2、ES6笔记之const声明变量

笔记代码如下

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>变量const</title>
</head>

<body>

</body>

<script>
    // const定义的是常量,定义的时候必须有值，不能后赋值，且后续不能更改
</script>

<script>
    const a = 12;
    a = 17; //报错信息Assignment to constant variable.
    console.log(a)


    const b; //报错信息Missing initializer in const declaration
    b = 12;
    console.log(b)
</script>

<script>
    const arr = ['1', '2'];
    arr[0] = '3';
    console.log(arr) //['3','2']

    // const arr=['1','2'];
    // arr.push('3')
    // console.log(arr)   //['1','2','3']
</script>

<script>
    // const arr = Object.freeze(['1', '2']);
    // arr.push('3') //报错Cannot add property 2, object is not extensible
    // console.log(arr)
</script>

<script>
    //对象是引用   可更改里面的值   如果是普通常量不可更改
    const person = {
        name: '小仙女',
        age: 12,
        sex: '女'
    }

    person.name = '小小小';
    console.log(person) //{name: "小小小", age: 12, sex: "女"}
</script>

</html>