What is the difference between let and var in js

The content of this article is about the difference between let and var in js. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1: Variable promotion or not

1: var:

console.log（a）； // undefined
var a = 'abc';
// 这段代码实际执行顺序是：
var a;  //变量声明提升至当前作用域顶部
console.log（a）；
a = 'abc';

2: let:

console.log（a）； // 报错： a is not defined
let a = 'abc';
// 这里， 用let声明变量， 变量声明不会提升， 完全按照文档流的执行顺序走

2: Scope problem

1. var：

for (var i = 0; i<10; i++) {    // code..  }
console.log(i); // 输出 10

2.let：

for (let i = 0; i<10; i++) {    // code..  }
console.log(i); // 报错： i is not defined
// let 作用于 代码块 {}

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