没有类型保护的打字稿判别联合

Question

The code below results in the error:

TS2322: Type 'RhinoData | HippoData' is not assignable to type 'RhinoData & HippoData'.
  Type 'RhinoData' is not assignable to type 'RhinoData & HippoData'.
    Property 'bar' is missing in type 'RhinoData' but required in type 'HippoData'.

The hope was that the AnimalPayload type provides enough information to guarantee that the assignment is legal. Is there a way to accomplish this in such a way that setAnimalState does not have to worry about specific types?

enum Animal {
  RHINO = 'RHINO',
  HIPPO = 'HIPPO',
}

interface RhinoData {
  foo: string,
}

interface HippoData {
  bar: string,
}

interface AnimalState {
  [Animal.RHINO]: RhinoData,
  [Animal.HIPPO]: HippoData,
}

type AnimalPayload = {
  type: Animal.RHINO,
  data: RhinoData,
} | {
  type: Animal.HIPPO,
  data: HippoData,
}

const state: AnimalState = {
  [Animal.RHINO]: null,
  [Animal.HIPPO]: null,
};

const setAnimalState = (payload: AnimalPayload) => {
  state[payload.type] = payload.data;
};

Answer 1

It sounds like typescript can't infer the fact that the type of payload.type is constrained by the type of AnimalState . Using mapped types can fix your problem here by making that relationship explicit.

enum Animal { RHINO = "RHINO", HIPPO = "HIPPO" }

interface RhinoData {
  foo: string,
}

interface HippoData {
  bar: string,
}

interface AnimalState {
  [Animal.RHINO]: RhinoData | null,
  [Animal.HIPPO]: HippoData | null,
}

type AnimalPayload<T extends Animal> = {
  type: T
  data: AnimalState[T],
}

const state: AnimalState = {
  [Animal.RHINO]: null,
  [Animal.HIPPO]: null,
};

const setAnimalState = <T extends Animal>(payload: AnimalPayload<T>) => {
  state[payload.type] = payload.data;
};