exhaustiveness-checking

Add a default case that calls an assertUnreachable(value: never): never helper. After all cases are handled, the remaining type is never; if a case is missing, the leftover variant type is not assignable to never and TypeScript raises a compile error.

Use the never-type exhaustiveness pattern in every switch over that union. When you add the new variant, each switch missing the case fails to compile with an error like 'Argument of type X is not assignable to parameter of type never', pointing to every location needing an update.

Yes, if the function has an explicit return type, every case returns, and strictNullChecks is enabled. A missing case then triggers 'Function lacks ending return statement'. This works only under those conditions, unlike the assertUnreachable approach.

A plain default case silently swallows new variants, so adding one produces no error and the new case is never handled. Using assertUnreachable in the default turns that silent omission into a compile-time type error.

Skip it when you intentionally handle only a subset of union variants, such as a function that processes common cases and deliberately ignores others. Adding assertUnreachable there would create false type errors for intentionally unhandled cases.