การพัฒนามิดเดิลแวร์ Express แบบกำหนดเอง

In Express.js, middleware functions are like a chain of helpers that process incoming requests before they reach your final route handler.

• They have access to the request (req) and response (res) objects.
• They can modify these objects, end the request-response cycle, or pass control to the next middleware.
• Custom middleware allows you to add specific functionality tailored to your application's needs, such as logging, authentication, or data parsing.

Every Express middleware function follows a specific signature: (req, res, next).

• req (request): The incoming HTTP request object.
• res (response): The HTTP response object that Express sends back.
• next: A function that, when called, passes control to the next middleware function in the stack. If you don't call next(), the request-response cycle stops, and no further middleware or route handlers will execute.

Let's create a simple custom middleware that logs details of every incoming request to the console. This helps in debugging and monitoring.

Try running the code and then accessing http://localhost:3000 in your browser. Check the console output!

You can apply middleware at different levels:

• Globally (Application-level): Using app.use(middlewareFunction) applies the middleware to ALL incoming requests.
• Route-specific: You can apply middleware to specific routes or groups of routes by passing it as an argument before the route handler.

This flexibility allows you to execute logic only where it's needed.

A powerful feature of middleware is its ability to add properties to the req object. These properties then become accessible to all subsequent middleware functions and the final route handler in the request-response cycle.

This is extremely useful for things like attaching user information after authentication, or parsed data before reaching the main logic.

Here's how you might attach a mock user object to the request. In a real application, this data would come from a database or a decoded JSON Web Token (JWT).

Notice how the /profile route handler can directly access req.user because the attachUserInfo middleware ran first.

You can chain multiple middleware functions together. Express executes them in the order they are defined. Each middleware must call next() to pass control to the next one.

• This allows you to break down complex request processing into smaller, manageable, and reusable functions.
• For example, you could have a middleware for authentication, then one for logging, then one for parsing, all before your final route handler.

Sometimes you only want middleware to run under certain conditions. You can achieve this by:

• Route-specific application: As seen, applying middleware directly to app.get(), app.post(), etc.
• Internal logic: Adding conditional checks (if statements) inside the middleware itself.
• Custom wrapper functions: Creating a function that returns a middleware based on parameters.

Let's build a middleware that measures how long it takes for a request to be processed and a response to be sent. This is very useful for performance monitoring.

This example uses res.on('finish', ...) to execute code AFTER the response has been sent to the client.

Consider a custom middleware function defined as const myMiddleware = (req, res, next) => { /* ... */ };

Which of the following statements about its behavior is TRUE?

You've learned how to create and use custom Express middleware!

• Custom middleware functions have the signature (req, res, next).
• Calling next() is crucial to pass control to the next middleware or route handler.
• Middleware can modify the req and res objects, making data available downstream.
• You can apply middleware globally with app.use() or to specific routes.
• They are powerful for adding reusable logic like logging, authentication, or request timing.

Mastering custom middleware is key to building modular and efficient Express applications.