TypeScript: Exhaustive Switch Helper

TypeScript

Make the compiler tell you when a union gains a case.

Chidi Okoro Sample Sep 27, 2026

TypeScript · 12 lines copied 9 times

function assertNever(x: never): never {
  throw new Error(`Unhandled case: ${JSON.stringify(x)}`);
}

function label(status: 'todo' | 'doing' | 'done') {
  switch (status) {
    case 'todo': return 'To do';
    case 'doing': return 'In progress';
    case 'done': return 'Done';
    default: return assertNever(status);
  }
}

function assertNever(x: never): never {
  throw new Error(`Unhandled case: ${JSON.stringify(x)}`);
}

function label(status: 'todo' | 'doing' | 'done') {
  switch (status) {
    case 'todo': return 'To do';
    case 'doing': return 'In progress';
    case 'done': return 'Done';
    default: return assertNever(status);
  }
}

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