How to Unwrap Type of a Promise

Issue

Say I have the following code:

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

const promisedOne = promiseOne()

typeof promisedOne // => Promised<number>

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

Solution

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

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 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

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

Answered By - Gerrit0