Function composeAsync

packages/forms/src/validators.ts:688–699

* Merges asynchronous validators into a single validator function. * See `Validators.composeAsync` for additional information.

(validators: (AsyncValidatorFn | null)[])

 * See `Validators.composeAsync` for additional information.
 */
function composeAsync(validators: (AsyncValidatorFn | null)[]): AsyncValidatorFn | null {
  if (!validators) return null;
  const presentValidators: AsyncValidatorFn[] = validators.filter(isPresent) as any;
  if (presentValidators.length == 0) return null;

  return function (control: AbstractControl) {
    const observables = executeValidators<AsyncValidatorFn>(control, presentValidators).map(
      toObservable,
    );
    return forkJoin(observables).pipe(map(mergeErrors));
  };
}

/**
 * Accepts a list of async validators of different possible shapes (`AsyncValidator` and