NodeJs Interview Questions & Answers

Basics of Node.js:

1. What is Node.js?

   • Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine that allows developers to execute JavaScript code on the server side.

2. Explain the event-driven architecture of Node.js.

   • Node.js follows an event-driven, non-blocking I/O model. It uses an event loop to handle asynchronous operations, allowing efficient processing of multiple requests.

3. What is the Node Package Manager (NPM)?

   • NPM is the default package manager for Node.js. It is used to install, manage, and share packages and dependencies for Node.js applications.

4. How does Node.js handle concurrency?

   • Node.js is single-threaded, but it uses an event-driven, non-blocking model to handle concurrency. Asynchronous operations, such as I/O operations, are performed using callbacks or Promises.

5. Explain the role of the V8 JavaScript engine in Node.js.

   • V8 is an open-source JavaScript engine developed by Google. Node.js uses V8 to execute JavaScript code efficiently on the server side.

Modules and CommonJS:

6. What is a module in Node.js?

   • A module in Node.js is a reusable piece of code encapsulated in a file. It can include variables, functions, and classes that can be exported and used in other modules.

7. How do you include a module in Node.js?

   • Modules are included using the require function. For example: const myModule = require('./myModule');

8. What is the purpose of the module.exports object?

   • The module.exports object is used to expose functions, objects, or variables from a module, making them accessible to other parts of the application.

9. Explain the CommonJS module format.

   • CommonJS is a module system for JavaScript used by Node.js. It specifies a simple format for modules, including the use of require and module.exports.

10. How do you create a custom event in Node.js?

    • The events module in Node.js is used to create and handle custom events. You can create an instance of the EventEmitter class and use its methods to emit and listen for events.

Asynchronous Programming:

11. What is callback hell? How can it be avoided?

    • Callback hell refers to the situation where multiple nested callbacks make the code difficult to read and maintain. It can be avoided by using Promises or async/await.

12. Explain the concept of Promises in Node.js.

    • Promises in Node.js are a way to handle asynchronous operations. They represent a value that may be available now, in the future, or never, resolving or rejecting with a value.

13. What is the purpose of the async and await keywords in Node.js?

    • async and await are used to work with asynchronous code more comfortably. The async keyword is used to define asynchronous functions, and await is used to wait for a promise to resolve.

14. How does the setImmediate function work in Node.js?

    • setImmediate is used to execute a callback function in the next iteration of the event loop. It is similar to setTimeout(fn, 0) but with some differences in execution order.

15. What is the purpose of the process.nextTick function?

    • process.nextTick is used to schedule a callback function to be executed in the next iteration of the event loop, immediately after the current operation.

Express.js:

16. What is Express.js?

    • Express.js is a web application framework for Node.js that simplifies the development of web applications by providing a robust set of features for routing, middleware, and handling HTTP requests and responses.

17. Explain the middleware concept in Express.js.

    • Middleware in Express.js are functions that have access to the request, response, and the next middleware function in the application’s request-response cycle. They can modify the request and response objects and terminate the request-response cycle.

18. How do you handle routing in Express.js?

    • Routing in Express.js is handled using the express.Router middleware. It allows developers to define routes and their corresponding handlers separately and then mount them on the main application.

19. What is the purpose of the app.use function in Express.js?

    • app.use is used to mount middleware functions in the Express.js application. It is called for every incoming request and can be used for tasks such as logging, authentication, and error handling.

20. Explain the difference between app.get() and app.post() in Express.js.

    • app.get() is used to handle HTTP GET requests, while app.post() is used to handle HTTP POST requests. They correspond to the HTTP methods they represent.

RESTful APIs and HTTP:

21. What is RESTful architecture?

    • REST (Representational State Transfer) is an architectural style that defines a set of constraints for designing web services. RESTful APIs use standard HTTP methods (GET, POST, PUT, DELETE) to perform CRUD operations.

22. How do you handle query parameters in Express.js?

    • Query parameters in Express.js can be accessed using req.query. For example, req.query.paramName retrieves the value of the parameter named paramName.

23. What is the purpose of the HTTP status code 404?

    • The HTTP status code 404 indicates that the requested resource could not be found on the server.

24. How can you secure Express.js applications?

    • Express.js applications can be secured by implementing measures such as using HTTPS, validating user input, setting secure HTTP headers, and implementing authentication and authorization mechanisms.

25. Explain the concept of CORS in Express.js.

    • CORS (Cross-Origin Resource Sharing) is a security feature implemented by browsers to restrict web pages from making requests to a different domain than the one that served the web page. In Express.js, CORS can be enabled using the cors middleware.

MongoDB and Mongoose:

26. What is MongoDB?

    • MongoDB is a NoSQL database that stores data in JSON-like documents. It is designed for scalability, flexibility, and performance.

27. Explain the concept of collections in MongoDB.

    • In MongoDB, a collection is a group of MongoDB documents. It is roughly equivalent to an RDBMS table.

