NodeJS

Node.js web site: https://nodejs.org/
Node.js is a free, open-source JavaScript runtime that runs on Windows, Mac, Linux, and more.
Built on Chrome's V8 JavaScript engine. it lets you execute JavaScript code outside of a web browser, enabling server-side.
Node.js uses an Non-blocking I/O, event-driven, single-threaded with event loop model.

🎉you can build with Node.js（Node.js的应用场景）

Web Servers: Create fast, scalable network applications
File Operations: Read, write, and manage files on the server
Database Interaction: Work with databases like MongoDB, MySQL, and more
APIs: Build RESTful services and GraphQL APIs
Real-time: Handle WebSockets for live applications
CLI Tools: Create command-line applications
IoT: Hardware control

🎉How the Event Loop Works（Node.js的事件循环是如何工作的）

Execute the main script (synchronous code)
Process any microtasks (Promises, process.nextTick)
Execute timers (setTimeout, setInterval)
Run I/O callbacks (file system, network operations)
Process setImmediate callbacks
Handle close events (like socket.on('close'))

console.log('1. Start');
process.nextTick(() => console.log('2. Next tick'));
Promise.resolve().then(() => console.log('3. Promise'));
setTimeout(() => console.log('4. Timeout'), 0);
setImmediate(() => console.log('5. Immediate'));
console.log('6. End');

The output will be：1.Start > 6.End > 2.Next tick > 3.Promise > 4.Timeout > 5.Immediate

🚀Run Node.js Code

node app.js

🚀Debugging Node.js Applications

Start your application with node --inspect app.js
Open Chrome and navigate to chrome://inspect
Click on "Open dedicated DevTools for Node"
Set breakpoints and debug your application

# Start with inspector (listens on default port 9229)
node --inspect app.js

# Break on first line of application
node --inspect-brk app.js

# Specify a custom port
node --inspect=9222 app.js

# Enable remote debugging (be careful with this in production)
node --inspect=0.0.0.0:9229 app.js

🚀环境变量

# Install dotenv package
npm install dotenv

// Load environment variables from .env file
require('dotenv').config();

// Get a specific environment variable
const apiKey = process.env.API_KEY;
const port = process.env.PORT || 3000;
const isProduction = process.env.NODE_ENV === 'production';

// List all environment variables
console.log('Environment variables:', process.env);

🚀JavaScript Concepts

Variables: let (mutable), const (immutable), var (legacy)
Functions: Regular, arrow functions, and methods
Objects & Arrays: Data structures for organizing data
Modules: require() (CommonJS) and import/export (ES6)
Error Handling: try/catch blocks

// Variables (let, const, var)
let name = 'Node.js';
const version = 20;

// Array
const colors = ['red', 'green', 'blue'];

例：
const obj = [
    { name: "apples", quantity: 2 },
    { name: "bananas", quantity: 0 },
    { name: "cherries", quantity: 5 },
];
const apples = obj.find(i => i.name === 'apples');   　//{ name: "apples", quantity: 2 } 抽出
const aaa = obj.filter(i => i.quantity >= 2);  // [ { name: 'apples', quantity: 2 }, { name: 'cherries', quantity: 5 } ]  抽出
obj.map(i => i.quantity=== 'apples');   　//改变原有obj 　

push(element1, ..., elementN): 配列の末尾に要素を追加
pop(): 配列の末尾から要素を削除し、その値を返す
unshift(element1, ..., elementN): 配列の先頭に要素を追加
shift(): 配列の先頭から要素を削除し、その値を返す

find( ): callback条件を満たす配列内の**最初の要素**を返す、見つからない場合は undefined を返す
filter( ):  callback条件を満たす配列内の**すべての要素**を返す、見つからない場合は 「」 を返す
reduce(function (previous, current, index, array):  配列要素の隣同士を演算して要素の初めから終わりまで繰り返す
map( ): 遍历数组，返り値あり
forEach(): 遍历数组， 返り値なし



// Object
const user = {
  name: 'Alice',
  age: 25,
  greet() {
    console.log(`Hi, I'm ${this.name}`);
  }
};

// Arrow Functions
const addArrow = (a, b) => a + b;
const double = num => num * 2;

// Destructuring  (ES6+)
const user = { name: 'Alice', age: 30, location: 'New York' };
const { name, age } = user;
console.log(name, age);

// Template Literals  (ES6+)
const name = 'Alice';
console.log(`Hello, ${name}!`);

