function isTarget(str: string | undefined) {
  return str === 'target';
}

function returnStr(str: string | undefined) {
  return isTarget(str) ? 'ok' : str || 'ng';
}

const str = undefined;

console.log(returnStr(str));

const str2 = 'Hello';

console.log(returnStr(str2));

const str3 = 'ok';

console.log(returnStr(str3));