【前端学习】—let const var之间的区别（十三）

<script>
        // 1、const let不存在变量提升 var 存在变量提升
        //var
console.log(`fullName`,fullName);//fullName undefined
//var fullName='cai';
/*
var 声明变量的方式会存在变量提升，var在声明变量之前，首先会将var fullName提升到代码块的最前面
*/
//let
console.log(`fullName`,fullName);//Uncaught ReferenceError: Cannot access 'fullName' before initialization
let fullName='cai';
//const
//console.log(`fullName`,fullName);
//const fullName='cai'//let const.html:13 Uncaught ReferenceError: Cannot access 'fullName' before initialization
    </script>

var fullName='cai';
function f(){
    console.log(`fullName`,fullName)//undefined
    if(false){
       var fullName='kiki'
    }
}
f();

let fullName='cai';
function f(){
    console.log(`fullName`,fullName)//cai  全局作用域
    if(false){
        //块级作用域 只在当前范围内生效
       let fullName='kiki'
    }
}
f();

// 暂时性死区 声明变量之前不能提前给变量赋值
var fullName='cai'
if(true){
  fullName='test'
  let fullName;//声明变量之前不能提前给变量赋值
}

//不能重复声明
function f(){
    let a='10';
    let a='20';
}

var fullName='cai'
console.log(`window`,window.fullName);//cai
//let 和const声明的变量不会挂载在window对象下面
const fullName1='cai'
console.log(`window`,window.fullName1);// undefined

//const声明的基本类型变量的值不能被改变，引用类型可以
const familyName='cai';
//familyName='cai1'
console.log(familyName)//Uncaught TypeError: Assignment to constant variable.
const fruit=['apple'];
fruit[0]='orange';
console.log(fruit);//['orange']