联合类型的通用打字稿：从其他属性中找出属性的类型

Question

我有一个具有payload和type属性的并集类型，其中type是我们唯一的属性。给定type时，我想推断出有效载荷类型。

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'>;

因此目标是PayloadOfTypeA等于APayload（或number）。

这可能吗？

Answer 1

您可以使用条件类型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'>;