JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Percentage Calculators</h1>
        
        
        <form name ="" id = "calc" method="post">            
			<select name = numberList id = "numberList">
			   <option value="1">What is Y% of X</option>
			   <option value="2">X is Y% of what</option>
			   <option value="3">What % of X is Y?</option>
			   <option value="4">X% of what is Y?</option>
			   <option value="5">Y% of X is what?</option>
			</select> <br>
            
            
        Wartość X: <input type="text" id="numField1" value=2><br>
        Wartość Y: <input type="text" id="numField2" value=4><br>
    
            <input type="submit" value="Oblicz" id = calculator>
            </form>
            <h3 id="resultField"></h3>

JavaScript

var numField1 = document.getElementById('numField1');
var numFeild2 = document.getElementById('numField2');
var result = document.getElementById('resultField');
var calculate = document.getElementById('select');

var frm = document.getElementById('calc');

frm.addEventListener('submit', function(event) {
    event.preventDefault();
    if (!numField1.value || !numFeild2.value) {
        alert("Proszę wprowadź dane");
    } else {
        var x = parseFloat(numField1.value);
        var y = parseFloat(numField2.value);
        
        var result = x / y;
        var percent = result * 100;
        
        resultField.innerText = "Answer " + percent + "%";
        event.preventDefault();
    
}});