JSFiddle - React, Tailwind, and code Playground

HTML

<body>

    <label>Mata in Celcius-grader</label>
    <input id="databox" type="number" /><br /><br />

    <button id="celcius"> Farenheit</button>

</body>

JavaScript

var inData = document.getElementById("databox").value;

var sendCel = document.getElementById("celcius");
sendCel.addEventListener('click', function() { convertFahr(inData); });

function convertFahr(value) {
    var conversion = value * (9/5) + 32;
    alert(conversion);
}