一、基本语法

1. 变量声明

• 使用 var

  var name = "Alice";

• 使用 let 和 const

  let age = 30;
  const birthYear = 1990;

2. 数据类型

• 原始数据类型

  let number = 42;          // Number
  let text = "Hello";       // String
  let isTrue = true;        // Boolean
  let undef = undefined;    // Undefined
  let empty = null;         // Null

• 对象

  let person = {
      name: "Bob",
      age: 25
  };

3. 运算符

• 算术运算符：+, -, *, /, %
• 比较运算符：==, ===, !=, !==, >, <, >=, <=
• 逻辑运算符：&&, ||, !

二、控制结构

1. 条件语句

• if 语句

  if (age > 18) {
      console.log("Adult");
  } else {
      console.log("Minor");
  }

• switch 语句

  switch (day) {
      case 1:
          console.log("Monday");
          break;
      case 2:
          console.log("Tuesday");
          break;
      default:
          console.log("Other day");
  }

2. 循环语句

• for 循环

  for (let i = 0; i < 5; i++) {
      console.log(i);
  }

• while 循环

  let i = 0;
  while (i < 5) {
      console.log(i);
      i++;
  }

三、函数

1. 函数声明

function greet(name) {
    return "Hello, " + name;
}

console.log(greet("Alice"));

2. 函数表达式

const greet = function (name) {
    return "Hello, " + name;
};

console.log(greet("Bob"));

3. 箭头函数

const greet = (name) => "Hello, " + name;

console.log(greet("Charlie"));

四、对象和数组

1. 对象

• 创建和访问对象

  let car = {
      brand: "Toyota",
      model: "Corolla",
      year: 2020
  };
  
  console.log(car.brand); // Toyota

• 修改对象

  car.year = 2021;
  car.color = "blue";

2. 数组

• 创建和访问数组

  let fruits = ["Apple", "Banana", "Cherry"];
  console.log(fruits[0]); // Apple

• 数组方法

  fruits.push("Durian"); // 添加元素
  fruits.pop();          // 删除最后一个元素

五、DOM 操作

1. 选择元素

• 通过 ID

  let element = document.getElementById("header");

• 通过类名

  let elements = document.getElementsByClassName("item");

• 通过选择器

  let element = document.querySelector(".item");
  let elements = document.querySelectorAll(".item");

2. 修改元素

• 修改内容和属性

  element.textContent = "New Content";
  element.setAttribute("class", "new-class");

• 修改样式

  element.style.color = "red";
  element.style.fontSize = "20px";

六、事件处理

1. 添加事件监听器

let button = document.querySelector("button");
button.addEventListener("click", function () {
    alert("Button clicked!");
});

2. 移除事件监听器

function handleClick() {
    alert("Button clicked!");
}

button.addEventListener("click", handleClick);
button.removeEventListener("click", handleClick);

七、ES6+ 新特性

1. 模板字符串

let name = "Alice";
let greeting = `Hello, ${name}!`;
console.log(greeting);

2. 解构赋值

• 数组解构

  let [a, b, c] = [1, 2, 3];

• 对象解构

  let { name, age } = { name: "Alice", age: 25 };

3. 扩展运算符

• 数组扩展

  let arr1 = [1, 2, 3];
  let arr2 = [...arr1, 4, 5];

• 对象扩展

  let obj1 = { a: 1, b: 2 };
  let obj2 = { ...obj1, c: 3 };

4. Promise 和异步编程

• Promise

  let promise = new Promise((resolve, reject) => {
      // 异步操作
      if (success) {
          resolve(result);
      } else {
          reject(error);
      }
  });
  
  promise.then(result => {
      console.log(result);
  }).catch(error => {
      console.error(error);
  });

• async/await

  async function fetchData() {
      try {
          let response = await fetch('https://api.example.com/data');
          let data = await response.json();
          console.log(data);
      } catch (error) {
          console.error(error);
      }
  }
  
  fetchData();

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