var, let 与 const

暂时性死区#

如果区块中存在let和const命令，这个区块对这些命令声明的变量，从一开始就形成了封闭作用域。凡是在声明之前就使用这些变量，就会报错。

let a = 1;
// 块级作用域
{
    a = 2;  // Cannot access 'a' before initialization
    let a = 3;
}

变量可以在var声明之前使用，值为undefined。

1 2	console.log(a); // undefined var a = 1;

1 2	console.log(a); // Cannot access 'a' before initialization let a = 1;

1
2
3

var a = 1;
var a = 2;
console.log(a); // 2

1 2	let a = 1; let a = 2; // Identifier 'a' has already been declared

{
    var a = 1;
}
console.log(a); // 1

{
    let a = 1;
}
console.log(a); // ReferenceError: a is not defined

node环境中

x = "哈哈哈哈";
var y = "呵呵呵呵";
let z = "嘿嘿嘿嘿";
console.log(global.x); // "哈哈哈哈"
console.log(global.y); // undefined
console.log(global.z); // undefined

brower环境中

x = "哈哈哈哈";
var y = "呵呵呵呵";
let z = "嘿嘿嘿嘿";
console.log(window.x); // 哈哈哈哈
console.log(window.y); // 呵呵呵呵
console.log(window.z); // undefined

总结let的属性特征