打字稿装饰器未设置属性值

Question

我正在尝试学习装饰器，但不确定下面的代码为什么不返回我期望的结果...

我想用字符串值修饰一个类，然后将其分配给该类中具有相同名称的属性。

Person.ts

@course({
    id: 'rainbow'
})
export class Person {
    id: string;
    firstName: string;
    lastName: string;

    constructor(firstName: string, lastName: string) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public returnNames() {
        console.log('Firstname: '+this.firstName+'. Lastname: '+this.lastName+'. ID: '+this.id);
    }
}

export function course(options: { id: string }) {
    console.log(options.id);
    return (target :any) => {
        target.id = options.id;
    }
}

main.ts

import { Person } from "./Person";

class Startup {
    public static main() {
        var test = new Person('Matt','OConnor');
        test.returnNames();
    }
}

Startup.main();

运行此命令的输出是...

$ node main.js
rainbow
Firstname: Matt. Lastname: OConnor. ID: undefined

为什么未定义ID，而不是“彩虹”？