What is Express middleware chaining and how does it work?

Middleware chaining allows you to run multiple middleware functions in sequence for a single route. You can chain them as multiple arguments: app.get('/admin', authenticate, authorize('admin'), handler). Or as an array: app.get('/admin', [authenticate, authorize('admin')], handler). Express calls them in order, and each must call next() to continue. If any calls next(err) or sends a response, the chain stops. This pattern is used to compose reusable middleware: authenticate verifies the JWT, authorize('admin') checks the role, and the final handler handles business logic. This keeps each middleware focused on a single concern and makes routes self-documenting about their access requirements.