type ApiResult = { ok: true; data: string[] } | { ok: false; error: string };
function printResult(result: ApiResult) {
  if (result.ok) {
    console.log(result.data.join(', '));
    return;
  }
  console.error(result.error);
}
function isError(value: unknown): value is Error { return value instanceof Error; }
try { printResult({ ok: true, data: ['a', 'b'] }); } catch (e) { if (isError(e)) console.error(e.message); }