MERN stack interview questions and answers

Node.js is a JavaScript runtime built on V8 that runs outside the browser.

npm is Node's default package manager.

dependencies are packages the app needs at runtime, such as express or mongoose.

JavaScript in Node runs on one main thread, but I/O is offloaded to the system and libuv.

require is CommonJS and loads modules synchronously.

process.env is an object of environment variables available to the process.

Synchronous code finishes before the next line runs and can block the event loop.

npm install reads package.json and the lockfile, then downloads packages into node_modules including transitive dependencies.

Express is a minimal Node framework for routing, middleware, and HTTP responses.

Middleware is a function with req, res, and next that runs during the request lifecycle.

You attach an HTTP method and path to an app or router, such as app.get('/health', handler).

req is the incoming request: params, query, body, headers, and cookies.

express.json() is middleware that parses JSON bodies into req.body.

Use res.status(code).json(payload) so the client gets the right status and Content-Type.

next() passes control to the next matching middleware or route.

MongoDB is a document database that stores BSON documents instead of rigid rows.

A document is one record, similar to a row but nested and flexible.

Mongoose is an ODM that maps MongoDB collections to Node models.

find returns matching documents as a query result list.

ObjectId is MongoDB's default unique identifier for _id.