// Array spread - combining arrays
const numbers = [1, 2, 3];
const moreNumbers = [4, 5, 6];
const combined = [...numbers, ...moreNumbers];
console.log(combined);

🚀Node.js vs Browser

https://www.w3schools.com/nodejs/nodejs_vs_browser.asp

Feature	Node.js	Browser
File System Access	Yes	No
Networking (TCP/UDP)	Yes	No
DOM Access	No	Yes
Global Object	Node.js uses global	browsers use window or self
Modules	CommonJS (require) / ES modules (import);	ES modules (import) / <script> tags 引用
Environment Variables	Yes (process.env)	No
Security	Full OS access	Sandboxed
Package Management	npm/yarn	CDN/Bundler

🚀Non-blocking 同步&异步（Async/Await vs Promises vs Callbacks）

https://www.w3schools.com/nodejs/nodejs_async_await.asp

Callback Hell

getUser(userId, (err, user) => {
  if (err) return handleError(err);
  getOrders(user.id, (err, orders) => {
    if (err) return handleError(err);
    processOrders(orders, (err) => {
      if (err) return handleError(err);
      console.log('All done!');
    });
  });
});

Promises

getUser(userId)
  .then(result => console.log(result))
  .catch(error => console.error(error))
  .finally(() => console.log('Operation completed'));

Async/Await (推薦)

async function readFiles() {
  try {
    console.log('1. Starting to read files...');
    const data1 = await fs.readFile('file1.txt', 'utf8');
    const data2 = await fs.readFile('file2.txt', 'utf8');
    console.log('2. Files read successfully!');
    return { data1, data2 };
  } catch (error) {
    console.error('Error reading files:', error);
  }
}
readFiles();

Node.js Modules

🚀CommonJS vs ES Modules

https://www.w3schools.com/nodejs/nodejs_modules_esm.asp

Feature	CommonJS	ES Modules
File Extension	.js (default)	.mjs (or .js with proper config)
Import Syntax	require()	import
Export Syntax	module.exports	export default / export
Import Timing	Dynamic (runtime)	Static (parsed before execution)
Top-level Await	Not supported	Supported
File URL in Imports	required for local files	import for local files
浏览器	不支持	支持
package.json	ー	"type": "module"

🚀Module Loading and Caching（路径分析：确定模块位置）

When you require a module, Node.js looks for it in this order:

Core Node.js modules (like fs, http)
Node modules in node_modules folders
Local files (using ./ or ../ prefix)
优先加载内置模块，即使有同名文件，也会优先使用内置模块。
不是内置模块，先去缓存找。
缓存没有就去找对应路径的文件。
不存在对应的文件，就将这个路径作为文件夹加载。
对应的文件和文件夹都找不到就去node_modules下面找。
还找不到就报错了。

文件夹加载，前提是找不到文件，但是不可能将整个文件夹都加载进来，加载文件夹的时候也是有一个加载顺序的：

先看看这个文件夹下面有没有package.json，如果有就找里面的main字段，main字段有值就加载对应的文件
如果没有package.json或者package.json里面没有main，就找index文件
如果这两步都找不到就报错了。

Tip

import、 require() 能加载什么文件
1、.js文件：通过module.exports 或者 exports 导出模块文件
2、.json文件：通过JSON.parse解析、把 JSON 文件解析成一个 JavaScript 对象
3、any文件：其他任意文件都会当作js文件解析

🚀加载.json文件

方法①（静的なファイル）：const data = require("./data.json");

方法②（静的なファイル）：import data from "./data.json" with { type: "json" };

方法③（サーバー経由）：
fetch('/path/to/data.json')
  .then(response => response.json())
  .then(data => console.log(data)

🚀fetch（请求跨域文件）

＜GETリクエストの例＞
fetch('https://example.com/api/data', {  method: 'GET',  credentials: 'include'  })  // クッキーや認証情報を含める場合
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));


＜GETリクエストの例＞
fetch('https://example.com/api/data?id=123')   // URL引数を含める場合
  .then(response => {
    if (!response.ok) {
      throw new Error('Network response was not ok');
    }
    return response.json();     // レスポンスをJSONとしてパース
  })


＜POSTリクエストの例＞
fetch("https://jsonplaceholder.typicode.com/posts", {
  method: "POST",
  body: JSON.stringify({
    title: "Hello World",
    body: "My POST request",
    userId: 900,
  }),
  headers: {
    "Content-type": "application/json; charset=UTF-8",
  },
})
.then((response) => response.json())
.then((json) => console.log(json));


