Top 50 MERN Stack Interview Questions and Answers

Introduction

The MERN stack—MongoDB, Express.js, React.js, and Node.js—has become a preferred choice for developers. It simplifies development by allowing JavaScript to be used across the stack, from front-end to back-end. Understanding the MERN stack is essential whether you’re gearing up for an interview or simply expanding your knowledge.

In this article, we’ve compiled 50 essential interview questions with answers to help you prepare. From beginner-level concepts to advanced scenarios, this guide covers it all to boost your confidence and knowledge.

MongoDB Questions

1. What is MongoDB?
   MongoDB is a NoSQL database that stores data in a flexible, JSON-like format called BSON.
2. What are the advantages of MongoDB?
   • Schema-less design.
   • High scalability.
   • Fast reads and writes.
3. What is the difference between MongoDB and MySQL?
   • MongoDB: NoSQL, schema-less, handles unstructured data.
   • MySQL: Relational database, structured schema.
4. How do you create a database in MongoDB?
   Use the command: use databaseName
5. What is the purpose of indexes in MongoDB?
   Indexes improve query performance by reducing the amount of data MongoDB scans.
6. Explain replication in MongoDB.
   Replication creates multiple copies of data across servers, ensuring availability and redundancy.
7. What is sharding in MongoDB?
   Sharding is a method to distribute data across multiple servers to handle large datasets efficiently.
8. What is the aggregation framework in MongoDB?
   It processes data through a pipeline to transform and aggregate results.
9. What is the purpose of ObjectId in MongoDB?
   ObjectId is a unique identifier for documents.
10. How do you delete a collection in MongoDB?
    Use the command: db.collectionName.drop()

Express.js Questions

11. What is Express.js?
    Express.js is a minimal Node.js framework for building web apps and APIs.
12. What are middleware functions in Express?
    Middleware functions handle requests, responses, and errors during request processing.
13. How do you define a route in Express.js? app.get('/route', (req, res) => { res.send('Hello, World!'); });
14. What is the role of next() in middleware?
    It passes control to the next middleware function in the stack.
15. How do you handle errors in Express?
    Use error-handling middleware with (err, req, res, next).
16. How can you serve static files in Express.js?
    Use the built-in middleware: app.use(express.static('public'));
17. What is res.json() in Express?
    It sends a JSON response to the client.
18. How do you handle form data in Express?
    Use middleware like body-parser or express.urlencoded().
19. What is the difference between app.get() and app.use()?
    • app.get(): Handles GET requests.
    • app.use(): Adds middleware for all request types.
20. How do you redirect a request in Express.js? res.redirect('/new-route');

React.js Questions

21. What is React.js?
    React is a library for building user interfaces.
22. What is JSX?
    JSX is a syntax extension that allows writing HTML within JavaScript.
23. What are props in React?
    Props are immutable data passed from parent to child components.
24. What is state in React?
    State is mutable and managed within a component.
25. What is a functional component in React?
    A JavaScript function that returns JSX.
26. What are React hooks?
    Hooks, like useState and useEffect, allow functional components to manage state and side effects.
27. What is the difference between useState and useEffect?
    • useState: Manages state.
    • useEffect: Handles side effects like API calls.
28. What is React Router?
    React Router enables navigation between different pages in a React application.
29. What is the Virtual DOM?
    A lightweight representation of the DOM, used for efficient updates.
30. How do you handle events in React?
    Use event handlers like: <button onClick={handleClick}>Click Me</button>

Node.js Questions

31. What is Node.js?
    Node.js is a runtime for executing JavaScript on the server-side.
32. What are modules in Node.js?
    Reusable pieces of code. Examples include fs, http, and custom modules.
33. What is npm?
    Node Package Manager (npm) is used to install and manage Node.js packages.
34. What is the event loop in Node.js?
    A mechanism that handles asynchronous operations.
35. What are streams in Node.js?
    Objects for reading or writing data continuously.
36. What is require() in Node.js?
    It imports modules into your application.
37. What is the difference between readFile and createReadStream?
    • readFile: Reads the entire file into memory.
    • createReadStream: Reads file in chunks.
38. How do you handle exceptions in Node.js?
    Use try-catch blocks or the process.on('uncaughtException') event.
39. What is the difference between synchronous and asynchronous code in Node.js?
    • Synchronous: Blocks execution until complete.
    • Asynchronous: Executes non-blocking operations.
40. What is package.json?
    A configuration file with metadata about a Node.js project.

Advanced Questions

41. What is the MERN stack?
    A JavaScript-based stack for full-stack web development.
42. Why choose the MERN stack?
    Uses JavaScript across front-end and back-end, ensuring consistency.
43. How does the MERN stack work together?
    • MongoDB stores data.
    • Express.js handles server-side logic.
    • React.js builds the front-end.
    • Node.js runs back-end JavaScript.
44. What is Redux, and why is it used?
    A state management library for predictable state handling.
45. How do you secure a MERN stack app?
    Use authentication (e.g., JWT), HTTPS, and validation techniques.
46. How do you deploy a MERN app?
    Use services like Heroku, AWS, or Netlify.
47. What is the role of CORS in a MERN app?
    CORS allows cross-origin requests, critical for connecting the front-end and back-end.
48. What is a REST API?
    An API that follows REST principles for CRUD operations.
49. How do you optimize a MERN app for performance?
    • Use caching.
    • Optimize database queries.
    • Minify front-end assets.
50. What is the future of MERN stack development?
    With JavaScript’s popularity, the MERN stack will continue to grow as a preferred solution for full-stack development.

Conclusion

Preparing for a MERN stack interview requires a solid understanding of its components and practical experience. This list of 50 questions and answers covers the essential topics you need to know. Take your time to practice and apply these concepts to real-world projects to boost your confidence. Good luck with your MERN stack journey!