Middleware

Middleware is a core concept in the Express.js framework. Middleware functions are functions that have access to the request object (`req`), the response object (`res`), and the `next` function in the application's request-response cycle. They act as a series of processing steps that a request goes through before it reaches its final route handler. Each middleware function can perform a specific task, making your application's logic more modular and reusable. When a request comes into your Express server, it passes through the middleware functions in the order they are defined. A middleware function can do one of the following: execute any code, make changes to the request and response objects (e.g., adding a property to `req`), end the request-response cycle by sending a response, or call the next middleware in the stack by invoking `next()`. If a middleware function does not call `next()` or send a response, the request will be left hanging. Common use cases for middleware include: logging every incoming request, parsing the body of incoming requests (e.g., JSON or URL-encoded data), checking if a user is authenticated before allowing them to access a protected route, adding CORS headers to the response, or handling errors in a centralized way. Express comes with some built-in middleware, and there is a vast ecosystem of third-party middleware packages available on npm.