}
  if (!numTwo) {
    $result.value = '';
  }
  numTwo += event.target.textContent;
  $result.value += event.target.textContent;
};
document.querySelector('#num-0').addEventListener('click', onClickNumber); 
document.querySelector('#num-1').addEventListener('click', onClickNumber); 
document.querySelector('#num-2').addEventListener('click', onClickNumber); 
document.querySelector('#num-3').addEventListener('click', onClickNumber); 
document.querySelector('#num-4').addEventListener('click', onClickNumber); 
document.querySelector('#num-5').addEventListener('click', onClickNumber); 
document.querySelector('#num-6').addEventListener('click', onClickNumber); 
document.querySelector('#num-7').addEventListener('click', onClickNumber);
document.querySelector('#num-8').addEventListener('click', onClickNumber); 
document.querySelector('#num-9').addEventListener('click', onClickNumber); 
const onClickOperator = (op) => () => { 
  if (numOne) {
    operator = op;
    $operator.value = op;
  } else {
    alert('숫자를 먼저 입력하세요.');
  }