MERN stack interview questions and answers

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.

A replica set is several MongoDB nodes that share the same data, with one primary for writes.

Transactions give ACID across multiple documents on a replica set or sharded cluster.

Write concern decides how many nodes must acknowledge a write, trading durability for latency.

explain shows whether MongoDB used an index, scanned how many documents, and what the winning plan was.

Avoid unbounded arrays, keep documents under the size limit, and index only hot query paths.

Change streams let Node watch inserts and updates in real time using the replica set oplog.

Reconciliation matches children between renders using position and key.

Concurrent features let React interrupt rendering to stay responsive and show fallback UI while data loads.

Effects and callbacks capture the props and state from the render that created them.

Profile with React DevTools to see which components commit often and why.

Keep server state in a cache such as React Query and UI state in local component state.

CSR renders in the browser after downloading JS, which is the classic MERN SPA.

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