Exhaustiveness

Make the compiler prove you handled every case. Assign the value to never in the default branch: never accepts nothing, so if any case is unhandled the assignment fails and the message names the type you forgot.

That is what makes adding a case to a union safe across a large codebase. Add a fourth state and every switch that does not handle it becomes an error, which is refactoring that finds its own work.

The empty case was added to the type and not handled. The error names it: Type '{ status: "empty"; }' is not assignable to type 'never'. That message is the compiler telling you exactly what you forgot.