Prepare for your Express.js interview with these 30 carefully curated questions covering basic, intermediate, and advanced concepts. This guide helps freshers, candidates with 1-3 years, and 3-6 years of experience master Express.js fundamentals, practical implementation, and real-world scenarios.
Basic Express.js Interview Questions
1. What is Express.js?
Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It simplifies handling HTTP requests, routing, middleware, and response rendering.[1][3][4]
2. What is the conventional port number used for Express.js applications?
Express.js does not have a fixed default port, but developers conventionally use port 3000 for development servers.[4]
const express = require('express');
const app = express();
app.listen(3000, () => {
console.log('Server running on port 3000');
});3. How do you create a basic Express.js application?
Create an Express app instance, define routes, add middleware, and start the server with app.listen().[3]
const express = require('express');
const app = express();
app.use(express.json());
app.get('/', (req, res) => res.send('Hello World'));
app.listen(3000);4. What are HTTP methods supported in Express.js?
Express.js supports standard HTTP methods: GET, POST, PUT, PATCH, DELETE, HEAD, and OPTIONS. GET retrieves data, POST submits data, PUT updates completely, PATCH updates partially, and DELETE removes resources.[3][4]
5. How do you parse JSON request bodies in Express.js?
Use the built-in express.json() middleware to parse incoming JSON requests with Content-Type application/json.[3]
app.use(express.json());
app.post('/data', (req, res) => {
console.log(req.body);
res.json({ received: true });
});6. What are route parameters in Express.js?
Route parameters are dynamic URL segments defined with a colon (:) that capture values from the URL path.[2][4]
app.get('/users/:id', (req, res) => {
const userId = req.params.id;
res.send(`User ID: ${userId}`);
});7. How do you access query parameters in Express.js?
Query parameters are available in req.query object for GET requests with ?key=value format.[2]
app.get('/search', (req, res) => {
const { q, category } = req.query;
res.json({ search: q, category });
});8. What is express.static middleware used for?
The express.static middleware serves static files like images, CSS, and JavaScript directly from a directory.[4]
app.use(express.static('public'));
9. How do you install Express.js in a project?
Run npm install express to add Express.js as a dependency to your Node.js project.[3]
10. What are the key features of Express.js?
Key features include routing, middleware support, HTTP utility methods, static file serving, and template engine integration.[4]
Intermediate Express.js Interview Questions
11. What is middleware in Express.js?
Middleware functions have access to request (req), response (res), and next() function to pass control to the next middleware.[1][3]
app.use((req, res, next) => {
console.log('Request logged');
next();
});12. What is the purpose of the next() function in middleware?
next() passes control to the next middleware function in the stack. Omitting next() hangs the request.[2][5]
13. How do you create custom middleware in Express.js?
Define a function with (req, res, next) signature and use app.use() to register it globally or for specific routes.[1]
14. What are the different types of middleware in Express.js?
Types include application-level (app.use()), router-level, error-handling (four arguments), built-in, and third-party middleware.[4]
15. How do you handle CORS in Express.js?
Use cors middleware or set Access-Control-Allow-Origin headers in responses to enable cross-origin requests.[5]
16. Explain error-handling middleware signature.
Error-handling middleware uses four arguments: (err, req, res, next) and must be defined after all routes.[1]
app.use((err, req, res, next) => {
res.status(500).json({ error: err.message });
});17. How do you prevent “Cannot set headers after they are sent” error?
Use return after res.send(), ensure single response path, and avoid calling next() after response is sent.[1]
18. What is the execution order of middleware in Express.js?
Middleware executes in the order defined. Routes mounted before middleware skip it, so order placement matters.[1]
19. How do you serve static files from multiple directories?
Chain multiple express.static calls in order of precedence.[4]
app.use(express.static('public'));
app.use(express.static('uploads'));
20. How do you implement authentication middleware?
Create middleware that checks auth tokens and calls next() if valid or sends 401 if invalid.[1]
Advanced Express.js Interview Questions
21. How do you prevent blocking the Node.js event loop in Express routes?
Offload CPU-intensive work to worker threads, child processes, or external queues to avoid blocking other requests.[1]
22. Scenario: At Zoho, authentication middleware skips some routes. How do you debug?
Check middleware registration order, route mounting sequence, and ensure next() is called conditionally.[1]
23. Why separate Express app creation from server listening?
Enables easier testing with supertest, cleaner deployment for serverless, and better code organization.[3]
const app = express();
// ... routes
module.exports = app;
24. Scenario: Multiple async operations in a route cause double responses at Paytm. Fix it.
Ensure only one response path executes using flags, promises.allSettled(), or early returns.[1]
25. How do you implement rate limiting in Express.js?
Use express-rate-limit middleware to restrict requests per IP or user within time windows.
26. Explain Express.js request-response lifecycle.
Middleware stack executes sequentially; matching route handler processes request; response sent finalizes cycle.[1]
27. Scenario: At Salesforce, 404 errors need custom handling. Implement it.
Define 404 middleware after all routes, then error-handling middleware for 500 errors.[5]
app.use((req, res) => {
res.status(404).json({ error: 'Route not found' });
});
28. How do you optimize Express.js for production at Atlassian scale?
Enable clustering, compression middleware, helmet for security headers, and proper logging with Morgan.[3]
29. What are RESTful principles for Express.js routes?
Use appropriate HTTP methods: GET for retrieval, POST for creation, PUT/PATCH for updates, DELETE for removal.[2]
30. Scenario: At Adobe, heavy computation blocks requests. Solution?
Move CPU-intensive tasks to background jobs using Bull Queue or child_process.fork() to maintain responsiveness.[1]
