Top 20 Interview Questions Every Developer Should Prepare For

Technical interviews can be challenging, but preparation is key. Here are the top 20 questions you should be ready to answer.

Technical Questions

1. Explain the difference between == and === in JavaScript The == operator compares values with type coercion, while === compares both value and type strictly. Always prefer === for predictable comparisons.

2. What is the time complexity of common operations? - Array access: O(1) - Array search: O(n) - Hash table lookup: O(1) average - Binary search: O(log n) - Sorting (efficient): O(n log n)

3. Explain REST API principles - Stateless - Each request contains all information needed - Client-server architecture - Separation of concerns - Uniform interface - Consistent resource naming - Cacheable - Responses can be cached - Layered system - Client doesn't know if connected directly to server

4. What is the difference between SQL and NoSQL? SQL databases are relational with fixed schemas, ACID compliance, and use structured query language. NoSQL databases are non-relational with flexible schemas, eventual consistency, and various data models (document, key-value, graph).

5. Explain the concept of closures A closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has returned. This enables data privacy and function factories.

6. What is the difference between GET and POST requests? GET requests retrieve data and are idempotent (can be repeated safely). POST requests submit data and may cause side effects. GET parameters are in the URL, POST parameters are in the request body.

7. Explain the concept of promises and async/await Promises represent eventual completion of async operations. Async/await is syntactic sugar that makes async code look synchronous, improving readability and error handling.

8. What is the difference between stack and heap memory? Stack stores local variables and function calls (LIFO, fast, limited size). Heap stores dynamically allocated objects (flexible size, slower, requires garbage collection).

9. Explain SOLID principles - Single Responsibility: One class, one purpose - Open/Closed: Open for extension, closed for modification - Liskov Substitution: Subtypes must be substitutable - Interface Segregation: Many specific interfaces over one general - Dependency Inversion: Depend on abstractions, not concretions

10. What is the difference between authentication and authorization? Authentication verifies identity (who you are). Authorization determines access rights (what you can do). Authentication comes first, then authorization.

11. Tell me about a challenging project Use the STAR method: Situation (context), Task (your responsibility), Action (what you did), Result (outcome and learnings).

12. How do you handle disagreements with team members? Focus on active listening, understanding different perspectives, presenting data-driven arguments, and finding common ground. Escalate professionally if needed.

13. Describe a time you failed and what you learned Be honest about the failure, take responsibility, explain what you learned, and describe how you've applied that learning since.

14. How do you stay updated with technology? Mention specific resources: tech blogs, podcasts, conferences, online courses, side projects, open source contributions, and professional communities.

15. How do you prioritize tasks when everything is urgent? Discuss frameworks like Eisenhower Matrix, communication with stakeholders, understanding true deadlines, and breaking down large tasks.

16. Design a URL shortener Key considerations: Base62 encoding for short URLs, database design (key-value store), handling collisions, analytics tracking, caching strategy, and horizontal scaling.

17. Design a rate limiter Discuss algorithms (token bucket, sliding window), storage (Redis), distributed systems considerations, and graceful degradation.

18. How would you design a cache system? Cover cache invalidation strategies (TTL, LRU, LFU), cache-aside vs write-through patterns, distributed caching, and cache stampede prevention.

19. Design a notification system Consider multiple channels (push, email, SMS), user preferences, delivery guarantees, rate limiting, and template management.

20. How would you scale a database? Discuss vertical vs horizontal scaling, read replicas, sharding strategies, connection pooling, query optimization, and caching layers.

Good luck with your interviews!