Node.js Backend Development Bootcamp · 课时

开发自定义 Express 中间件

编写您自己的 Express 中间件函数，以扩展功能并简化请求处理。

开发自定义 Express 中间件 是 CoddyKit 上的免费 Node.js Backend Development Bootcamp 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Node.js Backend Development Bootcamp 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Node.js Backend Development Bootcamp 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What is Custom Middleware?

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.

The Middleware Signature

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.

Your First Logger Middleware

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!

const express = require('express');
const app = express();
const port = 3000;

// Custom logging middleware
const requestLogger = (req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next(); // Pass control to the next middleware/route handler
};

// Apply the custom middleware globally
app.use(requestLogger);

// Define a simple route
app.get('/', (req, res) => {
  res.send('Hello from Express!');
});

// Start the server
app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}`);
});

Applying Middleware: Global vs. Route

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.

Passing Data Downstream

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.

Example: Attaching User Data

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.

const express = require('express');
const app = express();
const port = 3000;

// Middleware to attach user info (mock data)
const attachUserInfo = (req, res, next) => {
  req.user = { id: '123', name: 'Alice', role: 'admin' };
  console.log('User info attached to request.');
  next();
};

// Apply the middleware globally
app.use(attachUserInfo);

// Route that uses the attached user info
app.get('/profile', (req, res) => {
  res.send(`Welcome, ${req.user.name}! Your role is: ${req.user.role}`);
});

app.get('/', (req, res) => {
  res.send('Home page (no user info explicitly needed here)');
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}`);
});

Building Middleware Chains

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.

Conditional Middleware Execution

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.

Practical: Request Timer Middleware

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.

const express = require('express');
const app = express();
const port = 3000;

// Middleware to measure request duration
const requestTimer = (req, res, next) => {
  const start = Date.now(); // Record start time
  res.on('finish', () => { // Listen for the response 'finish' event
    const end = Date.now(); // Record end time
    const duration = end - start;
    console.log(`${req.method} ${req.originalUrl} took ${duration}ms`);
  });
  next(); // Pass control
};

// Apply the timer middleware
app.use(requestTimer);

// Define a route that simulates some work
app.get('/slow', (req, res) => {
  setTimeout(() => {
    res.send('This was a slow response!');
  }, 500); // Simulate 500ms delay
});

app.get('/', (req, res) => {
  res.send('Hello, fast world!');
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}`);
});

Middleware Function Quiz

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

Which of the following statements about its behavior is TRUE?

Custom Middleware Recap

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.

常见问题解答

「开发自定义 Express 中间件」课时是免费的吗？

是的 — 「开发自定义 Express 中间件」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Node.js Backend Development Bootcamp 课程的其余内容，请升级到 CoddyKit PRO。 Node.js Backend Development Bootcamp 课程共包含 4 节课。

「开发自定义 Express 中间件」这节课中我会学到什么？

编写您自己的 Express 中间件函数，以扩展功能并简化请求处理。你通过在浏览器中直接运行的动手代码来练习 Node.js Backend Development Bootcamp，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Node.js Backend Development Bootcamp 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Node.js Backend Development Bootcamp 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「开发自定义 Express 中间件」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Node.js Backend Development Bootcamp 课中编写并运行代码吗？

能。每节 Node.js Backend Development Bootcamp 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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