✅ Introduction

Whether you’re a fresher or an experienced developer, technical interviews can be intense. This guide contains 30 frequently asked technical questions, complete with concise model answers and tips—perfect for coding interviews in 2025.

🔍 Categories Covered:

• ✅ Programming Basics
• ✅ OOP & System Design
• ✅ Data Structures & Algorithms
• ✅ DBMS & SQL
• ✅ Web Development & APIs

🧠 Top 30 Technical Interview Questions and Answers

1. What is the difference between == and === in JavaScript?

🟢 Answer:
== compares values after type coercion, while === compares both value and type.

'5' == 5  // true  
'5' === 5 // false

2. What is a closure in JavaScript?

🟢 Answer:
A closure is a function that retains access to its lexical scope even when the function is executed outside that scope.

3. What are the four pillars of OOP?

🟢 Answer:

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

4. Explain the difference between stack and heap memory.

🟢 Answer:

• Stack: Static memory allocation, faster access.
• Heap: Dynamic memory allocation, more flexible, slower.

5. What is the difference between process and thread?

🟢 Answer:

• Process: Independent, has its own memory space.
• Thread: Lightweight, shares memory within a process.

6. Explain RESTful APIs.

🟢 Answer:
REST is an architectural style that uses HTTP methods like GET, POST, PUT, DELETE to manage resources identified by URIs.

7. What is a promise in JavaScript?

🟢 Answer:
A promise represents the eventual completion or failure of an asynchronous operation.

8. What is Big-O notation?

🟢 Answer:
It describes the time or space complexity of an algorithm in the worst-case scenario.

9. What’s the time complexity of binary search?

🟢 Answer:
O(log n)

10. How is an array different from a linked list?

🟢 Answer:
Arrays allow random access but have fixed size. Linked lists allow dynamic size but require sequential access.

11. What is a hash table?

🟢 Answer:
A data structure that maps keys to values using a hash function for constant time lookup on average.

12. Difference between SQL and NoSQL databases?

🟢 Answer:
SQL is relational and uses structured schemas; NoSQL is non-relational, flexible, and better for unstructured data.

13. What is normalization in databases?

🟢 Answer:
It’s the process of organizing data to reduce redundancy and improve integrity.

14. What is an index in SQL?

🟢 Answer:
An index improves the speed of data retrieval but slows down write operations.

15. Difference between HTTP and HTTPS?

🟢 Answer:
HTTPS is HTTP with encryption (SSL/TLS) for secure communication.

16. What is CORS?

🟢 Answer:
Cross-Origin Resource Sharing – a security feature that allows/disallows resources to be requested from another domain.

17. Explain event loop in JavaScript.

🟢 Answer:
The event loop manages the call stack and task queue, allowing asynchronous execution in JavaScript.

18. What is recursion? Give an example.

🟢 Answer:
A function that calls itself.
Example:

function factorial(n) {
  return n === 0 ? 1 : n * factorial(n - 1);
}

19. Difference between synchronous and asynchronous programming?

🟢 Answer:

• Synchronous: Tasks run one after another.
• Asynchronous: Tasks can run concurrently using callbacks, promises, or async/await.

20. What is memoization?

🟢 Answer:
An optimization technique where results of expensive function calls are cached.

21. What are pure functions?

🟢 Answer:
Functions that return the same output for the same input and have no side effects.

22. Explain the concept of middleware in Node.js.

🟢 Answer:
Functions that execute during the request-response cycle and can modify req/res or end the cycle.

23. How does garbage collection work in Java?

🟢 Answer:
It automatically deletes unused objects to free memory using algorithms like mark-and-sweep.

24. What is dependency injection?

🟢 Answer:
A design pattern where components are given their dependencies externally, promoting loose coupling.

25. Explain MVC architecture.

🟢 Answer:

• Model: Data logic
• View: UI
• Controller: Handles input and updates model/view

26. How do you optimize a slow SQL query?

🟢 Answer:
Use indexes, avoid SELECT *, analyze execution plans, reduce joins/subqueries.

27. What is a race condition?

🟢 Answer:
A situation where two or more threads access shared data and try to change it at the same time.

28. What are microservices?

🟢 Answer:
A design approach where the app is split into small, independent services that communicate via APIs.

29. What is JWT?

🟢 Answer:
JSON Web Token – a compact, secure way to transmit user identity between parties.

30. How do you handle security in web applications?

🟢 Answer:
Use HTTPS, sanitize input, apply rate limiting, avoid SQL injection, enable CORS policy, and use proper auth (e.g., OAuth2).

📘 Bonus Tip

Practice coding problems daily on platforms like LeetCode, HackerRank, and Codeforces to solidify these concepts.

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