การกำหนดเส้นทางและมิดเดิลแวร์ใน Express

Welcome back! In this lesson, we'll dive into routing and middleware in Express. These are core concepts for building powerful and organized web applications.

Routing helps your server understand what to do when it receives different requests, like showing a user profile or saving new data.

A route tells your Express app how to respond to a specific type of request to a particular URL path. You define routes using methods like app.get(), app.post(), app.put(), and app.delete().

Each route method takes a path and a handler function that executes when the route is matched.

Let's create a simple GET route that responds to requests at the root URL (/). This is often your homepage or a basic API status check.

The handler function receives req (request) and res (response) objects.

Often, you need to capture dynamic values from the URL, like a user's ID. This is done with route parameters, denoted by a colon (:) in the path.

You can access these parameters through the req.params object in your route handler.

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.

They can:

• Execute any code.
• Make changes to the request and the response objects.
• End the request-response cycle.
• Call the next middleware function in the stack.

The key to middleware is the next() function. If a middleware function does not end the request-response cycle (e.g., by calling res.send()), it must call next() to pass control to the next middleware function or route handler.

If next() is not called, the request will be left hanging.

You can apply middleware globally to all requests using app.use(). This is great for tasks like logging, parsing JSON bodies, or setting common headers.

Let's create a simple logger that runs for every incoming request.

Sometimes, you only want middleware to run for specific routes. You can apply route-level middleware by passing it as an argument before the route handler function.

You can even chain multiple middleware functions for a single route.

Middleware is incredibly versatile. Here are some common use cases:

• Authentication/Authorization: Checking user credentials or permissions.
• Logging: Recording request details for debugging or monitoring.
• Body Parsing: Extracting JSON or URL-encoded data from request bodies (e.g., express.json()).
• Error Handling: Catching and responding to errors (special middleware type).
• Input Validation: Ensuring request data meets specific criteria.

Let's test your understanding of Express routing and middleware!

Great job! You've learned how Express uses routing to direct requests to specific handlers based on URL paths and HTTP methods.

You also mastered middleware, powerful functions that can intercept and process requests at various stages, either globally with app.use() or for specific routes. Understanding these concepts is fundamental to building robust Express applications!