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
// 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