Question 1

What is Express middleware and how does the chain work?

Accepted Answer

Functions (req, res, next) that run in order; next() continues, next(err) jumps to error handlers. Order matters: parsers before routes, auth before protected routes, error handlers last. Forgetting next() hangs requests.

Question 2

How do you structure a large Express app?

Accepted Answer

Routers per domain, thin controllers, services for logic, centralized error middleware, and config modules. Avoid god app.js. Mount routers at paths. Keep middleware declarative. Consider TypeScript + validation layer.

Question 3

app.use(express.json()) pitfalls?

Accepted Answer

Default limits may be too high/low; applies to all routes if mounted globally; prototype pollution historically mattered. Scope parsers. Set explicit limit. Validate with schemas after parse. Don't parse bodies you don't need.

Question 4

Error-handling middleware signature?

Accepted Answer

Four args: (err, req, res, next) — Express detects by arity. Must be registered after routes. Map domain errors to HTTP statuses. Avoid leaking stack traces in production.

Question 5

Express vs Fastify vs bare http — trade-offs?

Accepted Answer

Express: ubiquity/middleware ecosystem; Fastify: schema/perf; bare: control/minimalism. Choose based on team, plugins, and validation needs. Perf differences matter at high RPS with profiling evidence.

Question 6

Scenario: memory leak in Express after adding session middleware.

Accepted Answer

In-memory session store grows unbounded in multi-instance or long-lived processes. Use Redis/external store. Set TTLs. Sticky sessions alone don't fix memory. Profile heap.

Express Concepts Node.js Interview Questions

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