Full Stack developer interview questions

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.

Each refresh issues a new refresh token and invalidates the previous one.

Server sessions are easy to revoke instantly and keep tokens out of the client payload.

The code flow keeps tokens off the URL fragment used by the implicit flow.

Session fixation tricks a user into authenticating inside an attacker-known session id.

After password success you require a second factor such as TOTP or a WebAuthn key before issuing a full session.

Rolling replaces instances gradually on the same environment.

Liveness should be cheap and detect a dead process, not a slow dependency.

Run multiple processes or containers behind a load balancer and keep the API stateless.

CI needs deploy credentials, not the production JWT secret, if the runtime can inject secrets itself.

A contract test asserts that the API still returns the shape the frontend depends on, without a full UI.

Flakes come from real timers, unordered async, shared MongoDB state, and snapshot races.