[Wixeman]

Всем большое спасибо за столь быстрые ответы! Вот рабочий код:

function calculation() {
  var firstNum = prompt('Enter first number: '),
    secondNum = prompt('Enter second number: '),
    operating = prompt('Enter operating with number: '),
    result;
  
  // string to number

  var nFirstNum = (+firstNum / 1), 
    nSecondNum = (+secondNum / 1); 
  
  switch(operating) {
    case '+':
      result = (nFirstNum + nSecondNum);
      break
    case '-':
      result = (nFirstNum - nSecondNum);
      break
    case '*':
      result = (nFirstNum * nSecondNum);
      break
    case '/':
      result = (nFirstNum / nSecondNum);
      break
    case '':
      result = 'ERROR: Values are empty!';
      console.log('ERROR: Values are undefined!');
      break
    default:
      result = 'ERROR: Operating is not correct!';
      console.log('ERROR: Operating is not correct!');
  } // switch

  switch(result) {
    case undefined:
      result = 'ERROR: Calcule is falied!';
      console.log('ERROR: Result is undefined!');
      break
    default:
      if(isNaN(result)) result = 'ERROR: Answer is not a number!';
  }
  alert(result);
};