var? let?? const???

function varExample() {
    if (true) {
        var x = 10; // 函数作用域
    }
    console.log(x); // 10 - 在if块外也能访问
}
varExample();

for (var i = 0; i < 3; i++) {
    // i在循环中
}
console.log(i); // 3 - 在循环外也能访问

function letExample() {
    if (true) {
        let x = 10; // 块级作用域
        const y = 20; // 块级作用域
    }
    console.log(x); // ReferenceError: x is not defined
    console.log(y); // ReferenceError: y is not defined
}

for (let i = 0; i < 3; i++) {
    // i在循环中
}
console.log(i); // ReferenceError: i is not defined

// 函数声明会被完全提升
console.log(add(2, 3)); // 5 - 可以在声明前调用

function add(a, b) {
  return a + b;
}

console.log(typeof multiply); //"undefined"
//console.log(multiply(2,3)); //multiply is not defined
//使用了const，无法提升
const multiply = function(a, b) {
  return a * b;
};

console.log(multiply(2, 3)); // 6 - 只能在定义后调用

// 使用var，提升为undefined
console.log(typeof myFunc); // "undefined"

myFunc();//报错：myFunc is not a function;

var myFunc = function() {
  console.log("函数表达式");
};

console.log(typeof myFunc); // "function"
myFunc(); // "函数表达式"

// const声明的函数表达式
console.log(typeof myFunc); // "undefined"
// myFunc(); // 报错：myFunc is not defined(变量根本没有被创建）

const myFunc = function() {//let也一样
  console.log("函数表达式");
};

console.log(typeof myFunc); // "function"
myFunc(); // "函数表达式"

// var - 变量提升，初始化为undefined
console.log(a); // undefined (不会报错)
var a = 5;

// 等价于：
var a; // 声明提升到顶部
console.log(a); // undefined
a = 5; // 赋值留在原地

// let/const - 暂时性死区(Temporal Dead Zone)
console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 5;

console.log(c); // ReferenceError: Cannot access 'c' before initialization
const c = 10;

// var允许重复声明（可能导致bug）
var x = 5;
var x = 10; // ✅ 允许
console.log(x); // 10

// let/const不允许重复声明
let y = 5;
let y = 10; // ❌ SyntaxError: Identifier 'y' has already been declared

const z = 5;
const z = 10; // ❌ SyntaxError: Identifier 'z' has already been declared

// var在全局作用域声明会成为全局对象的属性
var globalVar = 'I am var';
console.log(window.globalVar); // 'I am var' (在浏览器中)
console.log(globalThis.globalVar); // 'I am var'

// let/const在全局作用域声明不会成为全局对象的属性
let globalLet = 'I am let';
const globalConst = 'I am const';
console.log(window.globalLet); // undefined
console.log(window.globalConst); // undefined