Express.js 完整指南

Express.js 簡介

1.安裝與設定

Express.js 是一個最小且靈活的 Node.js Web 應用程式框架，為 Web 和行動應用程式提供了一組強大的功能。它簡化了伺服器端應用程式和 API 的建置、HTTP 請求的處理以及中間件的管理。

安裝與設定 Express.js 的步驟：

1. 安裝 Node.js：

   • Express.js 需要 Node.js。如果尚未安裝，請從官方網站下載並安裝 Node.js。

2. 建立一個新的專案目錄：
   

   mkdir my-express-app
   cd my-express-app

1. 初始化一個新的 Node.js 專案：

   npm init -y

此指令使用預設設定建立一個 package.json 檔案。

1. 安裝 Express.js：

   npm install express

此命令安裝 Express.js 並將其新增為 package.json 檔案中的依賴項。

1. 安裝nodemon：
   • nodemon 是一個在偵測到檔案變更時自動重新啟動應用程式的工具。

   npm install --save-dev nodemon

1. 將nodemon加入package.json腳本： 打開 package.json 並修改腳本部分以包括：

   "scripts": {
     "start": "nodemon app.js"
   }

1. 設定 JavaScript 導入語法：

   • Node.js 預設不支援直接在檔案中使用 ES6 模組語法（導入/匯出）。您需要透過在 package.json 中新增 "type": "module" 來啟用此功能。
   • 修改 package.json 以內含：

    "type": "module"
   

2. 建立入口點檔案：
   在專案目錄中建立一個名為 app.js（或 index.js）的檔案。此檔案將包含您的 Express 應用程式程式碼。

2.建立基本的 Express 應用程式

要建立基本的 Express 應用程序，請按照以下步驟操作：

1. 在app.js中初始化Express：

   import express from 'express';
   const app = express();
   const port = 3000;

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

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

• 從「快遞」進口快遞；使用 ES6 匯入語法匯入 Express 模組。
• express() 初始化 Express 應用程式。
• app.get() 定義了一個針對根 URL (/) 的 GET 請求的路由處理程序。
• app.listen() 啟動伺服器並監聽指定連接埠。

1. 運行應用程式：

   • 使用 npm start 透過 nodemon 運行應用程式：

    npm start
   

• 開啟網頁瀏覽器並導航至 http://localhost:3000。您應該看到“你好，世界！”顯示。

透過這些步驟，您已經使用 ES6 導入語法設定了一個基本的 Express.js 應用程序，並將 Nodemon 配置為在開發過程中自動重新啟動伺服器。此設定有助於簡化開發並有效處理程式碼變更。

Express.js 中的路由

Express.js 中的路由是定義應用程式如何回應特定端點的客戶端請求的過程。可以為不同的 HTTP 方法（GET、POST、PUT、DELETE 等）定義路由，並且可以組織處理複雜的 URL 結構。

基本 GET 路由

GET 路由用於從伺服器檢索資料。它通常用於根據查詢提供靜態內容或資料。

範例：

// Basic GET route
app.get('/home', (req, res) => {
  res.send('Welcome to the Home Page!');
});

基本郵寄路線

POST 路由用於將資料傳送到伺服器。它通常用於提交表單或建立新資源。

範例：

// Basic POST route
app.post('/submit', (req, res) => {
  res.send('Form submitted!');
});

基本 PUT 路線

PUT 路由用於更新伺服器上的現有資料。它通常用於修改資源或更新記錄。

範例：

// Basic PUT route
app.put('/update', (req, res) => {
  res.send('Data updated!');
});

基本刪除路線

DELETE 路由用於從伺服器中刪除資料。用於刪除資源或記錄。

範例：

// Basic DELETE route
app.delete('/delete', (req, res) => {
  res.send('Data deleted!');
});

