ES6--let声明变量

1.块作用域

<!--
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 * @Date: 2023-05-31 14:33:10
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 * @LastEditTime: 2023-05-31 14:42:09
 * @FilePath: \vscode\ECMA\01\01.html
 * @Description: 
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 * Copyright (c) 2023 by ${git_name_email}, All Rights Reserved. 
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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <ul>
        <li>111</li>
        <li>222</li>
        <li>333</li>
    </ul>
    <script>
        // {//块级作用域
        //     let i=100;
        // }

        // console.log(i)

        // for(let i=0;i<5;i++){

        // }
        // console.log(i)

        var oli=document.querySelectorAll("ul li")
        for(let i=0;i<oli.length;i++){
            oli[i].onclick=function(){
                console.log(i)
            }
        }
    </script>
</body>
</html>

2.不允许重复声明

3.变量提升

暂存性死区

4.不与顶层对象挂钩

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 * @Date: 2023-05-31 14:42:43
 * @LastEditors: Mei
 * @LastEditTime: 2023-05-31 14:52:25
 * @FilePath: \vscode\ECMA\01\02.html
 * @Description: 
 * 
 * Copyright (c) 2023 by ${git_name_email}, All Rights Reserved. 
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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        //不允许重复声明

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

        //3-变量提升
        //暂存性死区
        // console.log(myname)
        let myname="mez"
        // let myname="mez"  报错
        function test(){
            let myname="kunkun"
            console.log(myname)
            
        }
        test()

        //4.不与顶层对象挂钩
        // let myage=100  //undefined
        var myage=100
        console.log(window.myage)
    </script>
</body>
</html>

ES6--const声明变量

const声明的为常量，拥有以下特性：

1.常量

2.不能重复定义

3.块级作用域

4.暂存性死区，声明不提升

5.不与顶层对象挂钩

还有需要注意的点：所有的const声明都不能被修改吗？

答：const声明的对象不能被修改，如const obj={name:"test1"}   修改成const obj="test2"会报错。

但是对象里面的属性可以修改，如obj.name="kunkun".

<!--
 * @Author: RealRoad
 * @Date: 2023-05-31 14:55:04
 * @LastEditors: Mei
 * @LastEditTime: 2023-05-31 15:09:25
 * @FilePath: \vscode\ECMA\02\1.html
 * @Description: 
 * 
 * Copyright (c) 2023 by ${git_name_email}, All Rights Reserved. 
-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        //1.常量
        const myname="mez"
        // myname="kunkun" 报错
        // const myname2;  报错 ，必须在初始化赋值
        //2.不能重复定义
        // const myname="kunkun"

        //3.块级作用域
        {
            const myname2="Ikun"
        }
        // console.log(myname2)  报错

        //4.暂存性死区，声明不提升
        const myname3="mez"
        
        function test(){
            const myname3="kunkun"
            console.log(myname3)
            
        }
        test()
        //5.不与顶层对象挂钩
        const myage=100
        // console.log(window.myage) undefined

        const myobj={
            name:"rap",
            age:100
        }
        //myobj ="test"  报错
        myobj.name="ji哥"
        console.log(myobj)
        //freeze  不让修改对象
        const myobj2=Object.freeze({
            name:"chang",
            age:2.5
        })
        myobj2.name="test2"
        console.log(myobj2)
    </script>
</body>
</html>

ES6-ES13学习笔记（1.0）_学习

 那么，如果让对象里面的内容也不能修改呢？

答：可以使用freeze（）方法，使用Object.freeze({...})定义的对象不能够修改。