ExpressJS Core Concepts of ExpressJS Syllabus

How do you access multiple route parameters, e.g. /users/:userId/books/:bookId?

Through the req.params object: req.params.userId and req.params.bookId. Each named segment becomes a property on req.params.

What is the difference between route parameters and query parameters in Express?

Route parameters are part of the URL path (/users/:id → req.params) and are typically required; query parameters follow a ? in the URL (/users?sort=asc → req.query) and are typically optional filters.

What is a route handler in Express?

A callback function (with signature (req, res) or (req, res, next)) that executes when a route matches, responsible for processing the request and sending a response.

Can a single route have multiple handlers, and how do they chain?

Yes. You can pass multiple handler functions (or an array) to a route, e.g. app.get('/x', h1, h2, h3). Each handler must call next() to pass control to the next handler in the chain; otherwise the chain stops.

What is the Express Router?

An isolated, mini application instance created with express.Router() that groups related routes and middleware into a modular, mountable unit, which is attached to the main app via app.use().

How do you create and mount an Express Router?

const router = express.Router(); define routes like router.get('/', handler); then mount with app.use('/base', router). Requests to /base + route path are handled by that router.

What are the benefits of using express.Router()?

Modular organization of routes, ability to apply router-level middleware to a group, cleaner separation into files, and reusable path prefixes via mounting.

What is the req object in Express?

The HTTP request object representing the incoming request; it exposes properties and methods for reading the request such as req.params, req.query, req.body, req.headers, req.method, req.path, and req.cookies.

What do req.params, req.query, and req.body each contain?

req.params — route/path parameters (:id); req.query — parsed URL query string parameters (?key=value); req.body — the parsed request payload (requires a body-parsing middleware like express.json()).

How do you read a request header in Express?

Using req.get('Header-Name') (case-insensitive) or the req.headers object, e.g. req.get('Content-Type') or req.headers['content-type'].

What is the res object in Express?

The HTTP response object used to build and send the reply to the client; it exposes methods like res.send(), res.json(), res.status(), res.redirect(), res.sendFile(), and res.set()/res.append().

What is the difference between res.send() and res.json()?

res.send() sends a response of various types (string, Buffer, object, array) and infers the Content-Type; res.json() explicitly serializes the argument to JSON and sets Content-Type to application/json. res.json() also correctly handles values like null.

How do you set the HTTP status code of a response in Express?

With res.status(code), which is chainable, e.g. res.status(404).send('Not Found') or res.status(201).json(data).

What does res.redirect() do?

It sends a redirect response to the client, setting the Location header and a 3xx status (302 by default). Example: res.redirect('/login') or res.redirect(301, '/new-url').

How does Express handle different content types when receiving requests?

By using the appropriate body-parsing middleware matched to the request's Content-Type: express.json() for application/json, express.urlencoded() for form data, express.text() for text/plain, and express.raw() for binary/Buffer bodies.

How do you explicitly set the response Content-Type in Express?

Using res.type('...') or res.set('Content-Type', '...'), e.g. res.type('html') or res.set('Content-Type', 'application/xml').

What is content negotiation and how does res.format() support it?

Content negotiation serves different representations based on the client's Accept header. res.format() lets you define handlers per content type, e.g. res.format({ 'text/html': ()=>..., 'application/json': ()=>... }), and Express picks the best match.

List the main methods used to send responses in Express.

res.send() (generic), res.json() (JSON), res.sendFile() (a file), res.render() (a view/template), res.redirect() (redirect), res.end() (end without data), and res.download() (prompt file download).

Why can you only send one response per request in Express?

Because a response method (res.send/json/end) ends the response and writes headers/body; calling another afterward triggers the 'Cannot set headers after they are sent to the client' error since the HTTP response is already committed.

What does res.sendStatus() do?

It sets the response HTTP status code and sends its standard message text as the body in one call, e.g. res.sendStatus(200) sends 'OK', res.sendStatus(404) sends 'Not Found'.

What is Express's default (built-in) error handling behavior?

Express ships with a built-in default error handler. Errors thrown in synchronous code or passed to next(err) are caught and by default sent to the client with a stack trace in development, and a generic message plus 500 status in production.

How do you forward an error to Express's error-handling middleware?

By passing the error to next(), i.e. next(err). Any truthy value passed to next() (other than the string 'route') tells Express to skip remaining normal middleware and invoke error-handling middleware.

How does Express handle errors thrown in asynchronous code (e.g., in a Promise)?

In Express 4, async errors are NOT caught automatically — you must catch them and call next(err) yourself (or use a wrapper). In Express 5, rejected promises returned from handlers are automatically forwarded to error middleware.

What distinguishes an error-handling middleware function in Express?

It has exactly four arguments: (err, req, res, next). Express identifies error handlers by this arity and only calls them when an error is passed to next().

Write a basic custom error-handling middleware.

app.use((err, req, res, next) => { console.error(err.stack); res.status(err.status || 500).json({ error: err.message }); }); It must be defined after all other app.use() and routes.

Where must error-handling middleware be placed in the middleware stack?

Last — after all other app.use() calls and route definitions — so that errors from earlier middleware/routes propagate down to it.

How do you handle 404 (route not found) errors in Express?

Add a catch-all middleware after all routes, e.g. app.use((req, res) => res.status(404).send('Not Found')); it runs when no earlier route matched the request.

How can you set a custom status code on an error passed to next()?

Attach a property to the error object, e.g. const err = new Error('Forbidden'); err.status = 403; next(err); then read err.status in the error handler to call res.status(err.status).

What is the DEBUG environment variable used for in Express?

It enables Express's internal debug logging via the debug module. Setting DEBUG=express:* (e.g. DEBUG=express:* node app.js) prints detailed logs of routing and middleware execution to help diagnose issues.

What are effective debugging tips for an Express application?

Use DEBUG=express:* for internal logs; add request-logging middleware like morgan; use console.log or the Node inspector (node --inspect) with breakpoints; check middleware order; and always include an error-handling middleware to surface stack traces.

Why is a hanging (never-responding) request often a middleware bug, and how do you debug it?

It usually means a middleware neither sent a response nor called next(), so the chain stalls. Debug by logging entry/exit of each middleware to find where control stops, and ensure every non-terminal middleware calls next().