Question 1

Why do I need an async middleware wrapper?

Accepted Answer

Express 4 does not catch promise rejections from async middlewares. Wrap with Promise.resolve(fn(req, res, next)).catch(next) so rejections go to the error handler instead of being swallowed.

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.

Why do I need an async middleware wrapper?

More FAQs in How to Write Custom Middleware in Express

Still have questions?

Want to upskill yourself?

Small steps. Big possibilities.

Our courses are taking a Coffee break, but your curiosity shouldn't. Stay engaged with namastedev linkedin, youtube, discord and other resources while you wait.