const

const 用来声明常量

一、语法

const NAME = 'zhangsan';
const PI = 3.14;
const ARR = [1, 2, 3];
const OBJ = { name: 'zhangsan' };
const FN = function () {
    console.log('hello world');
}

注意

常量一般使用大写

二、一定要赋初始值

const name;

报错

Uncaught SyntaxError: Missing initializer in const declaration

三、不能被重新赋值

const name = 'zhangsan';
name = 'lisi';

报错

Uncaught TypeError: Assignment to constant variable.

四、块级作用域

if (true) {
    const name = 'zhangsan';
}
console.log(name);

报错

Uncaught ReferenceError: name is not defined

五、对于数组和对象，不算对常量的修改

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

注意

地址指针不变，所以可以修改

const obj = { name: 'zhangsan' };
obj = 1
console.log(obj);

报错

Uncaught TypeError: Assignment to constant variable.