Express.js 中的 app.use

Express.js 中的 app.use 方法用於註冊處理請求的中間件函數。中間件函數依照定義的順序執行，app.use 可用於全域應用中介軟體或特定路徑。

1.全域應用中介軟體

當在沒有路徑的情況下使用 app.use 時，中介軟體功能將套用於所有傳入請求。這對於設定全域功能（例如日誌記錄、解析請求正文或處理會話）非常有用。

範例：

// Middleware function applied globally
app.use((req, res, next) => {
  console.log(`Request URL: ${req.url}`);
  next(); // Pass control to the next handler
});

app.get('/', (req, res) => {
  res.send('Home Page');
});

• Global Middleware: The middleware function logs the request URL for every request received by the server.

2. Applying Middleware to Specific Paths

You can use app.use to apply middleware only to requests that match a specific path. This allows you to target middleware to certain routes.

Example:

// Middleware function applied to /admin paths
app.use('/admin', (req, res, next) => {
  console.log('Admin route accessed');
  next(); // Pass control to the next handler
});

app.get('/admin/dashboard', (req, res) => {
  res.send('Admin Dashboard');
});

app.get('/user/profile', (req, res) => {
  res.send('User Profile');
});

• Path-Specific Middleware: The middleware logs a message only for requests to paths starting with /admin.

3. Using app.use with Multiple Middleware Functions

You can chain multiple middleware functions together with app.use, allowing for sequential processing of requests.

Example:

// First middleware function
const firstMiddleware = (req, res, next) => {
  console.log('First Middleware');
  next(); // Proceed to the next middleware
};

// Second middleware function
const secondMiddleware = (req, res, next) => {
  console.log('Second Middleware');
  next(); // Proceed to the next handler
};

// Apply multiple middleware functions
app.use(firstMiddleware, secondMiddleware);

app.get('/', (req, res) => {
  res.send('Home Page');
});

• Chaining Middleware: firstMiddleware and secondMiddleware are executed sequentially for all requests.

The app.use method in Express.js provides flexibility for applying middleware functions globally or to specific routes, and for processing requests in a modular fashion.

Callback Functions in Express.js

In Express.js, callback functions are crucial for handling HTTP requests. They are used in middleware and route handlers to process requests and manage responses.

1. Parameters in Callback Functions

Callback functions in Express.js receive three parameters:

• req (Request): The request object containing details about the incoming request.
• res (Response): The response object used to send a response to the client.
• next (Next): A function to pass control to the next middleware or route handler.

Example:

function callback(req, res, next) {
  // Your code here
  next(); // Pass control to the next middleware or route handler
}

2. Middleware Callbacks

Middleware functions process requests before they reach route handlers. They utilize the req, res, and next parameters.

Example:

const logMiddleware = (req, res, next) => {
  console.log(`Request URL: ${req.url}`);
  next(); // Pass control to the next handler
};

app.use(logMiddleware); // Apply middleware globally

app.get('/', (req, res) => {
  res.send('Home Page');
});

3. Route Handler Callbacks

Route handlers define responses for specific routes, using callback parameters to manage requests and responses.

Example:

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

4. Chaining Middleware Functions

Multiple middleware functions can be chained together to handle requests sequentially.

Example:

const authenticate = (req, res, next) => {
  console.log('Authentication middleware');
  next(); // Proceed to the next middleware
};

const authorize = (req, res, next) => {
  console.log('Authorization middleware');
  next(); // Proceed to the route handler
};

app.get('/profile', authenticate, authorize, (req, res) => {
  res.send('User Profile');
});

Route Parameters in Express.js

Route parameters are dynamic segments of a URL used to capture values from the URL path. They allow you to define routes that can handle variable input, making your routes more flexible.

1. Basic Route Parameters

Route parameters are defined in the route path by using a colon : followed by the parameter name. You can access these parameters in your route handler through the req.params object.

Example:

// Route with a route parameter
app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  res.send(`User ID: ${userId}`);
});

• Route Parameter: The :id in the route path is a parameter that captures the value from the URL, accessible via req.params.id.

