> doubler2 is a partial function because it is not defined for all the inputs. If we pass in 5, it won’t return any value.

That's wrong. `doubler2` is defined for all the inputs, it just returns `null` for some of them, which is totally valid output. It would be partial if it were implemented like that:

int doubler2(int value) {

if (value == 1) return 2;

if (value == 2) return 4;

if (value == 3) return 6;

throw ArgumentError();

}