Question 1

What is a common mistake when writing Express middleware?

Accepted Answer

Forgetting to call next() or send a response. The request then hangs. Every middleware must call next() or send a response. Also, putting business logic in middleware makes it hard to test and reuse.

Question 2

How do I write custom middleware in Express?

Accepted Answer

Use the (req, res, next) signature. Do something with req or res, then call next() to continue or send a response to end the cycle. Keep middlewares small and one-job.

Question 3

When should I write custom middleware vs put logic in a handler?

Accepted Answer

Write middleware for cross-cutting concerns (auth, validation, logging, request IDs, permissions). Put business logic in handlers or services. Middleware shapes the request; logic acts on it.

Question 4

How do I make a configurable middleware?

Accepted Answer

Use a factory function. The factory takes options and returns a middleware. Example: validate(schema) returns a middleware that validates against that schema. requireRole(role) returns a middleware that checks that role.

What is a common mistake when writing Express middleware?

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