2. Multiple Route Parameters

You can define multiple route parameters in a single route path, allowing for more complex URL structures.

Example:

// Route with multiple route parameters
app.get('/post/:year/:month/:day', (req, res) => {
  const { year, month, day } = req.params;
  res.send(`Post date: ${year}-${month}-${day}`);
});

• Multiple Parameters: The :year, :month, and :day parameters capture parts of the date from the URL, accessible via req.params.year, req.params.month, and req.params.day.

3. Optional Route Parameters

Route parameters can also be optional. Use a question mark ? to indicate optional segments in the route path.

Example:

// Route with an optional route parameter
app.get('/product/:id?', (req, res) => {
  const productId = req.params.id || 'not specified';
  res.send(`Product ID: ${productId}`);
});

• Optional Parameter: The :id? parameter is optional, meaning the route can be accessed with or without this parameter.

Route parameters provide a way to build dynamic and flexible routes in Express.js, allowing you to handle various input values and create more sophisticated URL patterns.

req Object

In Express.js, the req object represents the incoming HTTP request from the client. It includes details about the request such as URL, headers, and body. Properly understanding the req object is crucial for handling requests effectively.

req.body

The req.body property contains data sent in the request body, typically used in POST and PUT requests. To access req.body, you need to use middleware for parsing the request data.

Handling JSON Data:

app.use(express.json()); // Middleware to parse JSON bodies

app.post('/submit', (req, res) => {
  const { name, age } = req.body;
  res.send(`Received data - Name: ${name}, Age: ${age}`);
});

• Explanation:
  • express.json(): Middleware to parse JSON data in request bodies.
  • req.body: Contains parsed JSON data.

Handling URL-encoded Data:

app.use(express.urlencoded({ extended: true })); // Middleware to parse URL-encoded bodies

app.post('/submit', (req, res) => {
  const { name, age } = req.body;
  res.send(`Received data - Name: ${name}, Age: ${age}`);
});

• Explanation:
  • express.urlencoded({ extended: true }): Middleware to parse URL-encoded data from forms.
  • req.body: Contains parsed URL-encoded data.

req.cookies

The req.cookies property contains cookies sent by the client. To use req.cookies, you need the cookie-parser middleware to parse cookies in requests.

Example:

import cookieParser from 'cookie-parser';
app.use(cookieParser()); // Middleware to parse cookies

app.get('/check-cookies', (req, res) => {
  const user = req.cookies.user; // Access a cookie named 'user'
  res.send(`Cookie value - User: ${user}`);
});

• Explanation:
  • cookieParser(): Middleware to parse cookies from request headers.
  • req.cookies: Contains cookies sent by the client.

req.method

The req.method property contains the HTTP method of the incoming request. This can be useful for handling different types of requests, such as GET, POST, PUT, DELETE, etc.

Example:

app.use((req, res, next) => {
  console.log(`Request Method: ${req.method}`); // Logs the HTTP method of the request
  next(); // Pass control to the next handler
});

app.get('/example', (req, res) => {
  res.send(`This is a GET request`);
});

app.post('/example', (req, res) => {
  res.send(`This is a POST request`);
});

• Explanation:
  • req.method: Contains the HTTP method used for the request (e.g., GET, POST).

req.params

The req.params property contains route parameters specified in the URL path. Route parameters are used to capture values from the URL and are typically defined in routes with a colon syntax (e.g., /users/:id).

Example:

app.get('/users/:id', (req, res) => {
  const userId = req.params.id; // Access the route parameter 'id'
  res.send(`User ID: ${userId}`);
});

• Explanation:
  • req.params: Contains key-value pairs of route parameters, where the key is the parameter name defined in the route, and the value is the actual value from the URL.

req.query

The req.query property contains query string parameters from the URL. These are typically used to pass data in the URL for GET requests.

