"your Redux reducer doesn't handle all of the possible action types so it will crash at run time"
And what are all these dr. Strange multiverse possibilities you speak of? Any semi seasoned JS dev has spidey sense for watching out for null and undefined, and generally you should know the type of what you are returning. Is there that much variability in what you are dealing with? If it’s an array of objects, that’s not hard to hold in your brain, just watch out for empty arrays or null/undefined items.