Exhaustiveness checking in flow

better.js

const checkType = (element: 'a' | 'b' | 'c'): string => {
 switch (element) {
   case 'a':
     return 'first';
   case 'b':
     return 'second';
   default:
     (element: empty) // This cast only succeeds if you've already handled every possible case. Otherwise, you will get a type error
     throw new Error()
 }
}