打字稿：从对象的属性中的属性确定对象类型

Question

我可能对 Typescript 要求太多了，但我想知道这样的事情是否可行：

interface ObjectType {
    type: 'this' | 'that';
}

interface SomeObject {
    objType: ObjectType
}

interface ThisObject extends SomeObject {
    objType: { type: 'this' }
    thisProp: 'anything'
}

interface ThatObject extends SomeObject {
    objType: { type: 'that' }; 
    thatProp: 'something'
}

function getProp(obj: ThisObject | ThatObject) {
    switch (obj.objType.type) {
        case 'this':
            return {
                type: obj.objType.type,
                prop: obj.thisProp
            };
        case 'that':
            return {
                type: obj.objType.type,
                prop: obj.thatProp
            };
    }
}

Typescript 能够正确缩小 obj.objType.type，但我试图在返回的对象中分配给 prop 的值没有进行类型检查。两者的错误相同（显然属性名称不同）：

TS2339: Property 'thatProp' does not exist on type 'ThisObject | ThatObject'. Property 'thatProp' does not exist on type 'ThisObject'.

这样的事情可能吗？我也尝试过这样的事情：

interface SomeObject {
    objType: ObjectType;
    thisProp: SomeObject['objType'] extends 'this' ? 'anything' : never;
    thatProp: SomeObject['objType'] extends 'that' ? 'something' : never;
}

这导致两个道具都是 never，以及类似这样的东西：

type PickObject<T> = T extends 'this' ? ThisObject : ThatObject;

function getProp<T extends 'this' | 'that'>(obj: PickObject<T>) {
    switch (obj.objType.type) {
        case 'this':
            return {
                type: obj.objType.type,
                prop: obj.thisProp
            };
        case 'that':
            return {
                type: obj.objType.type,
                prop: obj.thatProp
            };
    }
}

这会导致相同的错误：

TS2339: Property 'thisProp' does not exist on type 'PickObject '.

Answer 1

似乎不需要 SomeObject 接口。这是消除了 SomeObject 接口的可能解决方案：

TypeScript playground demo

interface ObjectType {
    type: 'this' | 'that';
}

interface ThisObject extends ObjectType {
    type: 'this'
    thisProp: 'anything'
}

interface ThatObject extends ObjectType {
    type: 'that'
    thatProp: 'something'
}

function getProp(obj: ThisObject | ThatObject) {
    switch (obj.type) {
        case 'this':
            return {
                type: obj.type,
                prop: obj.thisProp
            };
        case 'that':
            return {
                type: obj.type,
                prop: obj.thatProp
            };
    }
}

Answer 2

问题是 ObjectType 和 objType 键。 objType实际上并没有定义接口的类型。这是一个工作示例：

type ObjectType = 'this' | 'that'

interface SomeObject {
  type: ObjectType
}

interface ThisObject extends SomeObject {
  type: 'this'
  thisProp: 'anything'
}

interface ThatObject extends SomeObject {
  type: 'that'
  thatProp: 'something'
}

function getProp(obj: ThisObject | ThatObject) {
  switch (obj.type) {
    case 'this':
      return {
        type: obj.type,
        prop: obj.thisProp
      };
    case 'that':
      return {
        type: obj.type,
        prop: obj.thatProp
      };
  }
}

Answer 3

假设你的类型实际上是这样的：

interface ThisObject extends SomeObject { objType: { type: 'this' }; thisProp: 'anything' }
interface ThatObject extends SomeObject { objType: { type: 'that' }; thatProp: 'something' }

其中 type 属性实际上嵌套在 objType 属性中，那么您运行进入 microsoft/TypeScript#18758 where nested discriminated unions 提交的问题并不真正受支持在打字稿中。您不能将一个受歧视的联合用作另一个受歧视的联合中的判别式；判别式必须是单例类型的联合，例如 "this" | "that".

无需等待 microsoft/TypeScript#18758 得到解决，您可以编写自己的 user-defined type guard function that behaves like a nested discriminated union discriminator by passing a discriminant object and using the Extract utility type 来表示所需的缩小范围。像这样：

function nestedDiscrim<T extends object | PropertyKey, D extends object | PropertyKey>(
  val: T, discriminant: D): val is Extract<T, D>;
function nestedDiscrim(val: any, discriminant: any) {
  if ((typeof val === "object") && (typeof discriminant === "object")) {
    for (let k in discriminant) {
      if (!(k in val)) return false;
      if (!nestedDiscrim(val[k], discriminant[k])) return false;
    }
    return true;
  }
  if ((typeof val !== "object") && (typeof discriminant !== "object")) {
    return val === discriminant;
  }
  return false;
}

你会像这样使用它：

function getProp(obj: ThisObject | ThatObject) {
  if (nestedDiscrim(obj, { objType: { type: "this" } } as const)) {
    return {
      type: obj.objType.type,
      prop: obj.thisProp
    };
  } else {
    return {
      type: obj.objType.type,
      prop: obj.thatProp
    };
  }
}

你可以看到它有效：

console.log(getProp({ objType: { type: "this" }, thisProp: "anything" })); // {type: "this", prop: "anything"};
console.log(getProp({ objType: { type: "that" }, thatProp: "something" })); // {type: "that", prop: "something"};

想法是 nestedDiscrim(obj, { objType: { type: "this" } }) 走过 obj，查看 obj.objType.type 并将其与 "this" 进行比较。如果它们相同，那么我们将 obj 从 ThisObject | ThatObject 缩小到 ThisObject。否则，我们将 obj 从 ThisObject | ThatObject 缩小到 ThatObject.

好的，希望对您有所帮助；祝你好运！

Playground link to code

打字稿：从对象的属性中的属性确定对象类型

Typescript: determine object type from property in object's property

discriminated-union

typescript

conditional-types

打字稿：从对象的 属性 中的 属性 确定对象类型

Typescript: determine object type from property in object's property

discriminated-union

typescript

conditional-types

打字稿：从对象的属性中的属性确定对象类型