This part dives deep into the backend of the MERN stack — covering real-time Node.js concepts, Express.js best practices, MongoDB performance tricks, and even Managerial round questions for senior roles. Whether you’re a full-stack developer or aiming for a tech lead position, this guide will strengthen both your technical and leadership readiness.
NODE.JS, EXPRESS & MONGODB INTERVIEW QUESTIONS AND ANSWERS
1. Explain the event loop in Node.js
The event loop is what allows Node.js to perform non-blocking I/O operations. It continuously checks the call stack and callback queue to execute code, handle events, and run asynchronous tasks.
2. Explain the internal architecture of Node.js
Node.js uses a single-threaded event loop with non-blocking I/O. It’s built on the V8 JavaScript engine and uses libuv to handle threads, networking, and I/O.
3. What is npm and list out the uses of it.
NPM (Node Package Manager) is used to install, manage, and version control packages. It’s essential for adding libraries and tools in Node.js projects.
4. What are the different phases of event loop
Phases: timers → pending callbacks → idle/prepare → poll → check → close callbacks → nextTick/microtasks.
5. Explain the execution flow of Promise, async/await, setTimeout, setImmediate, process.nextTick()
Execution order:process.nextTick() (microtask queue)Promises / async-await (microtask queue)setTimeout, setImmediate (macrotask queue)
6. What is the purpose of package.json file in Node.js
It stores metadata about the project: name, version, dependencies, scripts, etc.
7. What is http module and create a server using it
The http module allows creation of web servers:
8. How do you create secure server setup using express and https
Use https module with SSL certificates:
9. How will you update any npm existing packages
Run npm update, or manually change the version in package.json and reinstall.
10. List out some of the global objects in Node.js__dirname, __filename, global, process, setTimeout, setImmediate, Buffer
11. Explain stream and buffer and their difference
Buffer: stores entire data in memory.
Stream: processes data in chunks, efficient for large data.
12. What is microtask and macrotask in Node.js
Microtasks: process.nextTick(), Promises
Macrotasks: setTimeout, setImmediate
13. Explain piping the data between streams
It sends data from one stream to another:
14. What are the features of Express.js
Routing, middleware support, error handling, template engine support, minimal and fast.
15. What happens if we pass parameter to next function inside middleware
If an error is passed to next(err), it triggers the error-handling middleware.
16. How we can jump from one middleware to another skipping some.
Use conditions inside middleware or next('route') to skip to the next route.
17. What is difference between PATCH and PUT API
PUT: replaces the entire object
PATCH: updates only specific fields
18. Explain the middleware concept in Node.js
Middleware functions execute before the final route handler, and can modify request or response objects.
19. How to create optional path API endpoints in Express.js
Use ? in route params like /user/:id? or regex patterns.
20. Explain error handling concept in Express.js
Use an error-handling middleware with 4 parameters:
21. What is JWT and explain the structure of JWT token
JWT (JSON Web Token) is used for authentication.
Structure: header.payload.signature
22. How can we modify the signature of JWT?
You can’t. The signature is hashed using a secret key. Changing it invalidates the token.
23. What is the authentication and authorization concept?
Authentication: verifying who the user is
Authorization: checking what the user can access
24. How do you implement role-based and permission-based access control in Express.js
Use middleware to check user roles before granting access:
25. Explain in detail best practice of API
Use versioning, proper HTTP methods, input validation, authentication, pagination, and consistent status codes.
26. Explain input validation and data sanitization concept in Express.js
Use libraries like joi, express-validator to validate inputs and prevent injection attacks.
27. Explain the concept of rate limiting in Express.js
Limit number of requests per user using express-rate-limit to prevent abuse.
28. Explain the concept of logging and monitoring in Node.js application
Use winston, morgan for logging. Use tools like ELK Stack or Prometheus for monitoring.
29. What is difference between stateless and stateful API
Stateless: doesn’t retain client session info (preferred in REST)
Stateful: retains session between requests (e.g., via cookies)
30. How can we achieve state management in Node.js application
Use cookies, sessions (express-session), JWT, or caching (Redis).
31. How do you scale Node.js application using cluster module
Use Node’s built-in cluster module to run multiple instances across CPU cores.
32. What is a worker thread? How does it handle CPU-intensive tasks
Worker threads allow parallel execution of JavaScript in separate threads.
33. Explain the working with concurrency and parallelism for async task
Concurrency: multiple tasks progressing at the same time
Parallelism: multiple tasks running exactly at the same time (on different threads)
34. How can you optimise database queries
Use indexes, avoid full scans, limit fields with projection, paginate results.
35. What is the Redis database? Explain the uses of it
Redis is an in-memory key-value store used for caching, sessions, pub/sub, and queues.
36. How we can increase the performance of Node.js application
Use async programming, clustering, caching, reduce middleware, and enable gzip.
37. What is the profiling concept in Node.js
Profiling helps analyze CPU usage and memory leaks using tools like clinic.js, node --inspect.
38. What is connection pooling concept for database
Maintains a pool of DB connections so connections can be reused instead of reopened.
39. Write a middleware for role-based access control (RBAC) in Node.js
40. Demonstrate the integration of OAuth2.0 for third-party authentication in a Node.js app
Use passport.js with OAuth2 strategy for services like Google, Facebook.
41. Implement a refresh token mechanism for JWT authentication in Node.js
On access token expiry, verify refresh token (stored securely) and issue a new access token.
42. What is role of indexes in performance optimization
Indexes speed up queries by reducing the number of documents to scan.
43. What is the aggregation pipeline in MongoDB? Explain in detail
Processes documents in stages ($match, $group, $sort, $project) for analytics and reporting.
44. What are transactions? How can we achieve it in MongoDB
Transactions allow multiple operations to succeed or fail together. Requires replica set and sessions.
45. How transaction helps in rollback in case of failure
If an operation fails, all previous operations in the transaction are rolled back to maintain consistency.
46. What is difference between hashing, encryption and encoding concept
Hashing: one-way, irreversible
Encryption: reversible with a key
Encoding: transforms data for transport (not secure)
47. How do you identify slow queries
Use .explain(), MongoDB logs, slow query profiler, or Atlas performance dashboard.
48. What is replica sets and sharding mechanism in MongoDB
Replica set: copies data across nodes for high availability
Sharding: splits large collections across multiple servers
49. How do you configure failover and disaster recovery in MongoDB
Use replica sets, regular backups, monitoring tools, and automated alerts.
50. What is load balancer and how to implement in Node.js
Use NGINX, HAProxy, or cloud LB services to distribute traffic across app instances.
51. How to avoid schema pitfalls
Use proper schema validation (Mongoose), avoid deeply nested documents, and normalize data where needed.
52. Measure care taken to write efficient queries
Use indexing, avoid $ne, paginate, project only required fields.
53. How to implement health checks in MongoDB
Use db.runCommand({ ping: 1 }), monitor replication lag, and check cluster health with monitoring tools.
54. What are the SOLID principles for software development?
Design principles for maintainable code:
S: Single Responsibility
O: Open/Closed
L: Liskov Substitution
I: Interface Segregation
D: Dependency Inversion
55. What is Kafka? Explain the use cases of Kafka
Apache Kafka is a distributed streaming platform.
Use cases: logs, messaging, event streaming, microservice communication.
56. How can we minimize middleware overhead
Only load necessary middleware, avoid synchronous/blocking operations, and cache responses where possible.
Conclusion:
Cracking backend and system-level interviews requires not just coding knowledge, but clarity on architecture, scalability, and real-world best practices. Use this set of Q&As to stay ahead and stand out in your next Node.js or full-stack interview.
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