express-interview-cheat-sheet.md

Express Interview Questions And Model Answers

This is the rapid-fire revision sheet for the Express folder.

Fundamentals

What is Express?

Express is a minimal web framework for Node.js that simplifies routing, middleware, and HTTP response handling.

Why use Express over raw Node http?

Because it removes repetitive boilerplate and provides a cleaner abstraction for common web server tasks.

What is middleware?

Middleware is a function in the request pipeline that can inspect the request, modify it, end the response, or pass control onward.

Routing

What is the difference between req.params, req.query, and req.body?

req.params comes from the path, req.query comes from the query string, and req.body comes from the parsed request payload.

Why does middleware order matter?

Because Express executes middleware in registration order.

APIs

What makes an Express API RESTful?

Clear resource-based routes, proper HTTP methods, consistent status codes, and predictable response design.

Why separate controllers and services?

To keep HTTP concerns separate from business logic and make the code easier to test and maintain.

Errors

How do you handle async errors in Express?

Wrap async handlers and forward rejections to centralized error middleware.

Why use centralized error handling?

It keeps error responses consistent and prevents duplicated logic across routes.

Production

How do you secure an Express app?

Use validation, authentication middleware, secure headers, rate limiting, dependency hygiene, and careful error responses.

What is a common Express performance mistake?

Blocking the event loop inside handlers or middleware with slow synchronous work.