25 important Node.js interview questions and answers for experienced developers

list of 25 important Node.js interview questions and answers for experienced developers, designed to be clear and easy to understand:

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Core Node.js Questions

1. What is the Event Loop in Node.js?

Answer:
The Event Loop is a mechanism that allows Node.js to perform non-blocking I/O operations by offloading tasks to the system kernel when possible. It processes asynchronous callbacks in phases (timers, I/O callbacks, idle/prepare, poll, check, close callbacks).

2. How does Node.js handle child threads?

Answer:
Node.js is single-threaded but uses worker threads (via the worker_threads module) to offload CPU-intensive tasks. Child processes (child_process module) can also be used to run separate instances of Node.js.

3. Explain the difference between process.nextTick() and setImmediate().

Answer:

• process.nextTick() executes before the next phase of the Event Loop.
• setImmediate() runs in the “check” phase of the Event Loop (after I/O callbacks).

4. What is the purpose of the cluster module?

Answer:
The cluster module allows Node.js to create multiple worker processes that share the same server port, improving performance by utilizing multi-core CPUs.

5. How does Node.js manage memory?

Answer:
Node.js uses V8’s garbage collector to manage memory. It employs a generational garbage collection strategy (Scavenge for young generation, Mark-Sweep-Compact for old generation).

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Asynchronous Programming

6. What are Promises and Async/Await?

Answer:

• Promises handle asynchronous operations with .then() and .catch().
• Async/Await is syntactic sugar over Promises, making asynchronous code look synchronous.

7. How do you handle unhandled Promise rejections?

Answer:
Use process.on('unhandledRejection', (err) => {}) to catch unhandled rejections.

8. What is the difference between setTimeout() and setImmediate()?

Answer:

• setTimeout() schedules execution after a specified delay.
• setImmediate() executes right after the current Event Loop phase.

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Modules & NPM

9. What is require.cache, and how can you clear it?

Answer:
require.cache stores loaded modules. To clear it:

delete require.cache[require.resolve('module-name')];

10. How do you create a custom Event Emitter?

Answer:

const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.emit('event');

11. What is the purpose of package-lock.json?

Answer:
It locks dependency versions to ensure consistent installations across environments.

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Error Handling

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

Answer:

• Use try/catch for synchronous code.
• Use .catch() for Promises.
• Listen for 'error' events on EventEmitters.

13. What is the difference between throw and EventEmitter.emit('error')?

Answer:

• throw stops execution unless caught.
• emit('error') triggers an error event but doesn’t stop execution unless unhandled.

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Performance & Optimization

14. How do you debug memory leaks in Node.js?

Answer:
Use tools like:

• node --inspect + Chrome DevTools.
• heapdump or clinic.js.

15. What is the Buffer class in Node.js?

Answer:
Buffer handles binary data. Example:

const buf = Buffer.from('hello');

16. How does Node.js handle slow I/O operations?

Answer:
It delegates I/O to the OS (via libuv) and continues processing other tasks, using callbacks when I/O completes.

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Security

17. How do you prevent SQL injection in Node.js?

Answer:
Use parameterized queries (e.g., with pg or mysql2):

db.query('SELECT * FROM users WHERE id = ?', [userId]);

18. What is the helmet middleware?

Answer:
helmet sets secure HTTP headers to protect against common vulnerabilities (e.g., XSS, CSRF).

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APIs & Frameworks

19. What is middleware in Express.js?

Answer:
Middleware are functions that process requests before they reach the route handler. Example:

app.use(express.json());

20. How do you manage sessions in Express?

Answer:
Use express-session or JWT (JSON Web Tokens) for stateless authentication.

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Testing

21. What is Mocha/Chai?

Answer:

• Mocha: Test framework.
• Chai: Assertion library.

22. How do you mock HTTP requests in tests?

Answer:
Use nock or supertest to mock API calls.

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Advanced Concepts

23. What is the stream module?

Answer:
Streams process data in chunks (e.g., reading large files):

fs.createReadStream('file.txt').pipe(res);

24. Explain the difference between exports and module.exports.

Answer:

• exports is a reference to module.exports.
• Overwriting exports breaks the reference; always use module.exports for new assignments.

25. How does Node.js support ES6 modules?

Answer:
Use .mjs extension or set "type": "module" in package.json.