MERN stack interview questions and answers

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.

Context passes data through the tree without prop drilling, such as theme or the current user.

A controlled input takes its value from React state and updates through onChange.

React.memo skips re-rendering a component when its props are shallowly equal.

Trigger the request in useEffect or a data library such as React Query, handle loading and error states, and cancel or ignore stale responses.

Lift state when two nearby components share data.

A custom hook is a function starting with use that calls other hooks.

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.