JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="textbox"/>
<br/>
<input type="button" id="calculate" value="Calculate"/>

JavaScript

document.getElementById('calculate').onclick = function() {
    var num = parseFloat(document.getElementById('textbox').value);
    var new_num = Math.round(num).toFixed(3);
    alert(new_num);
};