Example:

app.get('/search', (req, res) => {
  const query = req.query.q; // Access the query parameter 'q'
  res.send(`Search query: ${query}`);
});

• Explanation:
  • req.query: Contains key-value pairs of query string parameters. For example, for a URL like /search?q=example, req.query.q would be 'example'.

req.get()

The req.get() method is used to retrieve HTTP headers from the incoming request. It allows you to access specific headers by name. This is useful for extracting metadata about the request or for handling custom headers.

Example:

app.get('/headers', (req, res) => {
  const userAgent = req.get('User-Agent'); // Access the 'User-Agent' header
  const host = req.get('Host'); // Access the 'Host' header
  const acceptLanguage = req.get('Accept-Language'); // Access the 'Accept-Language' header
  const contentType = req.get('Content-Type'); // Access the 'Content-Type' header

  res.send(`
    User-Agent: ${userAgent}<br>
    Host: ${host}<br>
    Accept-Language: ${acceptLanguage}<br>
    Content-Type: ${contentType}
  `);
});

• Explanation:
  • req.get('User-Agent'): Retrieves the User-Agent header, which provides information about the client's browser or application.
  • req.get('Host'): Retrieves the Host header, which indicates the domain name of the server and port number.
  • req.get('Accept-Language'): Retrieves the Accept-Language header, which indicates the preferred language(s) for the response.
  • req.get('Content-Type'): Retrieves the Content-Type header, which specifies the media type of the request body.

res Object

In Express.js, the res object represents the HTTP response that is sent back to the client. It is used to set response headers, status codes, and to send data or files back to the client. Understanding the res object is essential for controlling the response sent from the server.

res.append()

The res.append() method is used to add additional headers to the response. It is useful when you need to modify or add headers dynamically before sending the response.

Example:

app.get('/set-headers', (req, res) => {
  res.append('Custom-Header', 'HeaderValue'); // Add a custom header
  res.append('Another-Header', 'AnotherValue'); // Add another header
  res.send('Headers have been set!');
});

• Explanation:
  • res.append(name, value): Adds a header with the specified name and value to the response. If the header already exists, the new value is appended to the existing values.

res.cookie()

The res.cookie() method is used to set cookies on the client's browser. It allows you to send cookies with specific options such as expiration, path, and secure flags.

Example:

app.get('/set-cookie', (req, res) => {
  // Set a cookie named 'username' with a value 'JohnDoe'
  res.cookie('username', 'JohnDoe', {
    maxAge: 24 * 60 * 60 * 1000, // Cookie expires after 1 day
    httpOnly: true,              // Cookie is not accessible via JavaScript
    secure: false,               // Cookie is sent over HTTP (not HTTPS)
    path: '/'                    // Cookie is valid for the entire domain
  });
  res.send('Cookie has been set');
});

• Explanation:
  • res.cookie(name, value, [options]): Sets a cookie with the specified name and value. The options parameter can include:
  • maxAge: Expiration time of the cookie in milliseconds.
  • httpOnly: If true, the cookie is not accessible via JavaScript (client-side).
  • secure: If true, the cookie is sent only over HTTPS connections.
  • path: The path for which the cookie is valid.

res.end()

The res.end() method is used to end the response process and send the response to the client. It is often used to send the final output or to close the response stream when no additional data needs to be sent.

Example:

app.get('/finish', (req, res) => {
  res.end('Response has been sent and the connection is closed.');
});

• Explanation:
  • res.end([data], [encoding]): Ends the response process. If data is provided, it is sent as the response body. The encoding parameter specifies the character encoding for the data. If no data is provided, an empty response is sent.

res.json()

The res.json() method is used to send a JSON response to the client. It automatically sets the Content-Type header to application/json and converts the provided data into a JSON string.

