operator = '';
    numTwo = '';
  } else {
    alert('숫자를먼저입력하세요.');
  }
};

const onClickOperator = (op) => () => {

  if (numTwo) {
    calculate();
    operator = op;
    $operator.value = op;
  } else if (numOne) {

    operator = op;
    $operator.value = op;
  } else {
    alert('숫자를먼저입력하세요.');
  }
};
document.querySelector('#plus').addEventListener('click', onClickOperator('+'));
document.querySelector('#minus').addEventListener('click', onClickOperator('-'));
document.querySelector('#divide').addEventListener('click', onClickOperator('/'));
document.querySelector('#multiply').addEventListener('click', onClickOperator('*'));

document.querySelector('#calculate').addEventListener('click', calculate);