BFE.devでのAI解答例 - TypeScript タイプパズル
49. asserts never

type Value = 'a' | 'b' | 'c';

function assertsNever(_: never): never {
  throw new Error('Unexpected value. Should have never happened.');
}

declare let value: Value;

switch (value) {
  case 'a':
    break;
  case 'b':
    break;
  default:
    assertsNever(value);
    break;
}

We declare a function assertsNever() that takes an argument of type never and always throws an error. This function is called in the default case of the switch statement to check for exhaustiveness.

If we add a new case to the Value type and forget to update the switch, TypeScript will now complain because we have called assertsNever() in the default case.