ES6(4) - const

const 也用來聲明變量，可是聲明的是常量，一旦聲明，常量值不改變。

 <script type="module">
        const Pi = 3.1415926535;
        Pi = 3;
        console.log(Pi);// error: Pi is read-only
 </script>

1） const 塊級做用域：

<script type="module">
        if(true){
            const Pi = 3.1415926535;
        };
        console.log(Pi);// error: Pi is not defined
</script>

2） const 暫時性死區：

<script type="module">
        if(true){
            console.log(Pi);//undefined
            const Pi = 3.1415926535;
        };
</script>

3） const  不可重複聲明：

<script type="module">
        {
            var a = 100;
            const a = 200;
            console.log(a);// ERROR: Duplicate declaration, a
        }
</script>

4） const 對象：

<script type="module">
   const person = {};
    person.name = "zhangsan";
    person.age = 30;

    console.log(person.name); // zhangsan
    console.log(person.age); // 30
    console.log(person); // Object {name: "zhangsan", age: 30}
</script>

錯誤方法：

<script type="module">
        const person = {};
        person.name = "zhangsan";
        person.age = 30;

        console.log(person.name); // zhangsan
        console.log(person.age); // 30
        console.log(person); // Object {name: "zhangsan", age: 30}

        person = {}; // Error:  person is read-only
</script>

5） const 數組：

 <script type="module">
        const arr = [];
        console.log(arr);
        console.log(arr.length);//0
        console.log("---------");
        arr.push("Hello World!");
        console.log(arr);
        console.log(arr.length);//1
        console.log("---------");
        arr.length = 0;
        console.log(arr);
        console.log(arr.length);//0
        console.log("---------");

        //錯誤用法
        //arr = ["Hello Everyone!"]; // error: arr is read-only
 </script>