I suspect you can adjust your approach a little and use something along the lines of the example here:

https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

I suspect you can adjust your approach a little and use something along the lines of the example here:

https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}