Top 50 MERN Stack Interview Questions for Freshers and Experienced

In this Full-Stack Tutorial, we will talk about some of the MERN stack job interview questions and their respective answers to help both, beginners as well as Experienced. Take a look at our interactive MERN: Full-Stack Web Developer Certification Training if you want to improve your skills in this area.

MERN Stack Interview Questions & Answers for Beginners

1. What is the MERN stack?

The MERN stack is a set of various technologies that the developers use to build web applications using JavaScript technologies. It is a full-stack JavaScript framework used for creating dynamic websites.

2. Explain what MongoDB is and its role in the MERN stack.

MongoDB is a NoSQL database which stores information in document collections that are similar to JSON objects. In the MERN stack, MongoDB serves as the database layer responsible for storing and handling data used by applications. It is essential in handling big data and also supports complex querying.

3. List some common tools or libraries used with the MERN stack.

Some common tools or libraries used with the MERN stack are:

• Mongoose
• Axios
• Redux
• React Router
• Body-Parser
• Nodemon
• JWT (JSON Web Token)

4. Explain the role of each component in the MERN stack.

• MongoDB- It is a NoSQL database that is used to store and manage application data.
• Express.js- It is a web application framework in Node.js that is used to build web and mobile applications.
• React.js- It is a JavaScript library that helps create user interfaces, especially for single-page applications.
• Node.js- It is a runtime environment based on JavaScript that is used for the execution of the server-side JavaScript code.

5. What is React.js, and why is it used?

React.js is a JavaScript Library that assists in building user interfaces. It permits developers to prepare maintainable UI components by letting them break down the development process into simpler components.

6. What is JSX in React?

JSX in React is a syntax extension used in JavaScript. It is used to write HTML-like markup in a JavaScript file. JSX is mostly used because of its conciseness and usability in most of the codebases

7. Explain the MVC architecture.

The MVC, also known as Model-View-Controller architecture, is a framework that divides an application into three main components:

1. Model
2. View
3. Controller

Each of these components is used to handle different aspects of the application.

8. What are the main features of MongoDB?

The main features of MongoDB are as follows:

• It is a schema-less database that can manage data without requiring any blueprint.
• The data is stored in documents, making it more flexible than SQL databases.
• Indexing is done appropriately so that query execution is better.
• It uses replication, which ensures data availability and stability are maintained.

9. What is Replication in MongoDB?

In MongoDB, Replication is a term that refers to copying the same data set in many MongoDB servers using a Replica Set.

10. What is Reconciliation in React JS?

Reconciliation in ReactJS is the process in which the diffing algorithm is used on the virtual DOM tree and the browser DOM is updated.

11. Explain the concept of middleware in Express.js.

In Express.js, middleware functions are the functions that are used for modifying the request and response objects. They can access the request object, the response object, and the next middleware function in the request-response cycle of the application. They can also end the request-response cycle or even call the next middleware function.

12. What is Sharding in MongoDB?

One way that sharding works in MongoDB is to break a large database up into smaller pieces, referred to as data shards, with each piece being dispersed over several computers.

13. How do you handle asynchronous operations in Node.js?

Callbacks, promises, and async/await syntax can be used to process async operations in Node.js. Although async/await and promises are most preferred simply because they are more readable and easier to work with than the rest.

14. What is the purpose of ExpressJS?

The main purpose of Express.js is to build robust server-side applications. It envelops all that comes with making HTTP requests and responses and assists with connecting URLs to functions on the server.

15. What is the role of package.json in a Node.js project?

A `package.json` in Node.js projects consists of a metadata file that stores information such as project dependencies, scripts, and metadata. Specifying project configuration, managing dependencies, defining scripts for processes such as application building, testing, and running are some of the uses of this file.

16. What are the data types in MongoDB?

The various data types in MongoDB are:

1. String
2. Integer
3. Double
4. Boolean
5. Array
6. Object
7. Date
8. Timestamp
9. Null
10. Binary

17. What is REPL In Node JS?

In Node.js REPL is short for Read-Eval-Print-Loop. It is an interactive programming environment that lets you type JavaScript code which gets evaluated and printed back to you immediately.

• Read-It reads the input from the user.
• Eval-The JavaScript engine interprets and evaluates the input.
• Print-The result is printed back to the user.
• Loop-This whole process repeats itself, making the interaction continuous.

18. What is meant by “Callback” in Node JS?

Callback in Node.js is a function that is passed as an argument to another function, which is executed later on. This is usually done after the asynchronous operation or some other event is completed.

