Node.js &amp; Express

What is Express middleware

Middleware are functions in a chain with req, res, next, they inspect or modify request and response, then call next to continue or send a response to end.

Parse JSON and URL-encoded bodies in Express

Express.json() for JSON, express.urlencoded() for form data, both registered via app.use().

Write a request-logging middleware in Express

Read req.method, log with a timestamp, then call next() to continue.

Application-level vs router-level middleware

App.use() binds to the app and runs everywhere; router.use() binds to a Router and runs only for that router's routes.

Express error-handling middleware signature

(err, req, res, next) with err first, identified by arity, placed last after all routes.

Middleware execution order and sharing data via req

Middleware runs top-down in registration order; each calls next(); attach data like req.user that later handlers read.

Write a JWT authentication middleware

Read the header, strip Bearer, jwt.verify with the secret, set req.user and next(), else send 401.

Conditionally apply middleware by request property

Use express.text({ type: 'application/xml' }) or a guard wrapper that checks req.is() then calls the parser or next().

Design an Express route to create a user

POST to a collection URL like /users, read the payload from req.body via express.json(), return 201 with the created resource.

req.params vs req.query vs req.body in Express

Req.params holds named route segments, req.query holds the URL query string, req.body holds the parsed payload.

Status codes for successful POST and GET

201 Created for a successful POST (ideally with a Location header), 200 OK for a successful GET returning data.

Modularize routes with express.Router

Create a Router per resource in its own file, define routes on it, export it, and mount with app.use('/products', router).

Idempotency: PUT vs POST in REST

Idempotent means repeated identical requests leave the same server state; PUT is idempotent, POST is not. Use PUT to overwrite a resource at a known URL.

404 vs 500: missing resource vs server failure

A missing resource returns 404 Not Found (client asked for something absent); a database failure returns 500 Internal Server Error (server-side fault).

Centralized error handling in an Express API

A final four-arg error middleware, an asyncHandler wrapper to funnel promise rejections via next, a custom error class with statusCode, returning uniform JSON.

API versioning: URL vs header strategies

Version via URL path (/v1/), a custom or Accept header, or a query param; URL is visible and cache-friendly, headers keep URLs clean but are less discoverable.

Define a Mongoose schema and model

New mongoose.Schema with field options (type, required, default), then mongoose.model('Product', schema) to get a model.

Mongoose populate() for referenced documents

Populate() replaces stored ObjectIds with the referenced documents, needs a ref in the schema, called via .populate('author').

Database migrations with the Sequelize CLI

Migrations are version-controlled scripts with up/down so teams apply identical schema changes; use sequelize-cli to generate, edit with addColumn, then db:migrate.