Node.js Fundamentals

1. What is Node.js?

   • Node.js is an open-source, back-end JavaScript runtime environment that executes JavaScript code outside a web browser.
   • It's ¹ built on Chrome's V8 JavaScript ² engine.  
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2. What are the key features of Node.js?

   • Asynchronous and Event-Driven: Handles multiple requests concurrently without blocking the main thread.
   • Non-blocking I/O: Performs I/O operations (like reading files or making network requests) asynchronously.
   • Cross-platform: Runs on various operating systems (Windows, macOS, Linux).
   • Large and Active Community: Strong community support, extensive documentation, and a vast ecosystem of libraries and frameworks.
   • Single-threaded: Primarily uses a single thread, but can utilize multiple cores through the cluster module.

3. Explain the event loop in Node.js.

   • The core of Node.js's asynchronous nature.
   • Handles incoming requests and distributes them to available threads.
   • Manages callbacks and ensures efficient resource utilization.

4. What is the role of the V8 engine in Node.js?

   • V8 is the JavaScript engine developed by Google for Chrome.
   • Node.js utilizes V8 to execute JavaScript code efficiently.

5. What is the difference between Node.js and JavaScript?

   • JavaScript: A language primarily used for front-end development within a web browser.
   • Node.js: A runtime environment that allows you to execute JavaScript code on the server-side.

Core Modules

6. What is the fs module in Node.js?

   • Provides methods for interacting with the file system (e.g., reading, writing, and deleting files).

7. What is the http module in Node.js?

   • Enables you to create HTTP servers and clients.

8. What is the path module in Node.js?

   • Provides utilities for working with file and directory paths.

9. What is the os module in Node.js?

   • Provides information about the operating system the Node.js process is running on.

10. What is the url module in Node.js?

    • Provides utilities for parsing and formatting URLs.

Asynchronous Programming

11. What are callbacks in Node.js?

    • Functions passed as arguments to other functions to be executed after an asynchronous operation completes.

12. What are Promises in Node.js?

    • Objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value.

13. What is async/await in Node.js?

    • Syntax for writing asynchronous code in a more synchronous-like manner.
    • Makes asynchronous code easier to read and write.

14. What is the difference between synchronous and asynchronous operations?

    • Synchronous: Operations that block the execution of subsequent code until they complete.
    • Asynchronous: Operations that do not block the execution of subsequent code, allowing the program to continue while waiting for the result.

Streams

15. What are streams in Node.js?

    • A sequence of data that can be read or written over time.
    • Enable efficient handling of large amounts of data without loading everything into memory at once.

16. What are the different types of streams in Node.js?

    • Readable: Streams that can be read from.
    • Writable: Streams that can be written to.
    • Duplex: Streams that can be both read from and written to.
    • Transform: Streams that modify the data as it passes through.

Modules

17. What is the require() function in Node.js?

    • Used to import modules into your Node.js application.

18. What is the module.exports object in Node.js?

    • Used to export members from a module.

19. What is the exports object in Node.js?

    • A reference to the module.exports object.

npm

20. What is npm?

    • Node Package Manager, used for managing dependencies in Node.js projects.

21. How do you install a package using npm?

    • npm install <package_name>

22. How do you save a package as a dependency in your package.json file?

    • npm install <package_name> --save

23. How do you save a package as a development dependency in your package.json file?

    • npm install <package_name> --save-dev

Express.js

24. What is Express.js?

    • A popular and minimal web framework for Node.js.
    • Used for building web applications and APIs.

25. What are middleware functions in Express.js?

    • Functions that have access to the request and response objects of an HTTP request/response cycle.
    • Can perform actions like logging, authentication, and parsing request bodies.

26. How do you create a basic HTTP server with Express.js?

    • Use the express() function to create an Express application and define routes using methods like get(), post(), put(), delete().

Testing

27. What is Mocha?

    • A popular test framework for Node.js.

28. What is Chai?

    • An assertion library commonly used with Mocha for writing test cases.

29. What is Sinon.js?

    • A test utility library for stubbing, mocking, and spying on functions.

Debugging

30. How can you debug Node.js applications?
    • Use the Node.js debugger.
    • Use a debugger in your code editor (e.g., VS Code).
    • Utilize logging statements to track the flow of execution.

Security

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

    • Cross-Site Scripting (XSS): Injecting malicious scripts into web pages.
    • SQL Injection: Injecting malicious SQL queries into database queries.
    • Cross-Site Request Forgery (CSRF): Tricking users into performing actions on a website.
    • Insecure Direct Object References: Accessing resources without proper authorization.

32. How can you prevent security vulnerabilities in Node.js applications?

    • Input Validation and Sanitization: Validate and sanitize user input to prevent malicious attacks.
    • Use Prepared Statements: Prevent SQL injection by using prepared statements.
    • Implement Proper Authentication and Authorization: Protect sensitive data and resources.
    • Keep Dependencies Updated: Regularly update dependencies to patch known vulnerabilities.

Advanced Topics

33. What is clustering in Node.js?

    • A technique for utilizing multiple CPU cores to improve performance.
    • Creates multiple worker processes that share the same server port.

34. What is a Node.js cluster?

    • A group of worker processes that share the same server port and communicate with each other.

35. What is the child_process module in Node.js?

    • Allows you to spawn child processes and communicate with them.

36. What is the domain module in Node.js?

    • Provides a way to handle errors and exceptions within a specific scope.

Interview Questions

37. Explain how you would handle a large number of concurrent requests in a Node.js application.
38. Describe the process of creating a RESTful API using Node.js and Express.js.
39. How would you implement authentication and authorization in a Node.js application?
40. Explain the difference between synchronous and asynchronous file reading in Node.js.
41. How would you debug a memory leak in a Node.js application?
42. What are some best practices for writing maintainable and scalable Node.js code?
43. How would you deploy a Node.js application to production?
44. Explain the role of the package.json file in a Node.js project.
45. How would you optimize the performance of a Node.js application?
46. What are some popular Node.js frameworks other than Express.js? (e.g., NestJS, Koa)
47. How would you handle errors and exceptions gracefully in a Node.js application?
48. Explain the concept of dependency injection in Node.js.
49. How would you use streams to process large files efficiently in Node.js?
50. Describe your experience with using Node.js in a real-world project.

I hope these questions are helpful for your Node.js interview preparation.