MERN stack interview questions and answers

Callbacks pass errors as the first argument and nest easily into callback hell.

Streams process data in chunks instead of loading an entire file or response into memory.

Wrap awaited calls in try/catch or use a wrapper that forwards rejections to next.

process.nextTick queues a callback to run after the current operation, before the event loop continues.

cluster forks worker processes so a multi-core machine can handle more concurrent connections.

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.

An index is a data structure that lets MongoDB find documents without scanning the whole collection.

Aggregation is a sequence of stages such as match, group, lookup, and project that transform documents on the server.

populate replaces stored ObjectId references with the related documents in a follow-up query.

Embed data that is read together and stays bounded, such as an address on a user.

lean() returns plain JavaScript objects instead of full Mongoose documents.

Offset pagination uses skip and limit, which is simple but slow on large skips.

Only call hooks at the top level of a React function, never inside loops or conditions.

useMemo caches an expensive calculation between renders.