Typed Explorer Step 5: Make the Impossible Unrepresentable

Go through your Explorer's state and find every value that can legitimately be absent. Type each one honestly as a union with null or undefined, rather than pretending it is always there. The compiler will now object in every place you were not handling it, and each objection is a bug you shipped. Fix them by narrowing, not by adding assertions. Count how many there were, because that number is the argument for this whole course.