javascript - 基于区分联合缩小 typescript 泛型类型

Question

我有一些类型定义，如下所示。

type PayloadType = 'A' | 'B';

interface Payload<T extends PayloadType> {
  type: T;
}

interface PayloadA extends Payload<'A'> {
  state: string
}

interface PayloadB extends Payload<'B'> {
  serialNumber: string;
}

type TPayload = PayloadA | PayloadB;


type PayloadInterpretation<T extends TPayload> = {
  payload: T;
  entries: T[]; // This property is only for demonstration purpose
};

type TPayloadInterpretation = PayloadInterpretation<PayloadA> | PayloadInterpretation<PayloadB>;

function f(interpretation: TPayloadInterpretation) {
  if (interpretation.payload.type === 'B') {
    const payload = interpretation.payload; // payload is of type PayloadB
    const entries = interpretation.entries; // entries is of type PayloadA[] | PayloadB[]
  }
}

评论表明，即使有效载荷的类型也可以正确缩小到 PayloadB基于歧视联合，但类型 T[]为 entries还在 PayloadA[] | PayloadB[] .
我在想 typescript 是否知道类型 T有效载荷是 PayloadA ，应该也能缩小 entries: T[]至 entries: PayloadB[] .我知道我可以进行类型转换，例如:

function f(interpretation: TPayloadInterpretation) {
  if (interpretation.payload.type === 'B') {
    const payloadBInterpretation = interpretation as PayloadInterpretation<PayloadB>;
    ...
  }
}

但我的问题是有没有其他方法可以做到这一点？
代码是 here在 typescript 游乐场。
谢谢!

Answer 1

当您检查 interpretation.payload.type 时，你只是缩小了interpretation.payload目的。您实际上并没有采取任何措施来缩小范围 interpretation.entries .
换句话说， typescript 不知道 interpretation.entries缩小时也可以缩小interpretation.payload .
如果您希望两者都缩小，则需要在 PayloadInterpretation 中使用另一个鉴别器类型:

// ...

type PayloadInterpretation<T extends TPayload> = {
  type: T['type']; // the new discriminator for the whole PayloadInterpretation
  payload: T;
  entries: T[];
};

// ...

function f(interpretation: TPayloadInterpretation) {
  if (interpretation.type === 'B') { // narrowing the whole interpretation instead of only interpretation.payload
    const payload = interpretation.payload; // payload is of type PayloadB
    const entries = interpretation.entries; // entries is of type PayloadB[]
  }
}