Top 20+ MERN Stack Interview Questions for Freshers

1. What is MongoDB?

MongoDB is a NoSQL-based database that helps in storing data in JSON-like BSON format. It is best suited for scalable web applications. 

2. What is MERN  Stack?

MERN Stack is a full-stack technology that consists of MongoDB, Express.js, React.js, and Node.js. They combine to build highly-functional web applications using JavaScript. 

3. Explain the role of React in the MERN Stack.

React in the MERN Stack is used for frontend functions. React helps in managing state, UI, and building reusable elements for highly interactive and faster user interfaces. 

4. Why is the MERN stack preferred over LAMP or MEAN Stack?

MERN uses end-to-end JavaScript that is from the frontend to the backend to the databases. So, no need to switch languages, and this speeds the development, reducing complexity. 

As compared to LAMP and MEAN stack, MERN is better for;

• Real-time web applications.
• Single Page Applications.
• Higher performance and scalability.

5. What is Node.js?

Node.js is a runtime environment that allows JavaScript to run on the server side with the help of Chrome’s V8 engine. This is the most popular unified programming language for backend and frontend. This non-blocking architecture manages multiple existing connections. 

6. What is Express.js?

Express.js is a framework for Node.js that helps in simplifying the process of making server-side logic. It handles HTTP requests and manages different tasks such as data parsing, logging, and authentication. It connects React with MongoDB, allowing easy communication. 

7. What is the MVC pattern in MERN?

MVC pattern in MERN stands for  Model View Controller.

• M for Model: Database Logic
• V for View: React UI
• C for Controller: APIs and business logic

8. What do you mean by React Hooks? Name some common React Hooks. 

React Hooks refers to special JavaScript functions. This allows functional elements to apply React and state features. They boost readability and code reusage. Some common Hooks are useContext, useState, useRef, and useReducer. 

9. What do you mean by Server-Side Rendering (SSR)?

SSR, or Server-Side-Rendering, refers to rendering React elements on the server side rather than the browser side. This boosts the page loading speed while improving SEO. 

10. What are React Fragments, and mention their functions. 

React Fragments help you group multiple elements together without adding any unnecessary nodes to the DOM. This helps in keeping a clean DOM structure with enhanced performance. 

Its functions are;

• It prevents CSS or layout issues using modern techniques like Flexbox or CSS Grids. 
• It helps in maintaining a cleaner DOM Structure, avoiding extra nesting. 

11. What are Webpack and Babel?

Webpack is a module bundler that helps in bundling and optimizing files such as CSS, JavaScript, and images that can be fast-loaded in browsers.

Babel is a JavaScript compiler that converts JSX code and ES6+ into JavaScript that is highly compatible with different browsers. In simple terms, it helps make code highly compatible with browsers. 

12. What do you mean by React Router?

React Router refers to the JavaScript library that helps in handling navigation and client-side routing. It helps to build multiple-view Single Page Applications (SPAs).

13. What are REST APIs?

REST APIs are an architectural style that helps in communication between the backend and the frontend with the help of HTTP methods like DELETE, POST, GET, and PUT. 

14. What is Aggregation in MongoDB?

Aggregation in MongoDB is a framework that is used to collect, process, and transfer data using stages such as $project, $match, and $group. They help in filtering and sorting documents. 

15. What is Replication in MongoDB?

Replication in MongoDB refers to copying data from different servers to ensure data safety and higher availability. 

16. What are Operators in MongoDB?

Operators in MongoDB are special symbols or keywords that help in filtering and modifying data in queries. Some operators are $lt, $eq, $gt, $and, $gte, $exists, $nin, and $near. 

17. What is the difference between $push and $addToSet?

$push adds value even if duplicates are present.

$addToSet helps in adding values only if they are not present there. 

18. What is the difference between SQL and NoSQL databases?

SQL databases refer to the table-based databases with a fixed schema, while NoSQL databases are document-based databases without a schema.

19. What is the difference between find() and findOne() ?

20. What is JSX in React?

JSX, or JavaScript XML, refers to a syntax extension used in React to show how a user interface should look. This syntax extension allows writing HTML inside a JavaScript framework. It helps improve readability and maintainability, offering a clear visual UI structure. 

21. How do you save large files (like Photos) into a MongoDB database?

Large files like photos are saved using cloud storage or GridFS, and you can only save the file URL in MongoDB.