Author here! Yeah I think you are right that discriminated unions are useful more broadly than just legacy JS code.
That being said, I still think TypeScript's solution to handling them of using "type guards" where the type of a variable changes in different scopes is definitely designed to match common JS patterns at the cost of added complexity. Most other languages I know of only have a single type for a variable (and if you want to pattern match you must give each case a new variable name).
That being said, I still think TypeScript's solution to handling them of using "type guards" where the type of a variable changes in different scopes is definitely designed to match common JS patterns at the cost of added complexity. Most other languages I know of only have a single type for a variable (and if you want to pattern match you must give each case a new variable name).