/Mocha, bundle with Webpack. Use semantic versioning and pre-deploy smoke tests for reliable releases.

26. What are worker_threads and when to use them?

`worker_threads` let you run JavaScript in separate threads for CPU-heavy operations. They communicate via messages or `SharedArrayBuffer`. Use them when clusters are insufficient for parallel tasks.

27. Explain CORS and how to configure it securely.

CORS ensures safe cross-origin communication. Configure `Access-Control-Allow-Origin`, methods, and headers carefully. Use express middleware or Kubernetes ingress to centralize control.

28. How do you design a REST API with pagination and filtering?

Use query params like `?page=2&limit=20` or `?cursor=xxx&limit=…`. Use efficient database queries with OFFSET/LIMIT or keyset pagination for large datasets.

29. What is JWT and how do you protect it?

JWTs encode user info using HMAC or RSA. Use short expiration times, secure HTTP-only cookies, and rotate secrets regularly. Use `jsonwebtoken` with refresh tokens for secure authentication.

30. Explain event sourcing in Node.js systems.

Event sourcing stores state changes as immutable events. Use with message brokers (Kafka) or event stores. Helps with auditability and rebuilding state. Handle schema migrations carefully.

31. Bonus: What are design patterns used in Node.js?

Explain Singleton for DB connections, Factory for middleware injection, Observer via EventEmitter, Decorator for enhancing imports, and Strategy for swapping algorithms or DB engines.

Final Thoughts

These **JavaScript & Node.js Interview Questions Experienced** cover deep technical aspects—from event loops and memory leaks to system design and security. Practice coding examples, debug live scenarios, and describe trade-offs to ace your interviews. With consistent preparation and real-world context, you’ll stand out as a senior JavaScript and Node.js engineer!