19. How does Node JS handle Child Threads?

Node.js handles child threads through the following methods:

1. Child Processes- A module 'child_process' is used where it creates new processes with 'spawn()', 'exec()', or 'execFile()' methods. These are executed simultaneously.
2. Worker Threads- The 'worker_threads' module is used to run JavaScript code in parallel.
3. Clustering- The 'cluster' module creates multiple instances of a Node.js process and distributes the load across CPU cores. This helps in improving the performance.

20. What are the advantages of using the MERN stack?

• MERN stack has a simple Model-View-Controller that is easier for developers to use when building online programs.
• It uses micro-services to share the MERN applications that enable each of the services to work separately without relying on many others.
• MERN works in both online and offline modes, so the developers can easily build effective in-browser applications without a browser window.
• The architectural design of MERN can handle programs of any size and complexity.
• It has an increasing and supportive community with an increase in popularity that can help each other learn and grow together.

21. What is prop drilling?

Prop drilling is the process of passing a prop from one component to the next. The data is passed over from the component much higher in the hierarchy to the one that is deeply nested.

22. How do you manage packages in your node.js project?

1. Initialize Project:

• 'npm init' or 'yarn init' creates a package.json file.

2. Install Packages:

• 'npm install <package-name>' or 'yarn add <package-name>' installs packages and adds them to dependencies'.
• 'npm install' or 'yarn install' installs all dependencies listed in 'package.json'.

3. Update Packages:

• 'npm update <package-name>' or 'yarn upgrade <package-name>' updates packages to their latest versions.

4. Uninstall Packages:

• 'npm uninstall <package-name>' or 'yarn remove <package-name>' removes packages from the project.

23. What is the significance of virtual DOM in React.js?

24. What is the difference between a class component and a functional component?

25. What is middleware in Node.js, and how is it used?

const express = require('express');
const app = express();

// Simple middleware function
app.use((req, res, next) => {
  console.log('Request received');
  next(); // Passes control to the next middleware
});

app.get('/', (req, res) => {
  res.send('Hello, World!');
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

MERN Stack Interview Questions and Answers for Intermediate

26. What is a RESTful API?

27. Explain the event loop in Node JS.

• Non-blocking I/O-Enables Node.js to handle multiple operations without waiting for any single operation to complete.
• Phases-The event loop has several phases (e.g., timers, I/O callbacks, idle, poll, check, and close callbacks) that process different types of callbacks.
• Callbacks-Functions are executed once their corresponding operations are completed.

28. What are node JS streams?

1. Readable streams
2. Writable streams
3. Duplex streams
4. Transform streams

29. What are Node JS buffers?

30. Why use Express.js over Node.js?

31. What is a Collection in MongoDB?

32. Explain the term “Indexing” in MongoDB.

db.collection.createIndex({ username: 1 });

In this example, the 1 specifies an ascending index on the username field, which speeds up queries searching for specific usernames.

33. What are forms in React?

34. Explain the lifecycle methods of components.

35. What is Redux?

36. What are the components of Redux?

• Store-Holds the global state of the application.
• Actions-Plain JavaScript objects that describe changes to the state.
• Reducers-Pure functions that specify how the state changes in response to actions.
• Middleware-Functions that extend Redux's capabilities, such as handling async actions.

37. What is the purpose of npm in Node.js development?

38. What is React Router?

39. Why do we need to React Router?

40. What is the difference between ShadowDOM and VirtualDOM?

MERN Stack Interview Questions and Answers for Experienced

41. What are Higher-Order Components (HOC) in React?

42. What do you mean by Temporal Dead Zone in ES6?

43. Can you elaborate on the MongoDB Aggregation Pipeline?

44. How can you use the like operator to query MongoDB?

45. Name a few techniques to optimize React app performance.

• Using PureComponent or React.memo for minimizing unnecessary renders.
• Memoizing values with useMemo or memo for caching computations.
• Splitting code using React.lazy and Suspense for lazy loading.
• Optimizing image loading with lazy loading or responsive images.
• Managing state efficiently with Redux or Context API to avoid unnecessary re-renders.

46. How do you handle authentication and authorization in a MERN stack application?

47. Can you explain CORS?

48. What is DOM diffing?

49. What are the benefits of using JSX in React?

• Improved readability and maintainability of React components.
• Facilitates the integration of UI and logic into cohesive components.
• Enables compile-time checks for errors and warnings.
• Enhances developer productivity by reducing the learning curve and enabling component-based architecture in React applications.

