JavaScript中const和var的区别详解

function test() {
    if (true) {
        var x = 10;
    }
    console.log(x); // 10 - 在if块外部可以访问
}

function test() {
    if (true) {
        const y = 20;
    }
    console.log(y); // ReferenceError: y is not defined
}

console.log(a); // undefined
var a = 5;

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

var x = 1;
var x = 2; // 不会报错

const y = 1;
const y = 2; // SyntaxError: Identifier 'y' has already been declared

var name = "Alice";
name = "Bob"; // 允许

const name = "Alice";
name = "Bob"; // TypeError: Assignment to constant variable

var x; // undefined

const y; // SyntaxError: Missing initializer in const declaration

var globalVar = "hello";
console.log(window.globalVar); // "hello"

const globalConst = "world";
console.log(window.globalConst); // undefined

const person = { name: "Alice" };
person.name = "Bob"; // 允许 - 修改对象属性
// person = { name: "Charlie" }; // 错误 - 不能重新赋值

const numbers = [1, 2, 3];
numbers.push(4); // 允许 - 修改数组内容
// numbers = [5, 6, 7]; // 错误 - 不能重新赋值

// 推荐写法
const PI = 3.14159;
let count = 0;

// 不推荐
var oldWay = "avoid this";