How do I test Express middleware?
Use Jest and supertest. Import the Express app, send requests with supertest, assert on the response. Test that auth rejects unauthenticated requests, that validation rejects bad input, and that happy paths work.
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More FAQs in Express Middleware Roadmap: Which to Learn First
Learn the (req, res, next) signature, use built-in middlewares, write a logger, write auth, write validation, learn error handling, learn async error handling, add security middlewares, add logging, add performance, apply per-router, compose per route, and test.
The (req, res, next) signature. Without it, nothing else makes sense. Then use built-in middlewares like express.json to see the pattern in action.
After you can write a custom middleware (auth or validation). Error handling needs the four-parameter signature and a custom ApiError class. It is a small step once you know the basic pattern.
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