23. PromiseAll

[astak16]

题目

题目链接：PromiseAll

实现 PromiseAll，满足下面功能

import type { Equal, Expect } from '@type-challenges/utils'

const promiseAllTest1 = PromiseAll([1, 2, 3] as const)
const promiseAllTest2 = PromiseAll([1, 2, Promise.resolve(3)] as const)
const promiseAllTest3 = PromiseAll([1, 2, Promise.resolve(3)])

type cases = [
  Expect<Equal<typeof promiseAllTest1, Promise<[1, 2, 3]>>>,
  Expect<Equal<typeof promiseAllTest2, Promise<[1, 2, number]>>>,
  Expect<Equal<typeof promiseAllTest3, Promise<[number, number, number]>>>,
]

答案

方法一

type TUtil1<T> = T extends Promise<infer R> ? R : T;
type TUtil2<T extends unknown[]> = T extends [infer F, ...infer R]
  ? [TUtil1<F>, ...TUtil2<R>]
  : [];
declare function PromiseAll<T extends unknown[]>(
  values: readonly [...T]
): Promise<TUtil2<T>>;

方法二

declare function PromiseAll<T extends unknown[]>(
  values: readonly [...T]
): Promise<{ [K in keyof T]: T[K] extends Promise<infer R> ? R : T[K] }>;

解析

遍历的方法有两种：

• 递归（方法一）
• 使用 in 变量（方法二）

方法一使用了可变元组类型：(Variadic tuple types)[https://github.com/microsoft/TypeScript/pull/39094]