Latest Top interview questions and answers for a Mern stack developer for experienced and freshers, get ready to crack any interview.

MongoDB interview question:

1. What is MongoDB, and why is it used in the MERN stack?
   • Answer: MongoDB is a NoSQL database that stores data in flexible, JSON-like documents. It’s used in the MERN stack for its scalability, flexibility, and ease of integration with Node.js.
2. Explain the difference between SQL and NoSQL databases.
   • Answer: SQL databases are relational and use structured query language, while NoSQL databases like MongoDB are non-relational and use various data models like document, key-value, or graph.
3. What is a Mongoose schema, and how is it used in MongoDB?
   • Answer: A Mongoose schema defines the structure of a document, specifying the fields and their types. It provides a way to model data before it is saved to a MongoDB database.
4. How do you perform a join operation in MongoDB?
   • Answer: MongoDB uses the $lookup stage in the aggregation pipeline to perform join operations between collections.
5. Explain the purpose of indexing in MongoDB.
   • Answer: Indexing in MongoDB improves query performance by creating data structures that allow the database to locate and access documents more efficiently.
6. What is the role of the ObjectId in MongoDB?
   • Answer: ObjectId is a 12-byte identifier typically employed as the primary key in MongoDB. It is unique across documents and collections and is automatically generated.
7. How does MongoDB ensure high availability and fault tolerance?
   • Answer: MongoDB achieves high availability through replica sets. A replica set consists of multiple MongoDB instances, ensuring automatic failover in case a primary node goes down.
8. Explain the concept of sharding in MongoDB.
   • Answer: Sharding is the process of dividing a large MongoDB database into smaller, more manageable pieces called shards. Each shard is distributed across different servers.
9. What is GridFS in MongoDB, and when is it used?
   • Answer: GridFS is a specification for storing and retrieving large files in MongoDB. It is used when documents exceed the BSON document size limit.
10. How do you perform CRUD operations in MongoDB using Mongoose?
    • Answer: Mongoose provides methods like create, find, update, and remove to perform CRUD operations in MongoDB.

Express.js interview question:

11. What is Express.js, and why is it used in the MERN stack?
    • Answer: Express.js is a web application framework for Node.js. It simplifies the process of building robust and scalable web applications in Node.js.
12. Explain the concept of middleware in Express.js.
    • Answer: Middleware functions in Express.js have access to the request, response, and the next middleware function. They can modify the request or response objects, end the request-response cycle, or call the next middleware.
13. How do you handle routing in Express.js?
    • Answer: Express.js uses the app.get, app.post, etc., methods to define routes. Each route can have a callback function to handle the request and response.
14. What is the purpose of the Express Router?
    • Answer: The Express Router allows grouping routes in a modular and reusable way. It helps in organizing routes and their handlers.
15. Explain the role of the template engines in Express.js.
    • Answer: Template engines like EJS or Pug in Express.js allow embedding dynamic data into HTML templates, enabling the creation of dynamic web pages.
16. How does error handling work in Express.js?
    • Answer: Error handling in Express.js is done through middleware functions with four parameters (err, req, res, next). If an error occurs, the middleware can handle it or pass it to the next error-handling middleware.
17. What is CORS, and how do you enable it in Express.js?
    • Answer: Cross-Origin Resource Sharing (CORS) is a security feature. In Express.js, you can enable CORS using the cors middleware.
18. Explain Express.js session management.
    • Answer: Express.js session management involves storing user-specific information on the server, typically in a session object, to maintain state across multiple requests.
19. How do you handle file uploads in Express.js?
    • Answer: File uploads in Express.js are handled using middleware like multer to process the uploaded files and store them on the server.
20. What is RESTful routing, and how is it implemented in Express.js?
    • Answer: RESTful routing is a convention for mapping HTTP methods and URIs to CRUD operations. In Express.js, it is implemented using various routes and HTTP methods.

React interview question:

21. What is React, and why is it used in the MERN stack?
    • Answer: React is a JavaScript library for building user interfaces. It is used in the MERN stack for creating reusable UI components and efficiently updating the user interface in response to data changes.
22. Explain the concept of a virtual DOM in React.
    • Answer: The virtual DOM is a lightweight copy of the real DOM that React maintains. It allows React to perform efficient updates by comparing the virtual DOM with the actual DOM and updating only the changed parts.
23. What is JSX in React, and why is it used?
    • Answer: JSX is a syntax extension for JavaScript recommended by React. It allows writing HTML elements and components in a syntax that looks similar to XML or HTML in JavaScript code.
24. How does state differ from props in React?
    • Answer: State is internal to a component and can be changed by the component, while props are external inputs to a component that are passed down from a parent component.
25. Explain the lifecycle methods in React.
    • Answer: React components have lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount. These methods allow developers to execute code at different phases of a component’s lifecycle.
26. What is the significance of the key attribute in React?
    • Answer: The key attribute is used to identify unique elements in a list. It helps React efficiently update the UI when the list changes.
27. How do you manage state in a functional component in React?
    • Answer: The useState hook is used in functional components to manage state. It returns an array with the current state value and a function to update the state.
28. What are React Hooks, and why are they used?
    • Answer: React Hooks are functions that allow functional components to use state and other React features. They provide a way to use state and lifecycle features in functional components.
29. What is React Router, and how is it used for navigation?
    • Answer: React Router is a standard library for routing in React applications. It enables navigation between different components when the URL changes.
30. Explain the concept of lifting state up in React.
    • Answer: Lifting state up is the process of moving the state from a child component to its parent component to share the state between multiple components.

Node.js interview question:

31. What is Node.js, and why is it used in the MERN stack?
    • Answer: Node.js is a JavaScript runtime built on the V8 JavaScript engine. It is used in the MERN stack to run server-side JavaScript, enabling a unified language for both client and server.
32. Explain the event-driven architecture in Node.js.
    • Answer: Node.js is designed to be event-driven and non-blocking. It uses an event loop to handle events asynchronously, making it highly scalable for handling concurrent connections.
33. How do you handle asynchronous operations in Node.js?
    • Answer: Asynchronous operations in Node.js are handled using callbacks, Promises, or the async/await syntax. The event-driven architecture allows non-blocking I/O operations.
34. What is npm, and how is it used in Node.js?
    • Answer: npm (Node Package Manager) is the default package manager for Node.js. It is used to install, manage, and share JavaScript packages and libraries.
35. Explain the role of the require function in Node.js.
    • Answer: The require function is used to import modules in Node.js. It allows using code from other files by including the module’s exports.
36. What is the purpose of the fs module in Node.js?
    • Answer: The fs (File System) module in Node.js provides functions for interacting with the file system. It allows reading, writing, and manipulating files and directories.
37. How do you create a basic server in Node.js using the http module?
    • Answer: You can create a basic server using the http module by creating an instance of http.Server and listening for requests. Example:
    javascriptCopy codeconst http = require('http'); const server = http.createServer((req, res) => { res.end('Hello, World!'); }); server.listen(3000, () => console.log('Server running on port 3000'));
38. What is middleware in the context of Node.js?
    • Answer: Middleware in Node.js are functions that have access to the request, response, and the next middleware function. They can modify request or response objects, end the request-response cycle, or call the next middleware.
39. Explain the purpose of the process object in Node.js.
    • Answer: The process object in Node.js provides information about the current Node.js process. It can be used to access command-line arguments, environment variables, and manage the process lifecycle.
40. What is the role of the Buffer class in Node.js?
    • Answer: The Buffer class in Node.js is used to handle binary data directly. It provides a way to work with raw binary data and is particularly useful for working with streams and file systems.