I think ICat | IDog is perfect. It means exactly that, the value can be of type ICat _OR_ IDog, so in that sense, we can only access properties that are in both

I think ICat | IDog is perfect. It means exactly that, the value can be of type ICat _OR_ IDog, so in that sense, we can only access properties that are in both of them until we know for sure if we are dealing with a ICat or a IDog specifically.