联合类型的打字稿泛型：从其他属性中找出属性的类型

Question

我有一个带有payload 和type 属性的联合类型，其中type 我们是uniq。我想在给定type 时推断有效负载类型。

type AAction = {type: 'A', payload: APayload};
type BAction = {type: 'B', payload: BPayload};

type APayload = number;
type BPayload = string;

type Actions = AAction | BAction;

const actions: Actions[] = [];

type PayloadOfType<T extends Actions['type']> = ????

type PayloadOfTypeA = PayloadOfType<'A'>;

所以目标是PayloadOfTypeA 等于APayload（或number）。

这可能吗？

Answer 1

可以使用条件类型Extract：

type AAction = {type: 'A', payload: APayload};
type BAction = {type: 'B', payload: BPayload};

type APayload = number;
type BPayload = string;

type Actions = AAction | BAction;

const actions: Actions[] = [];

type PayloadOfType<T extends Actions['type']> = Extract<Actions, { type: T }>['payload']

type PayloadOfTypeA = PayloadOfType<'A'>;