List some common middleware tasks?

0-2 years experience

1. 1How would you add a middleware to log each incoming request's method and URL in an Express app?
2. 2If you place a body‑parsing middleware after a route handler, what will happen when a client sends JSON data?
3. 3What does calling next() do inside a middleware function?

2-5 years experience

1. 1You need to protect certain routes with JWT authentication. Walk me through how you'd set up the middleware and what trade‑offs you consider.
2. 2During a recent deployment, requests started returning 500 errors after you added a new error‑handling middleware. How would you debug the issue?
3. 3Explain why the order of static file serving middleware and custom logging middleware matters in a typical Express app.

5-8 years experience

1. 1Our API gateway uses Express and runs on many instances. How would you design middleware for rate limiting that scales without becoming a bottleneck?
2. 2We have both synchronous and asynchronous error handling middleware. How do you ensure errors propagate correctly across async code paths?
3. 3Discuss the performance impact of using heavy body‑parsing middleware on high‑traffic endpoints and how you might mitigate it.

8+ years experience

1. 1We are migrating a monolithic Express service to a micro‑services architecture. How would you refactor shared middleware (e.g., auth, logging) to avoid duplication across teams?
2. 2Across multiple services, we need consistent request tracing. What architectural approach would you take to implement distributed tracing middleware while keeping services loosely coupled?
3. 3If a legacy Express app uses many custom middleware functions that are hard to test, how would you restructure them to improve maintainability and enable automated testing at scale?