4. js 兼容性

@babel/plugin-proposal-decorators: 装饰器编译成 es5

@babel/plugin-proposal-class-properties: 转换静态类属性及使用属性初始值化声明的属性

plugins: [
    ['@babel/plugin-proposal-descrators', { legacy: true }],
  ['@babel/plugin-proposal-class-properties', { loose: true }]
]

babel presets 预设是插件包，插件的集合，其中包含很多插件

function readonly (target, key, descriptor) {
    descriptor.writable = false;
}
class Person {
    @readonly PI = 3.14
}

const person = new Person();
person.PI = 3.15;