50. How do you handle state management in React?

To handle state management in React, we can use:

1. Stateful Component: Stateful components or class components have their own state that we can initialize in the constructor with ‘this.state = {/* initial state */}

   import React, { Component } from 'react';
   
   class Counter extends Component {
     constructor(props) {
       super(props);
       this.state = { count: 0 };
     }
   
     increment = () => {
       this.setState({ count: this.state.count + 1 });
     };
   
     render() {
       return (
         <div>
           <p>Count: {this.state.count}</p>
           <button onClick={this.increment}>Increment</button>
         </div>
       );
     }
   }
   
   export default Counter;
   

2. Functional Components: Functional Components are stateless by default, but with the updates in React 16.8, Hooks can be used to manage state with the 'useState' Hook.

   import React, { useState } from 'react';
   
   function Counter() {
     const [count, setCount] = useState(0);
   
     const increment = () => {
       setCount(count + 1);
     };
   
     return (
       <div>
         <p>Count: {count}</p>
         <button onClick={increment}>Increment</button>
       </div>
     );
   }
   
   export default Counter;
   

3. External State Management Libraries:There are external state management libraries such as Redux or Mobx. These can be used when creating applications with complex components and shared states between components that are not related in any way.

More Questions for  MERN Stack Interview:

1. What is Server-Side Rendering (SSR) in React?

Server-Side Rendering (SSR) is the process of generating the HTML for a React page on the server instead of in the browser. 

• The server runs your React code first.
• It generates a fully-formed HTML page with the component content.
• The browser immediately displays this HTML.
• React then “hydrates” the page by attaching event listeners to make it interactive.

Key Benefits of SSR

• Better SEO: Search engines can easily index pre-rendered HTML.
• Faster First Paint: Users see content sooner because HTML is already ready.                   
• Better Performance on Slow Devices: Less work for the client’s browser.

2. Compare GraphQL and REST APIs in the context of MERN Stack. 

3. Why Unit Testing is Important in MERN?

In a MERN application, unit testing ensures that:

• Backend logic (controllers, routes, and services) works correctly.
• Database models interact with MongoDB as expected.
• React components render correctly and respond to user events.
• Utility functions return correct outputs for given inputs.

Unit Testing is Important in MERN

• Early Bug Detection: Catches issues before they reach production.
• Code Confidence: Developers can refactor without fear of breaking features.
• Documentation: Tests serve as live documentation for how functions are supposed to behave.
• Better Collaboration: Multiple developers can work safely with fewer merge issues.

4. What are the best practices for securing a MERN app against common vulnerabilities?

Best Practices for Securing a MERN Application

• Secure Authentication & Authorization – Use JWT in HTTP-only cookies, enforce strong password hashing (bcrypt), and implement role-based access control (RBAC).
• Input Validation & Sanitization – Validate all incoming data, sanitize user input, and protect against XSS and NoSQL injection using libraries like express-validator.
• HTTPS & Secure Headers – Force HTTPS in production and apply security headers with helmet (CSP, X-Frame-Options, HSTS).
• CSRF & Brute-Force Protection – Use CSRF tokens (csurf middleware) and rate limiting to block malicious repeated requests.
• Database & Secrets Security – Store DB credentials in .env, restrict MongoDB access by IP, and never log or expose sensitive data.
• Continuous Monitoring & Updates – Run npm audit, patch vulnerabilities, use logging tools, and monitor suspicious activity in production

5. Compare MERN Stack with MEAN or MEVN stacks.

6. What is code splitting in React, and how does it improve performance? 

React apps built with tools like Webpack generate a single large bundle.js file containing your entire app. 

• This means users download all the code upfront — even for pages or components they may never visit.
• Larger bundle size → slower initial load time.

Code splitting solves this by:

• Splitting code into smaller chunks.
• Loading only the code needed for the current page.
• Fetching additional chunks on demand when users navigate.

Benefits of Code Splitting

• Faster Initial Load: Users download only what they need to see first.
• Better User Experience: Smaller initial JS improves Time-to-Interactive (TTI).
• Reduced Bandwidth Usage: Helpful for mobile or slow connections.
• Scalable for Large Apps: Avoids performance issues as your project grows.

7. What are Progressive Web Apps (PWAs), and how can MERN support PWA development?

Progressive Web App (PWA) is a web application that behaves like a native mobile app. It combines the reach of the web with features like offline access, push notifications, and installation on a device’s home screen.

Key Features of PWAs:

• Responsive: Works on any screen size (desktop, tablet, mobile).
• Offline Support: Caches resources and data for offline usage.
• Installable: Users can "Add to Home Screen" like an app.
• Fast & Reliable: Loads instantly, even on slow networks.
• Secure: Always served via HTTPS.

The MERN stack is perfect for building PWAs because:

• React provides fast, component-based UI and supports tools like create-react-app with built-in PWA templates.
• Express + Node.js can serve optimized, pre-cached static files and APIs.
• MongoDB provides a flexible backend to sync offline data when the user reconnects.

8. Explain Difference between Redux Thunk and Redux Saga.

Summary

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