Node.js has revolutionized web development with its asynchronous, event-driven architecture. This guide covers interview questions for all levels—beginner, intermediate, and advanced—helping candidates prepare for roles that require expertise in Node.js.

Beginner-Level Questions

1. What is Node.js?

Node.js is a runtime environment built on Chrome’s V8 JavaScript engine that allows JavaScript to be executed outside the browser, often used for server-side development.

2. What are the key features of Node.js?

• Asynchronous and Event-Driven

• Single-Threaded

• Cross-Platform

• Fast due to Chrome’s V8 engine

• Uses npm for package management

3. What is npm?

npm (Node Package Manager) is the default package manager for Node.js. It is used for:

• Installing packages.

• Managing dependencies.

• Publishing open-source libraries.

4. What is the difference between Node.js and JavaScript in the browser?

• Environment: Node.js runs outside the browser.

• APIs: Node.js provides server-side APIs like fs and http.

• Global Object: Node.js uses global instead of window.

5. How does Node.js handle asynchronous operations?

Node.js uses an event loop and callback functions to handle asynchronous operations like file I/O and HTTP requests.

6. What is the event loop in Node.js?

The event loop processes asynchronous operations by polling for events and executing callbacks.

7. What is a callback in Node.js?

A callback is a function passed as an argument to another function, executed after the operation completes.

Example:

fs.readFile('file.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

8. What are modules in Node.js?

Modules are reusable blocks of code in Node.js. Types:

• Core Modules: Built-in, like http and fs.

• Custom Modules: User-defined.

• Third-Party Modules: Installed via npm.

9. What is the require() function?

require() is used to include modules.

Example:

const fs = require('fs');

10. What is the difference between var, let, and const in Node.js?

• var: Function-scoped, can be re-declared.

• let: Block-scoped, cannot be re-declared.

• const: Block-scoped, immutable.

11. What is a buffer in Node.js?

Buffers are fixed-size memory allocations used to work with raw binary data.

12. What is the purpose of the process object in Node.js?

process is a global object that provides information and control over the Node.js process.

13. How do you handle exceptions in Node.js?

Use try-catch blocks for synchronous code and .catch() or async/await for promises.

14. What is the purpose of package.json?

It contains metadata about the project and dependencies.

15. How do you read command-line arguments in Node.js?

Use process.argv.

Example:

console.log(process.argv.slice(2));

Intermediate-Level Questions

16. What is event-driven programming in Node.js?

Event-driven programming synchronizes events through an event loop and callbacks.

17. What are streams in Node.js?

Streams allow reading or writing data sequentially. Types:

• Readable

• Writable

• Duplex

• Transform

18. What is the crypto module in Node.js?

The crypto module provides encryption, decryption, and hashing functionality.

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

Callback Hell refers to deeply nested callbacks. Avoid it using:

• Promises

• Async/Await

• Modularizing code

20. Explain the use of timers in Node.js.

Node.js timers manage delayed or repeated execution using:

• setTimeout

• setInterval

• setImmediate

21. What is the difference between setImmediate() and process.nextTick()?

• process.nextTick: Executes before I/O events.

• setImmediate: Executes in the next iteration of the event loop.

22. What is CORS in Node.js?

CORS (Cross-Origin Resource Sharing) allows resources to be accessed from different origins. Use the cors package to enable CORS.

23. What is body-parser in Node.js?

body-parser parses incoming request bodies in middleware. It’s commonly used with Express.

24. What is the difference between spawn() and fork()?

• spawn: Launches a new process.

• fork: Creates a new Node.js process.

25. Explain the use of Passport.js in Node.js.

Passport.js is an authentication middleware for Node.js supporting various strategies like OAuth, Google, and JWT.

26. How can you avoid blocking the event loop in Node.js?

• Use asynchronous methods.

• Offload heavy computations to child processes or worker threads.

27. What are middleware functions in Node.js?

Middleware functions process requests before reaching the route handler in Express.

28. What is a singleton pattern in Node.js?

A singleton ensures only one instance of a class/module is created and shared.

29. What are the types of HTTP methods supported by Node.js?

Common methods:

• GET

• POST

• PUT

• DELETE

30. Explain the concept of middleware in Express.

Middleware functions can modify requests, add headers, or handle errors.

Advanced-Level Questions

31. What is a cluster in Node.js?

Clusters allow multi-threading in Node.js by spawning multiple child processes to utilize multi-core CPUs.

32. What are child processes in Node.js?

The child_process module allows Node.js to spawn new processes.

33. How do you handle authentication in Node.js?

Authentication strategies:

• Session-based: express-session

• Token-based: JWT (JSON Web Token)

34. Explain WebSockets in Node.js.

WebSockets provide full-duplex communication between client and server, allowing real-time data exchange.

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

The util module provides utility functions like:

• util.promisify

• util.format

36. What is the purpose of Redis in Node.js?

Redis is an in-memory data store for caching, session management, and pub/sub messaging.

37. How do you manage environment variables in Node.js?

Use the dotenv package to load environment variables from a .env file.

38. Explain the difference between synchronous and asynchronous methods in Node.js.

• Synchronous: Blocks the event loop.

• Asynchronous: Non-blocking, uses callbacks or promises.

39. What is the difference between CommonJS and ES Modules?

• CommonJS: Uses require() for importing.

• ES Modules: Uses import/export syntax.

40. What are worker threads in Node.js?

Worker threads allow parallel execution of JavaScript code.

41. What is the purpose of the fs module in Node.js?

The fs module allows reading, writing, and managing files.

42. How do you implement logging in Node.js?

Use libraries like winston or morgan for structured logging.

43. Explain the TLS module in Node.js.

The tls module provides Transport Layer Security for encrypting communication.

44. What is tracing in Node.js?

Tracing provides insights into Node.js execution by monitoring events and system performance.

45. How do you optimize performance in Node.js applications?

• Use clusters or worker threads.

• Enable caching with Redis.

• Use asynchronous code.

• Profile and debug using tools like clinic.js.

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