Example:

app.get('/data', (req, res) => {
  const data = {
    name: 'John Doe',
    age: 30,
    city: 'New York'
  };
  res.json(data);
});

• Explanation:
  • res.json([body]): Sends a JSON response. The body parameter is an object or array that will be converted to a JSON string and sent as the response body. The Content-Type header is set to application/json automatically.

res.location()

The res.location() method sets the Location header of the response. It is commonly used to specify the URL to which a client should be redirected. However, this method does not send a response to the client by itself; it only sets the header.

Example:

app.get('/set-location', (req, res) => {
  res.location('/new-url');
  res.send('Location header has been set');
});

• Explanation:
  • res.location(url): Sets the Location header to the specified URL. This is often used in conjunction with res.redirect to indicate where the client should be redirected.

res.redirect()

The res.redirect() method sends a redirect response to the client. It sets the Location header and sends a status code (default is 302) to redirect the client to a different URL.

Example:

app.get('/redirect', (req, res) => {
  res.redirect('/new-url');
});

• Explanation:
  • res.redirect([status,] url): Redirects the client to the specified URL. The optional status parameter allows you to set a custom HTTP status code (e.g., 301 for permanent redirect, 302 for temporary redirect). If no status is provided, 302 is used by default.

res.send()

The res.send() method is used to send a response to the client. It can send a variety of response types, including strings, buffers, objects, or arrays. The method automatically sets the Content-Type header based on the type of the response.

Example:

app.get('/text', (req, res) => {
  res.send('This is a plain text response.');
});

app.get('/json', (req, res) => {
  const data = { message: 'This is a JSON response.' };
  res.send(data);
});

app.get('/buffer', (req, res) => {
  const buffer = Buffer.from('This is a buffer response.');
  res.send(buffer);
});

• Explanation:
  • res.send([body]): Sends the response to the client. The body parameter can be a string, buffer, object, or array. If an object or array is passed, it will be automatically converted to JSON. The Content-Type header is set based on the type of the body parameter.

res.sendFile()

The res.sendFile() method is used to send a file as the response to the client. It sets the appropriate Content-Type header based on the file type and streams the file to the client.

Example:

import path from 'path';

app.get('/file', (req, res) => {
  const filePath = path.join(__dirname, 'public', 'example.txt');
  res.sendFile(filePath);
});

• Explanation:
  • res.sendFile(path[, options], [callback]): Sends a file as the response. The path parameter is the absolute path to the file you want to send. The optional options parameter can be used to set additional options such as the Content-Type header or to handle errors. The optional callback parameter is a function that is called when the file has been sent.

res.sendStatus()

The res.sendStatus() method sets the HTTP status code and sends the corresponding status message as the response body. It is a shorthand for setting the status code and sending a response in one step.

Example:

app.get('/status', (req, res) => {
  res.sendStatus(404); // Sends a 404 Not Found status with the message 'Not Found'
});

• Explanation:
  • res.sendStatus(statusCode): Sets the HTTP status code and sends a response with the status message corresponding to the code. For example, 404 will send 'Not Found' as the response body.

res.set()

The res.set() method sets HTTP headers for the response. It can be used to specify various headers, including custom headers.

Example:

app.get('/headers', (req, res) => {
  res.set('X-Custom-Header', 'Value');
  res.set({
    'Content-Type': 'application/json',
    'X-Another-Header': 'AnotherValue'
  });
  res.send('Headers set');
});

• Explanation:
  • res.set(name, value): Sets a single HTTP header. name is the header name, and value is the header value.
  • res.set(headers): Sets multiple headers at once by passing an object where keys are header names and values are header values.

res.status()

The res.status() method sets the HTTP status code for the response. This method is used to define the status code before sending the response.

Example:

app.get('/error', (req, res) => {
  res.status(500).send('Internal Server Error'); // Sets status code to 500 and sends the message
});

• Explanation:
  • res.status(statusCode): Sets the HTTP status code for the response. The status code can then be followed by other methods (like res.send, res.json, etc.) to send the response body.

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