Exhaustive Discriminated Unions

Exhaustive Discriminated Unions: working example

Run this standalone example after compiling with TypeScript 5.4+ in strict mode, targeting ES2022. Expected result: The program prints Value: 42; adding an unhandled union member causes a type error at the never assignment.

type Result =
  | { kind: "success"; value: number }
  | { kind: "failure"; message: string };
function describe(result: Result): string {
  switch (result.kind) {
    case "success": return `Value: ${result.value}`;
    case "failure": return `Error: ${result.message}`;
    default: {
      const unhandled: never = result;
      throw new Error(`Unexpected result: ${unhandled}`);
    }
  }
}
console.log(describe({ kind: "success", value: 42 }));

1. Expected result: The program prints Value: 42; adding an unhandled union member causes a type error at the never assignment.
2. Each branch exposes only its own variant’s payload.
3. The throwing default also fails loudly if unvalidated JavaScript supplies an impossible value.