Backend EngineeringInterview questions and answers

Node.js is a JavaScript runtime built on Chrome's V8 engine that runs JS outside the browser, typically on servers. It uses a single-threaded event loop with non-blocking I/O — perfect for handling thousands of concurrent connections doing I/O-bound work like APIs, real-time chat, or proxying.

Express is a minimal, unopinionated Node.js web framework. It provides routing, middleware composition, and helpers around the raw HTTP module — but it stays out of your way about structure. Most Node tutorials use it because it's small enough to learn in an hour and flexible enough for big apps.

NestJS is an opinionated Node.js framework built with TypeScript and inspired by Angular and Spring. It gives you dependency injection, decorators, modules, guards, interceptors, and a CLI — ideal for large teams that want strict structure. Comes with first-class support for REST, GraphQL, microservices, and WebSockets.

Middleware is a function with access to (req, res, next) that runs before or after route handlers. Each one can read/modify req or res, then call next() to continue. Express composes them in order — typical examples: body parsing, authentication, logging, rate limiting, and error handling.

Node's event loop cycles through phases: timers (setTimeout), pending callbacks, poll (I/O), check (setImmediate), and close events. Between every phase, microtasks (promises, process.nextTick) drain. Knowing this order helps you predict ordering of callbacks and avoid blocking the loop.