.png)
🔥 Top 50 MERN Stack Interview Questions and Answers (2025)
The MERN stack (MongoDB, Express.js, React.js, Node.js) has gained significant popularity due to its flexibility and full-stack JavaScript capabilities. Whether you’re a fresher or an experienced developer, MERN stack interview questions often focus on key aspects of both frontend and backend development.
Based on my experience, here are the 50 most asked interview questions for the MERN stack that you should know before facing a real interview.
📌 What is the MERN Stack?
The MERN stack is the most popular JavaScript technology stack used for building full-stack web applications. It includes:
- MongoDB: A NoSQL database that stores data in JSON-like documents.
- Express.js: A lightweight framework for Node.js used to build backend web applications.
- React.js: A frontend JavaScript library used for building user interfaces. Many React packages make it even more versatile.
- Node.js: A runtime environment that allows you to run JavaScript on the server side.
🧠 Top 50 MERN Stack Interview Questions
Let's begin with M for MongoDB — a very popular NoSQL (Non-Relational) database designed for storing and managing large volumes of data. It stores data in a flexible, JSON-like format called documents, which are grouped into collections. Each document can have a different structure, making MongoDB highly scalable and adaptive.
📂 MongoDB (Database) Interview Questions
1. What is MongoDB, and how is it different from relational databases like MySQL?
Answer:
MongoDB is a NoSQL database that stores data in a JSON-like format (BSON), whereas MySQL uses rows and tables with a strict schema. In MongoDB, the structure of each document can vary, giving it more flexibility.
2. Explain the concept of collections and documents in MongoDB.
Answer:
- A document is a single record, stored in BSON format (Binary JSON).
Example: { "_id": "001", "name": "John Doe" } - A collection is a group of documents, similar to a table in MySQL.
In a blogging site, you might have collections like users, categories, blogs, comments, etc.
3. How do you perform CRUD operations in MongoDB?
Answer: CRUD stands for Create, Read, Update, and Delete.
Create: Insert a document into a collection.
Read: Find documents using filters.
Update: Modify existing documents.
Delete: Remove documents.
For detailed examples: codewithdeepak.in
4. What are the benefits of using MongoDB over other databases?
Answer:
- Flexible Schema: No need for a predefined schema.
- Scalability: Supports horizontal scaling via Sharding.
- High Performance: Optimized for read/write performance.
- Semi/Unstructured Data: Better suited than SQL for these types.
- Large Community and Ecosystem
5. How does Sharding work in MongoDB?
Answer:
Sharding is a method of distributing data across multiple servers.
- Each server is called a shard.
- Data is split based on a shard key.
- Example: In an e-commerce platform, shard keys might be user_id, order_id.
This improves scalability and performance.
6. What is Mongoose, and how does it simplify working with MongoDB?
Answer:
Mongoose is an ODM (Object Data Modeling) library for Node.js. While MongoDB is schema-less, Mongoose allows defining schemas, validation, and more:
- Schema Definition and Validation
- Middleware (Hooks)
- Data Type Casting
- Population for Relationships
7. Explain how you can create relationships between collections in MongoDB.
Answer:
There are two main ways:
| Feature | Embedded Documents | Reference-Based Relationship (Normalization) |
|---|---|---|
| Definition | Storing one document inside another | Storing the _id of another document as a reference |
| Structure | Denormalized | Normalized |
| Data Retrieval | One query is enough | Requires $lookup or multiple queries |
| Performance (Read) | Faster | Slower |
| Performance (Write) | Slower for large data | Faster |
| Data Consistency | Harder to maintain if duplicated | Easier to update in one place |
| Use Case | Tightly coupled data (e.g., comments) | Loosely coupled data (e.g., orders and users) |
| Atomic Operations | Supported | Not always atomic |
| Schema Complexity | Simpler | Better for complex data |
8. How do you index documents in MongoDB, and why is it important?
9. What are aggregation pipelines in MongoDB, and when would you use them?
10. How do you handle transactions in MongoDB?
11. What is the difference between MongoDB and Firebase?
🚀 Express.js (Backend Framework) Interview Questions
12. What is Express.js, and how does it work with Node.js?
13. Explain middleware in Express.js and its role.
14. How do you create a simple REST API using Express.js?
15. What is routing in Express.js, and how does it work?
16. How do you handle errors in Express.js applications?
17. What are CORS, and how do you handle them in an Express app?
18. What are sessions and cookies in Express.js?
19. How can you secure an Express.js application?
20. How do you connect Express.js with MongoDB?
21. Explain the purpose of app.use() in Express.
⚛️ React.js (Frontend Library) Interview Questions
22. What is React.js, and why is it popular?
23. Explain the concept of components in React.
24. What are props and state in React, and how do they differ?
25. What are hooks in React, and why are they used?
26. What is the virtual DOM in React, and how does it improve performance?
27. How do you manage forms in React?
28. Explain the difference between class components and functional components in React.
29. What are React lifecycle methods?
30. How do you perform API calls in React using fetch or axios?
31. What is React Router, and how do you implement routing in a React app?
32. How can you optimize the performance of a React application?
🖥️ Node.js (Runtime Environment) Interview Questions
33. What is Node.js, and why is it used in the MERN stack?
34. What is the difference between require() and import in Node.js?
35. Explain event-driven architecture in Node.js.
36. How do you handle asynchronous code in Node.js?
37. What are streams in Node.js, and how are they used?
38. How do you manage modules in Node.js?
39. Explain the role of NPM in Node.js projects.
40. What are the differences between synchronous and asynchronous functions in Node.js?
41. How do you use environment variables in Node.js applications?
42. What are child processes in Node.js, and when would you use them?
43. How do you implement authentication in a Node.js application?
✅ Conclusion
Preparing for a MERN stack interview requires a solid understanding of MongoDB, Express.js, React.js, and Node.js, along with the ability to solve real-world problems using these technologies.
This list of 50 commonly asked MERN stack interview questions will give you the head start you need to stand out and succeed in your interview.
