Typescript 根据类型脚本中的泛型属性 Select 类型

你很可能是有意

const dog = {
  type: AnimalType.DOG,
  legs: 4,
}

有{type: AnimalType.DOG, legs: number}型. 但是TypeScript将AnimalType.DOG的类型推断为更宽的enum类型AnimalType，而不是literal type AnimalType.DOG.

这意味着有人可以写dog.type = AnimalType.CAT，这不是很好.这也意味着typeof dog没有可以用来计算Animal<T>的信息.

要解决这个问题，您需要告诉编译器type应该被赋予一个更具体的文本类型.通常的方法是在有问题的字面上使用const assertion.如果您希望编译器强制要求dog.legs始终为4，则可以对完整的对象文字{type: AnimalType.DOG, legs: 4} as const执行此操作.但至少，我们应该为type号房产这么做.产生这样的结果:

const dog = { type: AnimalType.DOG as const, legs: 4 };
const cat = { type: AnimalType.CAT as const, tail: 1 };
const frog = { type: AnimalType.FROG as const, canSwim: true, tail: 0 };
const snake = { type: AnimalType.SNAKE as const, tail: 'looong', age: 2 };

现在，union type-typeof dog | typeof cat | typeof frog | typeof snake看起来像是:

type SomeAnimal = typeof dog | typeof cat | typeof frog | typeof snake;
/* type SomeAnimal = {
    type: AnimalType.DOG;
    legs: number;
} | {
    type: AnimalType.CAT;
    tail: number;
} | {
    type: AnimalType.FROG;
    canSwim: boolean;
    tail: number;
} | {
    type: AnimalType.SNAKE;
    tail: string;
    age: number;
} */

它允许我们通过联合过滤Extract utility type实现Animal:

type Animal<T extends AnimalType> =
  Extract<SomeAnimal, { type: T }>;

让我们测试一下:

type Dog = Animal<AnimalType.DOG>;
/* type Dog = {
    type: AnimalType.DOG;
    legs: number;
} */

看上go 不错.

Playground link to code个