Express Router Interview Questions and Answers

Express Router Interview Questions and Answers

Interviewers ask these Express Router questions often. Here are concise answers.

Q1: What is express.Router?

A mini-app you can mount with app.use('/path', router). It lets you group routes by feature in separate files. Routers have their own middleware stack and can be nested.

Q2: How do you mount a router?

app.use('/users', require('./routes/users')). The router's paths are then relative to /users. For example, router.get('/:id') responds to /users/:id.

Q3: How do you apply middleware to all routes in a router?

router.use(middleware) at the top of the router file. Every route in the router now runs that middleware. Useful for auth and role checks across a feature.

Q4: Can routers be nested?

Yes. Mount one router inside another: router.use('/:userId/posts', postsRouter). Now /users/:userId/posts/... hits postsRouter. Limit nesting to one level.

Q5: How do you version an Express API with routers?

Mount routers under /api/v1, /api/v2, etc. Each version has its own router. Bump on breaking changes. Non-breaking changes (adding fields) do not need a new version.

Q6: What is the difference between app and router?

app is the Express application. router is a mini-app you mount with app.use. Routers group routes by feature and keep app.js clean.

Q7: How do you handle public vs protected routes in a router?

Put public routes before router.use(auth). Then call router.use(auth) and put protected routes below. This keeps the structure clear and prevents accidental auth on public endpoints.

Q8: How do you handle async errors in router handlers?

Wrap with asyncHandler: Promise.resolve(fn(req, res, next)).catch(next). This catches promise rejections and passes them to the error handler. Express 5 catches async errors automatically.

Q9: What is the module pattern in Express?

Group each feature into a module: router, controller, service, model, validations in one folder (src/modules/users/). Mount modules in app.js. Gives cohesion, independence, testability, and team scalability.

Q10: What should app.js contain?

Create the app, apply global middlewares (json, cors, helmet), mount routers with app.use, and register the error handler. Everything else lives in feature folders. A thin app.js is a sign of a well-organized app.

The Takeaway

Express Router interview answers: Router is a mini-app mounted with app.use; apply middleware with router.use; nest routers for nested resources (limit to one level); version APIs with /api/v1; app vs router; public routes before router.use(auth); async errors with asyncHandler; module pattern for cohesion; thin app.js.