Top MERN Stack Interview Questions 2025

MERN is the most popular web framework known by its first letters: MongoDB, Express, React, and Node.js. Proven by top companies like Facebook, Netflix, and Instagram, MERN is a great focus to master, and to obtain a great IT profession, one must polish essential interview questions.

This article covers important MERN stack interview questions to help you prepare for full-stack development roles, boosting your chances of successfully clearing interviews.

Most Frequently Asked Mern Stack Questions in Interview

Q1: What is the MERN stack?

Q2: Explain the role of MongoDB in the MERN stack.

MERN stack is a set of four technologies that can be used for full-stack web development including MongoDB, Express.js, React, and Node.js. It makes development easier because it takes care of the database, server, and front end, therefore enjoyed by developers.

Q.2: Explain the role of MongoDB in the MERN stack.

MongoDB is a popular database among NoSQL of the MERN stack that is used as data storage. It works with data in a format close to JSON and can scale easily to work with big amounts of data in web applications.

Q.3: What are the data types in MongoDB?

MongoDB supports various data types, including:

1. String: Text data.
2. Number: Integer or float.
3. Boolean: True or false.
4. Array: List of values.
5. Object: Nested data.
6. Date: Date and time.

Q. 4: What is sharding in MongoDB?

MongoDB sharding is a process of partitioning data by distributing data in multiple servers to enhance the performance of the database. If it does so either horizontally or vertically, it splits data into different partitions called shards and is used for managing big data.

Q. 5: What is Express.js, and how does it function within the MERN stack?

Express.js is one of the Node.js web application frameworks that make it easier to work with these issues, including HTTP requests, routing, and middleware. In the MERN stack, it processes API requests, communicates with databases (MongoDB), and transmits data retrieved to the React front- end.

Q.6: Describe the key features of React.js.

In the MERN stack, React is used to develop the front-end user interfaces. It effectively refreshes the view, and more importantly, it communicates with the back end to get and show data.

Here are some key features of react.js:

• Component-Based: React uses components to construct interfaces that can be reused.
• Virtual DOM: It actually refreshes only those areas that have been modified on the page, hence enhancing performance.
• One-Way Data Binding: Data is only received in a one-way manner, and the process remains simpler as a result.
• Hooks: Out of those, functions that enable state management and have side effects in functional components are typical.

Q. 7: What are the advantages of using the MERN stack for web development?

Here are some important advantages of using the MERN stack in web development:

1. Single Language: Uses JavaScript for front end and back end.
2. Scalable: This is easily able to handle large applications as well as large data.
3. Fast Development: Provides efficient tools and libraries for rapid application development.
4. Performance: It is built to provide high response time with the utilization of the React and Node environments.
5. Flexible: Suits small and big project requirements equally.

Q. 8: How do you set up a basic MERN stack application?

To create a MERN stack application, one has to download Node.js and Npm, create a fresh project using Node.js with Express as the framework of the backend, use MongoDB for the database, and React for the front end. Link them through API routes in terms of data management.  

 In steps:

1. Install Node.js and npm: Install Node.js and npm for the back end and package manager.
2. Create Node.js Project: Fill in the nodes.js project via npm init.
3. Set up Express.js: This entails installing and configuring of Express for the backend server.
4. Install MongoDB: In the next step organize MongoDB for storing data.
5. Create React App: When developing the front end, one should use create-react-app to get started.
6. Connect Backend and Frontend: To implement data exchange, the Link React can be kept connected with Express APIs.

Q. 9: What is npm, and how is it utilized in MERN stack projects?

NPM (Node Package Manager) is a tool for managing dependencies and libraries used for JavaScript. In MERN stack projects, it aids in measuring libraries such as Express, React, and MongoDB drivers, which allows organizations to automate the building and maintenance of projects.

Q.10: How do you handle HTTP requests in Express.js?

In Express.js HTTP requests are managed by the router methods, including app.get(), app.post(), app.put(), and app.delete(). Each route names a path and behavioral pattern to handle the request, compute the data, and send the correct response to the client.

