Explain the concept of middleware chaining in Express.js and how it can be used to implement request-level authentication and logging.

Middleware chaining in Express.js refers to the sequential execution of middleware functions in the order they are defined. Each middleware function has access to the request object (`req`), the response object (`res`), and the `next()` function. The `next()` function is crucial; it's what allows the request to proceed to the next middleware in the chain. A middleware function can perform actions on the request or response, modify them, terminate the request-response cycle, or pass control to the next middleware by calling `next()`. Middleware chaining is a powerful mechanism for implementing cross-cutting concerns like authentication and logging. For request-level authentication, a middleware function can inspect the request headers (e.g., for an authorization token) or session data to verify the user's identity. If the user is authenticated, the middleware can call `next()` to allow the request to proceed to the route handler. If the user is not authenticated, the middleware can terminate the request-response cycle by sending an error response (e.g., a 401 Unauthorized status). For request-level logging, a middleware function can log information about the request, such as the HTTP method, URL, request headers, and the timestamp. This information can be useful for debugging, auditing, and monitoring application usage. The logging middleware can call `next()` to allow the request to continue to the next middleware or route handler. Example: `app.use(loggerMiddleware); app.use(authenticationMiddleware); app.get('/protected', routeHandler);` In this example, `loggerMiddleware` will be executed first, then `authenticationMiddleware`, and finally, if the user is authenticated, `routeHandler` will be executed. The order in which middleware is defined is critical, as it determines the order in which they are executed.