Node.js Interview Questions and Answers

Node.js is a JavaScript runtime built on Chrome's V8 engine that runs your JavaScript on a single main thread and uses non-blocking I/O, so one thread can handle many concurrent connections.

package.json lists your project's dependencies with allowed version ranges, while package-lock.json records the exact versions that were installed so every machine gets an identical dependency tree.

require is the CommonJS module system, which loads modules synchronously at runtime, while import is the ES module standard, which is statically analysed, asynchronous and supports top-level await and tree shaking.

Middleware are functions that receive the request, response and a next callback; they run in the order they are registered and can modify the request, end the response or pass control on with next().

process.nextTick runs its callback right after the current operation, before the event loop continues; setTimeout runs in the timers phase after a minimum delay; and setImmediate runs in the check phase right after the poll (I/O) phase.

Streams process data piece by piece as it arrives instead of loading it all into memory, which lets you handle large files and network data efficiently; they come in Readable, Writable, Duplex and Transform types.

Session authentication stores the login state on the server and gives the browser a session ID cookie, while JWT authentication gives the client a signed token that contains the user's claims so the server can verify it without storing state.

Validate all input, use parameterised queries, add security headers with helmet, rate-limit requests, restrict CORS, hash passwords, keep secrets in environment variables, and keep dependencies updated and audited.

Keep the main thread free by moving CPU-heavy work to worker threads or a job queue, and scale across cores and machines by running multiple processes (cluster or PM2) behind a load balancer with shared, external state.