Q.11: How does routing work in Express.js?

In Express.js, routing refers to defining paths (URLs) and handling requests for them. When a user sends a request to a specific URL, Express matches it to a defined route and executes the corresponding function to return a response, like data or a page.

Q.12: Explain the concept of middleware in Express.js.

Middleware in Express.js is basically a function that works on requests before it get to the routes. It can process request data or change it, perform preliminary work such as authentication, or deal with an error. Middleware is employed to incorporate additional capabilities as a parameter of logging, JSON parse, as well as user permissions in the request cycle.

Q.13: Explain how you can create a RESTful API using Express.js.

To create a new rest API in Express.js specify routes that will handle HTTP methods such as GET, POST, PUT, DELETE, which will correspond to CRUD operations. Handle requests and responses with the use of Express; integrate it with a database, such as MongoDB, and respond with data in JSON format.

Q.14: How do you manage state in a React application?

React state can be managed by using hooks, which include useState for factor components and this.setState for class factors. State data, which may vary with time, and when the state is altered, React recalculates the changed components on the page.

Code example:

import React, { useState } from 'react';

function Counter() {

  const [count, setCount] = useState(0);  // Initialize state

  const increment = () => setCount(count + 1);  // Update state

  return (

    <div>

      <p>Count: {count}</p>

      <button onClick={increment}>Increment</button>

    </div>

  );

}

export default Counter;

Here we manage the count state and update it when the button is clicked.

Q.15: Explain the concept of states and props in React.

Here we explain state and props by using table 

Aspect	State	Props
Definition	Holds data that can change over time.	Holds data passed from parent to child components.
Ownership	Managed within the component itself.	Managed by the parent component.
Mutability	Mutable (can be changed within the component).	Immutable (cannot be changed by the child).
Usage	Used for data that changes in the component (e.g., user input).	Used to pass data or functions to child components.
Re-render	Causes the component to re-render when updated.	Causes the child component to re-render when changed.

 

Q.16: What is the virtual DOM, and how does React use it to improve performance?

The virtual DOM is actually an in-memory representation of DOM. React uses it to update the DOM systematically compared to the real DOM and only apply changes. Type only changes, while other things remain the same, with performance enhanced by the reduction of direct DOM manipulations, which are slower.

Q.17: What is prop drilling?

Prop drilling is where data is passed through multiple layers of components using the use of props. Specifically in React, when a parent component wants to pass data deeper to some grandchild, the data has to travel the whole intermediate components, which is quite inconvenient in large applications.

Q.18: What is CORS, and how do you enable it in an Express.js app?

CORS ( Cross-Origin Resource Sharing) is a measure used when a browser allows or restricts a particular request from domains. To have CORS enabled in an Express.js app, you have to install the cors package and use app.use(cors()) to allow cross-origin requests from certain origins.

Q.19: How can you connect a React frontend with an Express.js backend?

To link a React frontend to an Express.js backend, call the Express routes from a React frontend using either fetch or axios. The backend contains all the routes to handle requests from the front-end, and interact with the database; also, the backend sends data to the front-end in JSON format.

Q.20: Write a basic API in Express.js to handle CRUD operations for a “User” resource

In this code example, we define an Express app with routes for CRUD operations. It manages users in memory, allowing GET, POST, PUT, and DELETE requests to handle user data.

const express = require('express');

const app = express();

app.use(express.json());

let users = [];

app.get('/users', (req, res) => res.json(users));

app.post('/users', (req, res) => {

  users.push(req.body);

  res.status(201).send(req.body);

});

app.put('/users/:id', (req, res) => {

  const user = users.find(u => u.id === req.params.id);

  Object.assign(user, req.body);

  res.send(user);

});

app.delete('/users/:id', (req, res) => {

  users = users.filter(u => u.id !== req.params.id);

  res.status(204).send();

});

app.listen(3000, () => console.log('Server running'));


Advanced-level Mern Stack Interview Questions

Q21: How can you use the like operator to query MongoDB?

