Typescript IS for async

Functions that are tagged async must be typed as returning a promise, since JS guarantees that the function will return a promise when invoked, regardless of its body.

This is a little bit more complex because you are using a typeguard, which cannot be async. One strategy to deal with this is to make your typeguard synchronous and have it take in pre-awaited results that it might otherwise await.

As an example

// what we'd like to do, obviously a contrived example
async isSomething(t: Something | OtherThing): t is Something {
  const result = await someCall(t); // some async result we need to determine what t is
  return result.isSomething;
}

This could be changed up to

async isSomethingAsyncPart(t: Something | OtherThing): IsSomethingAsyncResultSet {
  // do some await'ing here
  return asyncResults; 
}
isSomethingGuard(t: Something | OtherThing, asyncResults: IsSomethingAsyncResultSet): t is Something {
  // some synchronous evaluation using t and/or asyncResults
}

// kinda yucky but :shrug_emoji:
if (isSomethingGuard(t, await isSomethingAsyncPart(t)) { . . .}

Not sure this is the best way to handle stuff (typeguards requiringg async work seems a little suspect overall, honestly). You may be trying to shoehorn the type system into doing something it isn't designed for.

If your typeguard really is just () => true, you could just make it not async as well.