};
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', () => {
  if (numTwo) {
    switch (operator) { 
      case '+':
        $result.value = parseInt(numOne) + parseInt(numTwo);
        break;
      case '-':
        $result.value = numOne - numTwo;
        break;
      case '*':
        $result.value = numOne * numTwo;
        break;
      case '/':
        $result.value = numOne / numTwo;
        break;
      default:
        break;
    }