Node.js interview questions and answers

nodemon watches files and restarts on every save, which is what you want locally.

Create an express.Router per resource, such as users or orders, and mount them on paths like /api/users.

Register a four-argument middleware last: err, req, res, next.

helmet sets security-related HTTP headers such as Content-Security-Policy defaults, X-Content-Type-Options, and frame guards.

Build the React app, then use express.static to serve the dist or build folder.

Use a schema library such as Zod, Joi, or express-validator before business logic runs.

Apply a limiter early, especially on login and signup, to slow brute force and abuse.

Timers run setTimeout callbacks, then pending callbacks, idle/prepare, poll for I/O, check for setImmediate, and close callbacks.

Backpressure is the signal that a writable destination is slower than the readable source.

Global caches without bounds, event listeners that are never removed, and closures holding large request objects are common causes.

Worker threads share memory and suit CPU-heavy tasks such as image processing inside one process.

An unhandled promise rejection means async work failed with no catch.

Routes only parse HTTP, controllers map requests to use cases, services hold business rules, and a data layer talks to Mongoose.

On SIGTERM stop accepting new connections, finish in-flight requests, close the HTTP server, then close MongoDB and Redis.

A proxy timeout stops hung clients from holding sockets forever.

Attackers can upload huge files, unexpected MIME types, or stored XSS in SVG.

Generate or accept a request id, put it on logs, and return it on error responses.

Node handles many concurrent I/O waits on one thread with a non-blocking event loop.

It pins exact dependency versions for reproducible installs.

It parses JSON bodies into req.body.