 

In MongoDB, the $regex operator is used for searching similar to what we use in the SQL LIKE operator. For example, to find documents where a field contains a specific string:

db.users.find({ name: { $regex: /john/i } });

Q.22: What is DOM diffing?

 

DOM diffing refers to a way of finding the difference between the current Virtual DOM and a new Virtual DOM. React uses this technique to update only the actual DOM and takes efficiency control to minimize redundant rendering and direct manipulation of the DOM structure.

Q.23: How does server-side rendering (SSR) work in a MERN application, and what are its advantages?

Server rendering in the MERN application specifically means that the React components that are to be sent to the client are first rendered on the server. It enhances the performance and SEO since it serves completely rendered HTML to the browser.

Here are some advantages 

• Faster page load times.
• Better SEO with pre-rendered content.
• Improves user experience on slower networks.
• Reduced load on the client’s device.

Q24: What is the role of WebSockets in a MERN application, and how would you implement them for real-time updates?

Web sockets allow data to be sent in real-time by both the server and the client. WebSockets can be used in a MERN application for things like live chat or notifications. Use them through frameworks like socket.io to create a persistent socket connection for the communication of real-time data.

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Q.25: How can you handle authentication and authorization in a MERN stack application using JWT tokens?

In a MERN stack application, you should use JWT tokens for the process of user authentication and authorization. If a user has logged in, to authenticate him further, he gets something on the server called JWT token and it is sent to the client side. For protected routes check for the token using middleware which will confirm if the user is authenticated.

Q.26: Explain the difference between the traditional REST API and GraphQL.

 

Aspect	REST API	GraphQL
Data Fetching	Fetches fixed data from endpoints.	Fetches specific data based on queries.
Requests	Multiple requests for different resources.	One request can fetch multiple resources.
Flexibility	Limited flexibility, fixed responses.	More flexible, clients define the response structure.
Versioning	It requires versioning as the API changes.	No versioning; schema evolves naturally.
Over-fetching/Under-fetching	This can result in over-fetching or under-fetching.	Avoids over/under-fetching with precise queries.

 

Q27: Describe the concept of containerization and how you can deploy a MERN stack app using Docker.

Containerization puts a Java MERN app together with its requisites into individual containers with the help of Docker. To apply, write Dockerfiles for both the backend and frontend, design services in docker-compose.yml, make images, and launch containers. This helps in making it constant across the environments but also helps in the ease of the deployment process.

Q.28: How to Connect Node.js to a MongoDB Database? 

To connect NodeJS to MongoDB, now use Mongoose. One can install it via npm install mongoose; then they can import mongoose and then use mongoose.connect(uri) This creates a connection with MongoDB for posing queries and executing data manipulation operations.

const mongoose = require('mongoose');

// Replace with your MongoDB connection string

const uri = 'mongodb://localhost:27017/mydatabase';

mongoose.connect(uri, { useNewUrlParser: true, useUnifiedTopology: true })

  .then(() => console.log('Connected to MongoDB!'))

  .catch(err => console.error('Error connecting to MongoDB:', err))

Note: Replace mydatabase with your database name.

Q. 29:  What do you mean by Temporal Dead Zone in ES6?

The Temporal Dead Zone (TDZ) in ES6 is the time between when a `let` or `const` variable is declared and when it’s initialized. During this time, accessing the variable causes an error because it hasn’t been assigned a value yet.

Q.30: How do you secure a MERN stack application against common security vulnerabilities, such as SQL injection or cross-site scripting (XSS)?

To secure a MERN stack app:

• To prevent SQL injection, MongoDB uses parameterized queries.
• Protection against Cross-Site Scripting attacks entails sanitization of user inputs.
• Use HTTPS for securely exchanging information with clients.
• Sensitive data use environment variables.
• For authentication, to validate JWTs.