What is Middleware in Express and How It Works

Short summary

Express middleware are functions intercepting HTTP requests in a sequential pipeline with access to req, res, and next callbacks. Application-level middleware (app.use) applies globally or to path prefixes; router-level middleware scopes to specific routes. Middleware can inspect/modify requests, authenticate, or send early responses; order matters because sending a response halts the chain.

• •Middleware are functions in the request-response pipeline with access to req, res, and next
• •Application-level middleware uses app.use() for global or path-prefixed scope; router-level scopes to specific routes
• •Middleware order is critical—sending a response before next() halts the chain and prevents further middleware execution

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