Basic calculator, dynamic operator

HTML

<input id="item1" type="text">

<select id="selectOperator">
    <option value="add">add</option>
    <option value="subtract">subtract</option>
    <option value="divide">divide</option>
    <option value="multiply">multiply</option>
</select>
    
<input id="item2" type="text"><br>
Tax: <input id="item3" type="text">%<br>
Discount: <input id="item4" type="text"><br><br>
    
<button id="submitBtn">RESULT</button><br><br>
    
Result of math: <div class="math-result"></div>

JavaScript

(function(){
    var submitBtn = document.getElementById("submitBtn");
    var resultBox = document.querySelector(".math-result");
    var mathOperator;
    
    console.log(submitBtn, resultBox);
    
    var getKeys = function(obj){
        var keys = [];
        for(var key in obj){
            keys.push(key);
        }
        return keys;
    }
    
    function doTheMath(operator, value1, value2){
        var mathResult;
        value1 = parseInt(value1);
        value2 = parseInt(value2);
        
        switch(operator) {
            case 'add':
                console.log("add");
                mathResult = value1 * 2;
                break;
            case 'subtract':
                console.log("sub");
                mathResult = value1 - value2;
                break;
            case 'divide':
                console.log("divide");
                mathResult = value1 / value2;
                break;
            case 'multiply':
                console.log("multiply");
                mathResult = value1 * value2;
                break;
            default:
                alert("please select operator");
        }
        
        return mathResult;
    }
    
    submitBtn.onclick = function(e){
        var value1 = document.getElementById("item1").value || 0;
        var value2 = document.getElementById("item2").value || 0;
        var taxVal = document.getElementById("item3").value || 0;
        var discount = document.getElementById("item4").value || 0;
        
        mathOperator = document.getElementById("selectOperator").value;
        
        var mathResult = doTheMath(mathOperator, value1, value2);
            mathResult = mathResult * (taxVal / 100 + 1);
            mathResult = mathResult.toFixed(2);
            mathResult = mathResult - discount;
        
        resultBox.innerHTML = mathResult;
    };
})();