区分类的联合

Question

使用 Typescript 3.4 我试图根据位于每个子类上的单个只读字段（鉴别器）来区分 typescript 的并集类；本身看起来很简单，但我似乎无法解决它。

下面是一个 link 的游乐场，其中 createFruit 方法应该能够采用通用参数来通过鉴别器过滤下一个属性。任何见解将不胜感激；但是，它似乎允许评估为 never。

abstract class Fruit {
    abstract readonly fruitType: string;
}

class Banana extends Fruit {
    fruitType = 'banana';
    length = 2;
    color = 'yellow';
}

class Pear extends Fruit {
    fruitType = 'pear';
    roundness = 'very round';
}

class Apple extends Fruit {
    fruitType = 'apple';
    fallOfMan = true;
    hasWorms = true;
}

type KnownFruits = Banana | Pear | Apple;

type FruitTypes = KnownFruits['fruitType'];

type FruitDiscriminator<T extends FruitTypes> = Extract<KnownFruits, { fruitType: T }>;

let createFruit = <T extends FruitTypes>(fruitType: T, props: FruitDiscriminator<T>) => { }

createFruit('pear', {} ) // Argument of type '{}' is not assignable to parameter of type 'never'.

TS Playground

Answer 1

问题是派生的类中的 fruitType 是 string 类型，而不是与字符串文字关联的字符串文字类型。在初始化字段时，Typescript 默认将文字类型扩展为基本类型。要解决此问题，您可以使用 as const 断言，或使 w 字段 readonly （或手动指定字符串文字类型，但这将复制字符串）

readonly 解决方案最有意义，因为类型不应更改：

abstract class Fruit {
    abstract readonly fruitType: string;
}

class Banana extends Fruit {
    readonly fruitType = 'banana';
    length = 2;
    color = 'yellow';
}

class Pear extends Fruit {
    readonly fruitType = 'pear';
    roundness = 'very round';
}

class Apple extends Fruit {
    readonly fruitType = 'apple';
    fallOfMan = true;
    hasWorms = true;
}

type KnownFruits = Banana | Pear | Apple;

type FruitTypes = KnownFruits['fruitType'];

type FruitDiscriminator<T extends FruitTypes> = Extract<KnownFruits, { fruitType: T }>;

let createFruit = <T extends FruitTypes>(fruitType: T, props: FruitDiscriminator<T>) => { }

// ok
createFruit('pear', {
    fruitType: 'pear',
    roundness: ""
})

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