TypeScript 没有对结构体的属性初始化或者没有给对象属性初始化 未初始化的字段都是undefined

https://gitee.com/smallinsect/MyJS/blob/master/TypeScript/%E5%AF%B9%E8%B1%A1%E5%B1%9E%E6%80%A7%E6%9C%AA%E5%88%9D%E5%A7%8B%E5%8C%96.ts

interface Animal {
    name: string;
    age: number;
    egg?: number;
}

let animal: Animal = <Animal>{};
console.log('------------------------');
console.log(animal.name);
console.log('------------------------');
console.log(animal.age);
console.log('------------------------');
console.log(animal.egg);
console.log('------------------------');