JavaScript 核心语法与 ES6 新特性全解析

{
  let name = 'Alice'; // 块级作用域
  const PI = 3.14; // 常量
  // PI = 3.1415; // 报错：Assignment to constant variable.
}
// console.log(name); // 报错：name is not defined

let str = '123';
let num = Number(str); // 123
let price = 19.99;
let formatted = price.toFixed(1); // "20.0" (字符串)

let score = 85;
if (score >= 90) {
  console.log('优秀');
} else if (score >= 60) {
  console.log('及格');
} else {
  console.log('不及格');
}

// for 循环
for (let i = 0; i < 5; i++) {
  console.log(i);
}
// for...of 遍历数组（ES6）
let fruits = ['apple', 'banana', 'orange'];
for (let fruit of fruits) {
  console.log(fruit);
}

let userName = '张三';
let age = 25;
let message = `欢迎 ${userName}，您今年 ${age} 岁。`;
console.log(message); // 欢迎 张三，您今年 25 岁。
// 多行字符串
let html = `
<div class="user-card">
  <h2>${userName}</h2>
  <p>年龄：${age}</p>
</div>
`;

let arr = [1, 2, 3];
arr.push(4); // [1, 2, 3, 4]
arr.splice(1, 1); // 从索引1删除1个元素 -> [1, 3, 4]
arr.forEach(item => console.log(item)); // 1, 3, 4
let doubled = arr.map(item => item * 2); // [2, 6, 8]
console.log(arr.length); // 3
// 本地存储还原实例
let data = [{id: 1, name: 'A'}, {id: 2, name: 'B'}];
localStorage.setItem('myData', JSON.stringify(data));
let restored = JSON.parse(localStorage.getItem('myData'));
let names = restored.map(item => item.name); // ['A', 'B']

// 普通函数声明
function greet(name) {
  return `Hello, ${name}!`;
}
// 工厂函数：返回一个闭包，创建独立的计数器
function createCounter() {
  let count = 0; // 私有变量
  return function() {
    count++;
    return count;
  };
}
let counter1 = createCounter();
console.log(counter1()); // 1
console.log(counter1()); // 2
let counter2 = createCounter(); // 新的独立作用域
console.log(counter2()); // 1

// 传统函数
let add = function(a, b) {
  return a + b;
};
// 箭头函数
let addArrow = (a, b) => a + b;
// 事件回调简写
document.getElementById('btn').addEventListener('click', () => {
  console.log('按钮被点击了');
});
// 箭头函数没有自己的 this，继承自外层作用域
let obj = {
  value: 10,
  traditional: function() {
    console.log(this.value); // 10
  },
  arrow: () => {
    console.log(this.value); // undefined（此处 this 指向 window 或全局对象）
  }
};

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
  // 实例方法
  introduce() {
    return `我叫${this.name}，今年${this.age}岁。`;
  }
}
let p1 = new Person('李四', 30);
console.log(p1.introduce()); // 我叫李四，今年30岁。

class Calculator {
  constructor() {
    this.result = 0;
  }
  add(x) {
    this.result += x;
    return this;
  }
  subtract(x) {
    this.result -= x;
    return this;
  }
  getResult() {
    return this.result;
  }
}
let calc = new Calculator();
calc.add(5).subtract(2);
console.log(calc.getResult()); // 3

class Student extends Person {
  constructor(name, age, grade) {
    super(name, age); // 调用父类构造函数
    this.grade = grade;
  }
  study() {
    return `${this.name}正在学习，年级：${this.grade}`;
  }
}
let stu = new Student('王五', 18, '高三');
console.log(stu.introduce()); // 我叫王五，今年18岁。
console.log(stu.study()); // 王五正在学习，年级：高三

function School() {
  // 私有变量，外部无法直接访问
  let stuList = [];
  // 公开方法，形成闭包
  return {
    addStudent(name) {
      stuList.push(name);
    },
    getStudentCount() {
      return stuList.length;
    },
    getStudents() {
      // 返回副本，防止外部直接修改原数组
      return [...stuList];
    }
  };
}
let mySchool = School();
mySchool.addStudent('小明');
mySchool.addStudent('小红');
console.log(mySchool.getStudentCount()); // 2
console.log(mySchool.getStudents()); // ['小明', '小红']
// mySchool.stuList = []; // 无效，stuList 是私有变量

function createUniqueCounter() {
  let count = 0;
  return {
    increment() {
      count++;
      return count;
    },
    reset() {
      count = 0;
    },
    getCount() {
      return count;
    }
  };
}
let counterA = createUniqueCounter();
let counterB = createUniqueCounter();
counterA.increment();
counterA.increment();
console.log(counterA.getCount()); // 2
counterB.increment();
console.log(counterB.getCount()); // 1
// counterA 和 counterB 的 count 变量相互独立，不会污染

let user = {
  name: '赵六',
  age: 28,
  hobbies: ['读书', '编程']
};
let jsonString = JSON.stringify(user);
console.log(jsonString); // {"name":"赵六","age":28,"hobbies":["读书","编程"]}
// 存储到 localStorage
localStorage.setItem('userInfo', jsonString);

// 从 localStorage 读取
let storedString = localStorage.getItem('userInfo');
if (storedString) {
  let restoredUser = JSON.parse(storedString);
  console.log(restoredUser.name); // 赵六
  console.log(restoredUser.hobbies); // ['读书', '编程']
}
// 注意：JSON.parse 无法还原函数、undefined 等特殊类型
let obj = { fn: function() {}, x: undefined };
let str = JSON.stringify(obj); // "{}"
let parsed = JSON.parse(str); // {}