＜PUTリクエストの例＞
fetch("https://jsonplaceholder.typicode.com/posts/1", {
  method: "PUT",
  body: JSON.stringify({
    id: 1,
    title: "My PUT request",
    body: "Updating the entire object",
    userId: 1,
  }),
  headers: {
    "Content-type": "application/json; charset=UTF-8",
  },
})
.then((response) => response.json())
.then((json) => console.log(json));


＜Express内使用実例１＞
app.get('/list', (req, res) => {
    fetch('https://dummyjson.com/recipes')
        .then(response => response.json())
        .then(data => {
            res.json(data.recipes);　　// res.json() 返回json对象
        })
        .catch(error => console.log(error))
});


＜Express内使用実例２＞
app.get('/list/:id', (req, res) => {
    fetch('https://dummyjson.com/recipes')
        .then(response => response.json())
        .then(data => {
            const item = data.recipes.find(item => item.id == req.params.id);
            console.log(item);
            res.json(item);
        })
        .catch(error => console.log(error))
});


＜HTML内使用実例１＞
<ul id="bbb"></ul>
<script>
    const ul_user = document.getElementById("bbb");
    fetch('https://dummyjson.com/users')
        .then(response => response.json())
        .then(data => {
            data.users.forEach(element => {
                ul_user.innerHTML += `<li><img src='${element.image}'/>${element.firstName} ${element.age}</li>`;
            });
        })
        .catch(error => console.error(error))
</script>

JSON文字列をObjに変換

Tip

JSON.parse( )　　JSON文字列⇒Obj
例: JSON.parse({"name":"Taro","age":30}) は { name: "Taro", age: 30 }

JSON.stringify( )　　Obj⇒JSON文字列
例: JSON.stringify({ name: "Taro", age: 30 }) は {"name":"Taro","age":30}

※JSON文字列：key 必须双引号，只保存属性，不保存方法，如果 Obj里有方法，转换后会失去

🚀npm

npm init    // 初始化一个新项目，在当前目录创建 package.json 文件

npm install    // 根据 package.json 文件安装项目的所有依赖
npm install <package-name>        // 当前项目安装指定package
npm install -g <package-name>    // 全局安装指定package

npm uninstall <package-name>        // 当前项目删除指定package
npm uninstall -g <package-name>    // 全局删除指定package

npm update                    // 根据 package.json 文件更新项目的所有依赖
npm update  <package-name>    // 当前项目更新指定package
npm update -g <package-name>  // 更新package

npm ls        // 查看当前项目package
npm ls -g    // 查看全局package

npm view        // 查看当前项目所有依赖项的最新版本
npm outdated    // 仅列出当前项目已过时的依赖项
npm audit   // 检查脆弱性

🚀常用第三方库

npm install express            //ExpressJSフレームワーク

npm install cors    // CORS

npm install helmet    // Security middleware

npm install log4js    // ログを記録するミドルウェア

npm install http-proxy-middleware    // 代理

npm install cheerio // 网页爬虫

npm install @apollo/server graphql  //GraphQL Server

npm install -g json-server   // 快速构建 json REST API  server
json-server --watch abc.json
http://localhost:3000

npm install -g http-server   // 快速构建static HTTP server
http-server
http://localhost:8080/

🚀core 库 Core Built-in Modules

os 库

https://www.w3schools.com/nodejs/nodejs_os.asp

const os = require('os');

// Basic system information
console.log(`OS Platform: ${os.platform()}`);
console.log(`OS Type: ${os.type()}`);
console.log(`OS Release: ${os.release()}`);
console.log(`Kernel Version: ${os.version()}`);
console.log(`CPU Architecture: ${os.arch()}`);
console.log(`Hostname: ${os.hostname()}`);

// Memory information
const totalMemGB = (os.totalmem() / (1024 * 1024 * 1024)).toFixed(2);
const freeMemGB = (os.freemem() / (1024 * 1024 * 1024)).toFixed(2);
console.log(`Memory: ${freeMemGB}GB free of ${totalMemGB}GB`);

// User information
const user = os.userInfo();
console.log('User Information:');
console.log(`- Username: ${user.username}`);
console.log(`- User ID: ${user.uid}`);
console.log(`- Group ID: ${user.gid}`);

// On Windows, you can also get the user's domain
if (os.platform() === 'win32') {
    console.log(`- Domain: ${user.domain || 'N/A'}`);
}
// Note: user.shell is only available on POSIX platforms
if (user.shell) {
    console.log(`- Default Shell: ${user.shell}`);
}

