TypeScript: Error: TS1055: Type 'Promise' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

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I’m having the same issue with axios’ AxiosPromise typing (see: https://github.com/axios/axios/blob/master/index.d.ts#L87 )

I’m getting:

Type 'AxiosPromise' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value. ts(1055)

As you can see in the provided link, it’s just:

export interface AxiosPromise<T = any> extends Promise<AxiosResponse<T>> {}

Here’s the code I use:

const getAsync = async (): AxiosPromise<any> => {
  ...
}

If I copy the type, it works as expected:

const getAsync = async (): Promise<AxiosResponse<any>> => {
  ...
}

Who should we tag to reopen this, I found only one name?

Hello, @RyanCavanaugh, this is not a question, but clearly a bug that needs fixing. If it’s not a duplicate, please change the issue type and re-open.

Regards

It seems to me that if this expression is valid typescript:

const f = <T> async ():Promise<T> => {...}

(and it is)… then the following expression should also be valid typescript:

type P<T> = Promise<T>
const f = <T> async ():P<T> => {...}

the nature of the implementation of choosing a promise-compatible constructor to use during desugaring seems to be, or at least seems like it should be, an independent concern that doesn’t bear on the case where the return type aliases directly and inarguably to the already acceptable Promise type.

Said differently, In cases where the literal word Promise is acceptable, why shouldn’t any other type alias which resolves deterministically to it and only it be also acceptable?

Forgive me if I am overlooking something. 😃

With arrow functions it might work by moving the type declaration to the const object.

//doesn't work on target: ES5
const a = async () : Promise<any> => 3

//works for any target
const b : () => Promise<any> = async () => 3```

Don’t use "target": "es5" in tsconfig.json, use something like esnext should work!

“target”: “esnext”,

For me, this issue arose with a conditional type. I changed

const foo = <T extends boolean>(): T extends true ? Promise<string[]> : Promise<string[][]> => {
    //...
}

to

const foo = <T extends boolean>(): Promise<T extends true ? string[] : string[][]> =>{
    // ...
}

In other words, wrapping the whole conditional in a promise worked when splitting the promises inside the conditional didn’t.

@RyanCavanaugh ping

The same with built-in ReturnType helper:

async function fooBar(): Promise<void> {

}

async function barBaz(): ReturnType<typeof fooBar> { // Type 'ReturnType' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

}

https://www.typescriptlang.org/play/index.html#src=async function fooBar()%3A Promise<void> { } async function barBaz()%3A ReturnType<typeof fooBar> { }