Experiments with the new Awaited Type in Typescript

awaited.ts

declare function MaybePromise<T>(value: T):  T | Promise<T> | PromiseLike<T>

declare function MaybeNumberPromise(value: number):  number | Promise<number> | PromiseLike<number>
declare type MaybePromiseType = ReturnType<typeof MaybeNumberPromise>


//Does not work at all!
async function doSomethingSpecial(): Awaited<[number, number]> {
    const result = await Promise.all([MaybePromise(100), MaybePromise(200)]);
    // Error!
    //
    //    [number | Promise<100>, number | Promise<200>]
    //
    // is not assignable to type
    //
    //    [number, number]
    return result;
}

//Works in 4.5 not in 4.4
async function doSomething(): Promise<[number, number]> {
    const result = await Promise.all([MaybePromise(100), MaybePromise(200)]);
    // Error!
    //
    //    [number | Promise<100>, number | Promise<200>]
    //
    // is not assignable to type
    //
    //    [number, number]
    return result;
}

//Works in 4.5 not in 4.4
async function doSomethingWithAwaited(): Promise<[number, number]> {
  //4.4 Does not know Awaited
  const result: Awaited<[number,number]> = await Promise.all([MaybePromise(100), MaybePromise(200)]);
  return result;
}

//Works in 4.5 and in 4.4
async function doSomethingWithAnyTypes(): Promise<[number, number]> {
  const promiseAllResult: Promise<[any, any]> =  Promise.all([MaybePromise(100), MaybePromise(200)]);
  const result:[any, any] = await promiseAllResult
  return result;
}

//Works in 4.5 not in 4.4
async function doSomethingWithExplicitTypesIn44(): Promise<[number, number]> {
  const promiseAllResult: Promise<[number,number]> =  Promise.all([MaybePromise(100), MaybePromise(200)]);
  const result =  await promiseAllResult
  return result;
}

//Works in 4.5 not in 4.4
async function doSomethingWithExplicitTypesIn45(): Promise<[number, number]> {
  const promiseAllResult: Promise<[Awaited<number>,Awaited<number>]> =  Promise.all([MaybePromise(100), MaybePromise(200)]);
  const anotherPromiseAllResult: Promise<[number, number]> = promiseAllResult
  //const result1:[MaybePromiseType<number>, MaybePromiseType<number>] = await promiseAllResult
  //const result2:[number, number] = await promiseAllResult
  //const result3:[Promise<number>, Promise<number>] = await promiseAllResult
  const result: Awaited<[number,number]> = await anotherPromiseAllResult
  return result;
}

//Works in 4.5 not in 4.4
async function doSomethingWithImplicitTypes(): Promise<[number, number]> {
  const promiseAllResult =  Promise.all([MaybePromise(100), MaybePromise(200)]);
  //const result1:[MaybePromiseType<number>, MaybePromiseType<number>] = await promiseAllResult
  //const result2:[number, number] = await promiseAllResult
  //const result3:[Promise<number>, Promise<number>] = await promiseAllResult
  const result: Awaited<[number,number]> = await promiseAllResult
  return result;
}

async function doSomethingNonGeneric(): Promise<[number, number]> {
    const result = await Promise.all([MaybeNumberPromise(100), MaybeNumberPromise(200)]);
    // Error!
    //
    //    [number | Promise<100>, number | Promise<200>]
    //
    // is not assignable to type
    //
    //    [number, number]
    return result;
}