// Get the home directory
console.log(`Home Directory: ${os.homedir()}`);

// Get the system default temp dir
console.log(`Temporary Directory: ${os.tmpdir()}`);

path 库

https://www.w3schools.com/nodejs/nodejs_path.asp

Tip

关于__dirname、__filename
末尾にスラッシュ（/）は含まれません。Node.jsのグローバル変数であり、ブラウザでは使用できません。

在ESmodule下，需 import定义
const __dirname = import.meta.dirname;
const __filename = import.meta.filename;

// Ensure directories exist
async function ensureDirectories() {
  try {
    await Promise.all([
      fs.mkdir(paths.logs, { recursive: true }),
      fs.mkdir(paths.public, { recursive: true }),
      fs.mkdir(paths.uploads, { recursive: true, mode: 0o755 })
    ]);
    console.log('All directories ready');
  } catch (error) {
    console.error('Error creating directories:', error);
  }
}

// Example: Load configuration
async function loadConfig() {
  try {
    const configData = await fs.readFile(paths.config, 'utf8');
    return JSON.parse(configData);
  } catch (error) {
    console.error('Error loading config:', error.message);
    return {};
  }
}

// Example: Log to application log
async function logToFile(message) {
  try {
    const logFile = path.join(paths.logs, `${new Date().toISOString().split('T')[0]}.log`);
    const logMessage = `[${new Date().toISOString()}] ${message}\n`;
    await fs.appendFile(logFile, logMessage, 'utf8');
  } catch (error) {
    console.error('Error writing to log:', error);
  }
}

fs 库

https://www.w3schools.com/nodejs/nodejs_filesystem.asp

文件操作

### 读文件
fs.readFile(filename, [options], callback);
fs.readFileSync(filename, [options]); 
fs.createReadStream(path, [options]);


### 写文件
fs.writeFile(filename, data, [options], callback);
fs.writeFileSync(filename, data, [options]);
fs.appendFile(filename, data, [options], callback);
fs.appendFileSync(filename, data, [options]);
fs.createWriteStream(path, [options]); 


### 重命名&移动
fs.rename(oldPath, newPath, callback);  
fs.renameSync(oldPath, newPath);


### 文件删除
fs.unlink(path, callback);   
fs.unlinkSync(path); 
fs.rm(path, callback);   
fs.rmSync(path);

文件夹操作

### 创建文件夹
fs.mkdir(path, [mode], callback); 
fs.mkdirSync(path, [mode]);  
fs.mkdir('./a/b/c', {recursive:true}, err=>{console.log(err)});    // 创建子文件 


### 读取文件夹
fs.readdir(path, callback); 
fs.readdirSync(path);     // 不递归，只读取当前文件夹


### 删除文件夹
fs.rmdir(path, callback); 
fs.rmdirSync(path);
fs.rmdir('./a', {recursive:true}, err=>{console.log(err)});    // 递归删除子文件 

fs.rm(path, callback); 
fs.rmSync(path);
fs.rm('./a', {recursive:true}, err=>{console.log(err)});    // 递归删除子文件

文件状态

fs.stat('./abc', (err, data) => {
    if (!err) {
        if (data.isFile()) { console.log('iFile') }
        if (data.isDirectory()) { console.log('isDirectory') }
    }
});

fs.createReadStream使い方

const fs = require('fs');
const path = require('path');
const rs = fs.createReadStream(path.resolve(__dirname, './test.txt'));
rs.on('data', (chunk) => { 
    console.log(`読み込まれたデータ: ${chunk}`); 
});
rs.on('end', ()=> { 
    console.log('ファイル読み込みが完了しました。'); 
});
rs.on('error', (err)=> { 
    console.error('エラーが発生しました:', err);
});

イベント処理:
on('data', ...): ファイルからデータが読み込まれる場合
on('end', ...): ファイルの読み込みが完了した場合
on('error', ...): 読み取り中にエラーが発生した場合

fs.createWriteStream使い方

const fs = require('fs');
const path = require('path');
const ws= fs.createWriteStream(path.resolve(__dirname, './test.txt'));
ws.write('これは最初の部分です。\n');
ws.write('これは2番目の部分です。\n');
ws.end();  // ストリームを閉じる（これがないとfinishイベントが発火しない場合があります）

ws.on('finish', () => {
  console.log('ファイルへの書き込みが完了しました。');
});
ws.on('error', (err) => {
  console.error('書き込み中にエラーが発生しました:', err);
});