If the `something` function had `(num: string | number)` I would not be able to call `assertIsInRange(num: number)` because `Argument of type 'string | number'

If the `something` function had `(num: string | number)` I would not be able to call `assertIsInRange(num: number)` because `Argument of type 'string | number' is not assignable to parameter of type 'number'.` So If I wanted to do that I would have to if (typeof num === 'number') { assertIsInRange(num); } anyway. I think I still don't get it.