Top 50 Most Asked Interview Questions for MERN Stack Developers (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 an real interview.

In this blog, we’ll explore the top 50 most frequently asked interview questions in MERN stack interviews to help you prepare effectively.

What is the MERN Stack?

The MERN stack is the most popular JavaScript Combo(Stack) used for building full-stack web applications.

• MongoDB: A NoSQL database that stores data in JSON-like documents.
• Express.js: A lightweight framework for Node.js used to build web applications.
• React.js: A frontend JavaScript library used for building user interfaces. There are so many packages available for react that makes it's 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 get Start with M (MongoDB). MongoDB is a very popular NoSQL(Non-Relational) database designed for storing and managing large volumes of data. It is more flexible, and store data in JSON-like format called documents. Grouped documents are called Collections,  and each document can have different structure, making MongoDB highly scalable and adaptive.

MongoDB (Database) 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, whereas in MySQL, data is stored in the form of rows and tables with a strict schema. In MongoDB, the structure of each document can be different.

2. Explain the concept of collections and documents in MongoDB.

   Answer: A document in MongoDB a single record in a database similar to a row in relational database. And a document is stored in JSON-like format called BSON(Binary JSON). Documents consists of filed and value pair.

         {

               “_id”: “001”,

               “name”: “John Doe”

         }

   A collection is a group of documents, similar to a table in a MySQL database. Suppose you have a blogging website with several collections for different purposes, such as one for users, one for categories, one for blogs, one for comments, and others

3. How do you perform CRUD operations in MongoDB?

   Answer: CRUD (Create, Read, Update and Delete)

   

   Find:

   

   Update: 

   

   Delete:

   

    

4. What are the benefits of using MongoDB over other databases?

   Answer: There are following benefits of using MongoDB database over other databases.

   1. Flexible Schema (Schema-less): Means you don't need to define any fixed schema before inserting data. you can save what you want.

   2. Scalability: It supports horizontal scaling through Sharding. Which Splits data across multiple servers automatically, to handle high traffic.

   How Sharding works in MongoDB?

   It's an database architecture where data is divided across mutiple servers called as shards, Each Shard hold some portion of the database. Instread of storing all the data on the server. This is happened based on the sharding key. A Sharding key is a specific field or fields in MongoDB documents that determine how to distribute data.

   For Example we have an e-commerce platform, the shards key could be user_id, order_id. 

   Sharding is a big topic in itself; you can refer to my blog for more details

   3. High Performance.                                 

   4. Better for Unstructured and Semi-Structured Data.             

   5. Community and Ecosystem Support.

5. What is Mongoose, and how does it simplify working with MongoDB?

   Answer: As we discussed above MongoDB is schema-less database. But some application required strict schema. In that case we can use Mongoose. Mongoose is Object Data Modeling (ODM) library for MongoDB and Nodejs. It provide straightforward and schema-based solutions to model application data. What are the things provided by mongoose.

   1. Schema Definition and Validation.

   2. Middleware(Hooks)

   3. Data Validation and Type Casting.

   4. Relationships and Population

   If you want to see how it works in one of my blog I show you how to define schema, validation and setup Middleware. Read This Scroll to end of the blog.

6. Explain how you can create relationships between collections in MongoDB.
7. How do you index documents in MongoDB, and why is it important?
8. What are aggregation pipelines in MongoDB, and when would you use them?
9. How do you handle transactions in MongoDB?
10. What is the difference between MongoDB and Firebase?

Express.js (Backend Framework) Questions:

1. What is Express.js, and how does it work with Node.js?
2. Explain middleware in Express.js and its role.
3. How do you create a simple REST API using Express.js?
4. What is routing in Express.js, and how does it work?
5. How do you handle errors in Express.js applications?
6. What are CORS, and how do you handle them in an Express app?
7. What are sessions and cookies in Express.js?
8. How can you secure an Express.js application?
9. How do you connect Express.js with MongoDB?
10. Explain the purpose of app.use() in Express.

React.js (Frontend Library) Questions:

1. What is React.js, and why is it popular?
2. Explain the concept of components in React.
3. What are props and state in React, and how do they differ?
4. What are hooks in React, and why are they used?
5. What is the virtual DOM in React, and how does it improve performance?
6. How do you manage forms in React?
7. Explain the difference between class components and functional components in React.
8. What are React lifecycle methods?
9. How do you perform API calls in React using fetch or axios?
10. What is React Router, and how do you implement routing in a React app?
11. How can you optimize the performance of a React application?

Node.js (Runtime Environment) Questions:

1. What is Node.js, and why is it used in the MERN stack?
2. What is the difference between require() and import in Node.js?
3. Explain event-driven architecture in Node.js.
4. How do you handle asynchronous code in Node.js?
5. What are streams in Node.js, and how are they used?
6. How do you manage modules in Node.js?
7. Explain the role of NPM in Node.js projects.
8. What are the differences between synchronous and asynchronous functions in Node.js?
9. How do you use environment variables in Node.js applications?
10. What are child processes in Node.js, and when would you use them?
11. 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.

Good luck!