const (常量的使用)

const 声明常量

    1、不能重复声明常量名 

// var num = 10;
    // const num = 15;
    // console.log(num) //Identifier 'num' has already been declared

 

    2、声明后必须赋值 字符串，数字 布尔值等基本数据类型 

// var num1;
    // const num1 = 20;
    // console.log(num1); // Missing initializer in const declaration

   

 

    3、常量特点不能修改;（Assignment to constant variable）

 // const person  = { num:10,age:20} 
    // // console.log(person.num,person.age)
    // person.num = 40;
    // console.log(person.num) //40

    // person = {num:100};
    // console.log(person.num)  //Assignment to constant variable.

    // const arr = [1,2,3,4];
    // arr[1] = 20;
    // console.log(arr)
    // arr = [1,20,3,4]

    // const num  = 10;
    // num = 20;
    // console.log(num);  //Assignment to constant variable.

// const num  = 10;
    // num = 20;
    // console.log(num);  //Assignment to constant variable.

 

 　4、没有变量提升

// function fn(){
    //     console.log(num)
    //     const num=10;
    // }
    // fn()  //Cannot access 'num' before initialization

 

　 5、具有块级作用域

for( var j =0;j<5;j++){
        const num = 10;
    }
    console.log(num);//num is not defined