the lack of typing work on typescript ? should it say that the parameters are not typed ?

Out of curiosity based on how i refactored my solution, why is this more popular of a solution than return (cast) (a+b)? It seems declaring another variable, even a local one, is more work than needs to be done when you can just return the value.

Is there a best practice that says a declared local variable is less prone to bugs or is it just preference as ease of understanding?

a and b already are of the type byte, so casting them will not do anything. However, the addition operator + returns an int (as bytes are not necessarily used as numbers, so a + b implicitly does is (int) a + (int) b), hence we do need to cast the result.

It is often considered bad practice to use exceptions as flow control, but honestly, in this case, I will make an exception as Java sort of forces your hand.

It is indeed. But as you said: excersise.