28. What is Mongoose in the context of Node.js?

    • Mongoose is an ODM (Object-Document Mapper) library for MongoDB and Node.js. It provides a schema-based solution for modeling application data and interacting with MongoDB.

29. How do you perform CRUD operations in MongoDB using Mongoose?

    • Mongoose provides methods like save, find, update, and remove to perform CRUD operations on MongoDB documents.

30. What is the purpose of the Mongoose schema in Node.js?

    • Mongoose schema defines the structure of documents in a MongoDB collection. It specifies the fields, types, and constraints for the documents.

Authentication and Authorization:

How can you implement user authentication in Node.js? - User authentication in Node.js can be implemented using various strategies such as username/password, JWT (JSON Web Tokens), OAuth, and third-party authentication providers.

32. Explain the purpose of JWT (JSON Web Tokens) in Node.js authentication.

    • JWT is a compact, URL-safe means of representing claims to be transferred between two parties. In the context of Node.js authentication, JWTs are commonly used to encode user information and verify the authenticity of requests.

33. What is OAuth, and how does it relate to Node.js authentication?

    • OAuth is an open standard for access delegation, commonly used as a way for Internet users to grant websites or applications access to their information on other websites without giving them their credentials. In Node.js, OAuth can be used for third-party authentication.

34. How do you handle user sessions in Express.js?

    • User sessions in Express.js can be handled using middleware like express-session. Sessions are often used to store user authentication status and other user-related information.

35. Explain the concept of role-based access control in Node.js.

    • Role-based access control is a method of regulating access to a system based on roles assigned to users. In Node.js, roles can be used to determine the permissions and access levels of users.

WebSocket and Real-time Communication:

36. What is WebSocket, and how is it different from HTTP?

    • WebSocket is a communication protocol that provides full-duplex communication channels over a single, long-lived connection. It is different from HTTP, which follows a request-response model.

37. How can you implement WebSocket in Node.js?

    • WebSocket can be implemented in Node.js using libraries like ws or the WebSocket support provided by frameworks like Express.js.

38. Explain the concept of real-time communication in Node.js.

    • Real-time communication in Node.js involves bidirectional communication between clients and servers, allowing instant data updates without the need for manual refreshing.

Testing in Node.js:

39. What are unit tests, and how can they be implemented in Node.js?

    • Unit tests are tests that focus on a specific unit of code, such as a function or module, to ensure that it behaves as expected. Unit tests in Node.js can be implemented using testing frameworks like Mocha or Jest.

40. What is Mocha, and how does it facilitate testing in Node.js?

    • Mocha is a feature-rich JavaScript test framework for Node.js and browsers. It provides support for asynchronous testing, before/after hooks, and various reporting options.

Deployment and DevOps:

41. How can you deploy a Node.js application?

    • Node.js applications can be deployed using various platforms, cloud services, and deployment tools. Popular choices include AWS, Heroku, and Docker.

42. What is PM2, and how is it used in Node.js deployment?

    • PM2 is a process manager for Node.js applications. It ensures that applications are always running and provides features like automatic restarts, load balancing, and monitoring.

43. Explain the concept of continuous integration in Node.js development.

    • Continuous Integration (CI) is a software development practice where changes to the codebase are automatically tested and integrated into the main code repository. Tools like Jenkins, Travis CI, and GitHub Actions can be used for CI in Node.js projects.

44. How do you handle environment variables in Node.js?

    • Environment variables in Node.js can be handled using the process.env object. Libraries like dotenv can be used to manage and load environment variables from a configuration file.

Security in Node.js:

45. What are some common security vulnerabilities in Node.js applications?

    • Common security vulnerabilities in Node.js applications include injection attacks, XSS (Cross-Site Scripting), CSRF (Cross-Site Request Forgery), and insecure dependencies.

46. How can you prevent SQL injection in Node.js applications?

    • SQL injection can be prevented by using parameterized queries or prepared statements when interacting with databases. Libraries like knex provide mechanisms for safe query construction.

47. Explain the concept of Cross-Origin Resource Sharing (CORS) in Node.js.

    • CORS is a security feature implemented by browsers to restrict web pages from making requests to a different domain than the one that served the web page. In Node.js, CORS can be handled using middleware or headers.

48. How can you secure file uploads in Node.js?

    • File uploads in Node.js can be secured by validating file types, setting file size limits, and storing files in a secure location. Libraries like multer can be used to handle file uploads.

Miscellaneous:

49. What is the purpose of the child_process module in Node.js?

    • The child_process module in Node.js is used to spawn child processes, allowing the execution of external commands or scripts from within a Node.js application.

50. How do you handle errors in Node.js?

    • Errors in Node.js can be handled using try-catch blocks for synchronous code and using .catch() or try/catch for asynchronous code. Additionally, middleware like express-error-handler can be used to handle errors in Express.js applications.