Express Middleware Explained: Key Patterns, Gotchas, and Best Practices

Express middleware consists of a sequential stack of functions that can access request and response objects, execute code, and pass control via next(). Developers can chain single-purpose middlewares — such as authentication and role checks — directly within route definitions instead of bundling all logic into one controller. A common pitfall is forgetting to call next() or send a response, which causes the client browser to hang indefinitely. Error-handling middleware must declare exactly four parameters to be recognized by Express, and in Express 4, async errors must be explicitly forwarded using next(err). Express 5 addresses this by natively catching rejected promises in async middleware.