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.');
  }
};
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', () => {}); 
document.querySelector('#clear').addEventListener('click', () => {});
</script>