MERN stack interview questions and answers

CRUD is Create, Read, Update, and Delete.

A schema defines fields, types, required flags, defaults, and indexes for a collection.

React is a library for building user interfaces from components.

JSX is a syntax that looks like HTML inside JavaScript.

Props are inputs passed from a parent and should be treated as read-only.

useState returns the current value and a setter that queues a re-render.

useEffect runs side effects after render, such as fetching data or attaching listeners.

A component is a function that returns UI from props and state.

The virtual DOM is React's in-memory tree of UI.

Class components use this.state and lifecycle methods.

key helps React identify which item changed, was added, or was removed.

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.