标签：erro 更改 声明 UNC fine not red es6 div
在ES6中，新出了let和const这两个新的声明变量的命令。与之前的var相比，let和const有几个不同的特性。
var
<!--可以重复声明-->
var a = "this is a";
var a = "this is another a"; //重复声明了a
console.log(a); //输出this is another a
<!--存在变量提升-->
console.log(b); //变量提升，var b 被提升至所在作用域顶部，输出undefined
var b = "this is b";
<!--无法限制修改-->
var c = "this is c";
c = "this is cc";
c = "this is ccc";
console.log(c); //输出this is ccc
<!--没有块级作用域-->
{
var d = "this is d";
console.log(d); //输出this is d
}
console.log(d); //输出this is d
let
1.不能重复声明，且不存在变量提升
2.块级作用域
<!--不能重复声明--> let a = "this is a"; let a = "this is another a"; //这里会报错：Uncaught SyntaxError: Identifier ‘a‘ has already been declared <!--没有变量提升--> console.log(b); let b = "this is b"; //这里会报错：Uncaught ReferenceError: b is not defined <!--块级作用域--> { let c = "this is c"; console.log(c); //输出this is c } console.log(c); //这里会报错：Uncaught ReferenceError: c is not defined
const
const包含let的所有特性，区别是声明的变量不可以修改（const保证变量指向的内存不可改动，而不是声明的值不能改动）
<!--不能更改值--> const a = "this is a"; a = "b"; console.log(a); //这里报错：Uncaught TypeError: Assignment to constant variable.
【ES6专题】——var、let、const的区别和使用场景
标签：erro 更改 声明 UNC fine not red es6 div
原文地址：https://www.cnblogs.com/sanyi2019/p/10357518.html
