如何解开 Promise 的类型 [英] How to Unwrap Type of a Promise

问题描述

假设我有以下代码:

async promiseOne() {
  return 1
} // => Promise<number>

const promisedOne = promiseOne()

typeof promisedOne // => Promised<number>

我将如何提取承诺结果的类型(在这种简化的情况下是一个数字)作为它自己的类型?

How would I go about extracting the the type of the promise result (in this simplified case a number) as it's own type?

推荐答案

截至 Typescript 2.8，现在可以利用 条件类型的，这里有一个基本的示例:游乐场

As of Typescript 2.8, this is now possible by taking advantage of conditional types, here's a basic example: Playground

function promiseOne() {
  return Promise.resolve(1)
}
    
const promisedOne = promiseOne()
    
// note PromiseLike instead of Promise, this lets it work on any thenable
type ThenArg<T> = T extends PromiseLike<infer U> ? U : T
    
type PromiseOneThenArg = ThenArg<typeof promisedOne> // => number
// or
type PromiseOneThenArg2 = ThenArg<ReturnType<typeof promiseOne>> // => number

这不是一个完美的解决方案.承诺不能包含其他承诺.Promise> 不是真实的，如果你 await 得到的类型应该是 string，但是 ThenArg 将给出 Promise.

This isn't a perfect solution. Promises cannot contain other promises. Promise<Promise<string>> isn't real, and the type you get if you await it should be string, but ThenArg will give Promise<string>.

TypeScript 4.1 包括对递归类型别名的支持，这使得编写以下内容成为可能.游乐场

TypeScript 4.1 includes support for recursive type aliases, which makes it possible to write the following. Playground

type ThenArgRecursive<T> = T extends PromiseLike<infer U> ? ThenArgRecursive<U> : T

type Str = ThenArgRecursive<Promise<Promise<Promise<string>>>> // string

对于禁止递归类型的旧版 TypeScript，您可以通过使用递归类型别名来解决此问题，但请注意，这些版本官方不支持(尽管实际上，通常很好).游乐场

For older versions of TypeScript, which ban recursive types, you can work around this issue by using a recursive type alias, but be aware that this is officially not supported in those versions (though practically, usually fine). Playground

type ThenArgRecursive<T> = T extends PromiseLike<infer U>
  ? { 0: ThenArgRecursive<U>; 1: U }[U extends PromiseLike<any> ? 0 : 1]
  : T

type Str = ThenArgRecursive<Promise<Promise<Promise<string>>>> // string

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