awaitedType.js

//// [awaitedType.ts]
type T1 = Awaited<number>;
type T2 = Awaited<Promise<number>>;
type T3 = Awaited<number | Promise<number>>;
type T4 = Awaited<number | Promise<string>>;
type T5 = Awaited<{ then: number }>;
type T6 = Awaited<{ then(): void }>; // never (non-promise "thenable")
type T7 = Awaited<{ then(x: number): void }>; // never (non-promise "thenable")
type T8 = Awaited<{ then(x: () => void): void }>; // unknown
type T9 = Awaited<any>;
type T10 = Awaited<never>;
type T11 = Awaited<unknown>;
type T12 = Awaited<Promise<Promise<number>>>;
type T13 = _Expect<Awaited<Promise<Promise<number>> | string | null>, /*expected*/ string | number | null>; // otherwise just prints T13 in types tests, which isn't very helpful
type T14 = _Expect<Awaited<Promise<Promise<number>> | string | undefined>, /*expected*/ string | number | undefined>; // otherwise just prints T14 in types tests, which isn't very helpful
type T15 = _Expect<Awaited<Promise<Promise<number>> | string | null | undefined>, /*expected*/ string | number | null | undefined>; // otherwise just prints T15 in types tests, which isn't very helpful

interface BadPromise { then(cb: (value: BadPromise) => void): void; }
type T16 = Awaited<BadPromise>; // error

interface BadPromise1 { then(cb: (value: BadPromise2) => void): void; }
interface BadPromise2 { then(cb: (value: BadPromise1) => void): void; }
type T17 = Awaited<BadPromise1>; // error

// https://github.com/microsoft/TypeScript/issues/33562
type MaybePromise<T> = T | Promise<T> | PromiseLike<T>
declare function MaybePromise<T>(value: T): MaybePromise<T>;

async function main() {
    let aaa: number;
    let bbb: string;
    [
        aaa,
        bbb,
    ] = await Promise.all([
        MaybePromise(1),
        MaybePromise('2'),
        MaybePromise(true),
    ])
}

// helps with tests where '.types' just prints out the type alias name
type _Expect<TActual extends TExpected, TExpected> = TActual;


//// [awaitedType.js]
async function main() {
    let aaa;
    let bbb;
    [
        aaa,
        bbb,
    ] = await Promise.all([
        MaybePromise(1),
        MaybePromise('2'),
        MaybePromise(true),
    ]);
}