I’m struggling with something I’m certain should be simple in Typescript. I’m trying to define a helper function that runs some function and logs a custom message if it errors (this is a simplification of the actual functionality, but it suffices for the example).

async function logIt<T extends () => Promise<any>>(message: string, fn: T): ReturnType<T> {
  try {
    const result = await fn();
    return result;
  }
  catch (e) {
    console.log(message);
    throw e;
  }
}

I then use it:

class Thing extends Whatever {
  // as the name suggests, this function is overwritten from `Whatever` - it's returnType is `Promise<number>`
  functionOnWhatever() {
    return logIt("hello", () => super.functionOnWhatever());
  }
}

The problem is, the logIt function complains that ReturnType<T> should be Promise<ReturnType<T>> – and when I make that change, functionOnWhatever complains that 'Promise<Promise<number>>' is not assignable to 'Promise<number>'.