[ruslan_mart]

function calculation() {
    var firstNum = prompt('Enter first number:'),
        secondNum = prompt('Enter second number:'),
        operating = prompt('Enter operating with number:'),
        result = /[+*\/-]/.test(operating) ? eval(+firstNum + operating + +secondNum) : 'ERROR: ' + {'': 'Values are empty!', NaN: 'Answer is not a number!'}[operating] || 'Calcule is falied!';
    alert(result);
};
calculation();