Top 50+ Node.js Interview Questions and Answers for 2023

Basics of Node.js:

1. What is Node.js?

   • Node.js is a server-side JavaScript runtime environment built on the V8 JavaScript engine.

2. How does Node.js differ from traditional JavaScript?

   • Node.js is not a language; it allows JavaScript to be executed server-side, outside the browser.

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

   • Node.js operates on a non-blocking, event-driven model where events trigger asynchronous callbacks.

4. What is an EventEmitter in Node.js?

   • EventEmitter is a class in Node.js that allows objects to emit and listen for events.

Node.js Modules:

5. What is a module in Node.js?

   • A module is a reusable piece of code that encapsulates functionality.

6. How can you include external libraries in Node.js?

   • Use require('module') to include external libraries.

npm (Node Package Manager):

7. What is npm?

   • npm is the Node Package Manager used for package management and distribution.

8. How do you install packages using npm?

   • Use npm install package-name to install packages locally and npm install -g package-name for global installation.

9. Explain the purpose of package.json.

   • package.json is a file containing metadata about a Node.js project, including dependencies.

Callbacks and Promises:

10. What is a callback function in Node.js?

    • A callback is a function passed as an argument to another function, to be executed later.

11. Explain the concept of callback hell.

    • Callback hell refers to the nesting of multiple callbacks, making the code hard to read and maintain.

12. What are Promises in Node.js?

    • Promises are objects representing the eventual completion or failure of an asynchronous operation.

13. How do you handle errors in Promises?

    • Use .catch() or .then(null, errorHandler) to handle errors in Promises.

File System:

14. How do you read and write files in Node.js?

    • Use fs.readFile for reading and fs.writeFile for writing files.

15. Explain the difference between fs.readFileSync and fs.readFile.

    • fs.readFileSync is synchronous, blocking execution, while fs.readFile is asynchronous.

16. What is the purpose of the fs.createReadStream method?

    • It creates a readable stream to efficiently read large files.

Express.js:

17. What is Express.js?

    • Express.js is a web application framework for Node.js, simplifying the process of building robust web applications.

18. How do you install Express.js?

    • Use npm install express to install Express.js.

19. Explain the routing in Express.js.

    • Routing in Express.js defines how the application responds to client requests.

20. What is middleware in Express.js?

    • Middleware functions are functions that have access to the request, response, and the next middleware function in the application’s request-response cycle.

RESTful APIs:

21. What is RESTful architecture?

    • RESTful architecture is a style of designing networked applications using simple HTTP methods.

22. How do you create a RESTful API using Express.js?

    • Use Express.js to define routes and handlers for HTTP methods.

23. Explain the HTTP methods used in RESTful services.

    • GET (read), POST (create), PUT (update), DELETE (delete).

Asynchronous Programming:

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

    • The event loop is a core concept in Node.js for handling asynchronous operations.

25. How does Node.js handle asynchronous code?

    • Through callbacks, Promises, and async/await.

26. What is the purpose of the setImmediate function?

    • setImmediate is used to execute a script once the current event loop cycle completes.

Streams:

27. What are streams in Node.js?

    • Streams provide an efficient way to read or write data in chunks.

28. Explain the difference between readable and writable streams.

    • Readable streams allow reading, while writable streams allow writing.

29. How do you pipe streams in Node.js?

    • Use the pipe method to connect the output of one stream to the input of another.

WebSocket:

30. What is WebSocket?

    • WebSocket is a communication protocol providing full-duplex communication channels over a single TCP connection.

31. How do you implement WebSocket in Node.js?

    • Use the ws library or the socket.io library for WebSocket implementation.

MongoDB and Mongoose:

32. What is MongoDB?

    • MongoDB is a NoSQL database.

33. How do you connect to a MongoDB database using Node.js?

    • Use the mongodb driver or an ODM like Mongoose.

34. What is Mongoose?

    • Mongoose is an ODM (Object-Document Mapper) for MongoDB and Node.js.

35. Explain the schema in Mongoose.

    • A schema defines the structure of documents in a collection.

Testing in Node.js:

36. What testing frameworks are commonly used in Node.js?

    • Mocha, Jasmine, Jest.

37. How do you write unit tests in Node.js?

    • Use testing frameworks like Mocha and assertions libraries like Chai.

Security:

38. How can you prevent common security vulnerabilities in Node.js applications?
    • Validate input, use parameterized queries, sanitize user inputs, and keep dependencies updated.

Debugging:

39. How do you debug a Node.js application?
    • Use the debugger statement or tools like node-inspector and built-in debugging in VSCode.

Performance Optimization:

40. What techniques can you use to optimize the performance of a Node.js application?
    • Caching, load balancing, minimizing blocking code, and using a reverse proxy.

Child Processes:

41. Explain the use of the child_process module in Node.js.
    • It allows running external processes, enabling interaction with the operating system.

Global Objects in Node.js:

42. What is the global object in Node.js?

    • The global object represents the global namespace in Node.js.

43. Explain the purpose of __dirname and __filename.

    • __dirname is the name of the directory containing the currently executing script, and __filename is the file name of the current module.

Error Handling:

44. How do you handle errors in Node.js?
    • Use try-catch blocks, callback error-first pattern, and Promise .catch().

Deployment:

45. What are some common deployment strategies for Node.js applications?
    • Using containers (Docker), cloud platforms (AWS, Azure, Heroku), and continuous integration.

Scalability:

46. How can you scale a Node.js application?
    • Horizontal scaling with load balancing, using a reverse proxy, and optimizing code.

Middleware:

47. Explain the concept of middleware in Express.js.
    • Middleware functions are functions that have access to the request, response, and the next middleware function in the application’s request-response cycle.

Cross-Origin Resource Sharing (CORS):

48. What is CORS, and how can you handle it in Express.js?
    • CORS (Cross-Origin Resource Sharing) is a security feature implemented by browsers. In Express.js, you can use the cors middleware to handle it.

Template Engines:

49. What is a template engine, and which ones are commonly used with Node.js?
    • A template engine processes templates to produce HTML. Common ones for Node.js are EJS, Handlebars, and Pug.

WebSockets:

50. How do WebSockets differ from traditional HTTP communication?
    • WebSockets provide full-duplex communication, allowing real-time data transfer, unlike the request-response nature of HTTP.