JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap">
            <h1 class="heading">
                Internet speed converter
            </h1>
            <h3 class="subheading">Megabyte !== Megabit</h3>
            <form class="form" name="units" method="post" id="form">
                <div class="inputs">
                    <input type="text" name="from" id="from">
                    <select name="fromWhat" id="fromWhat">
                        <option>Convert from</option>
                        <option value="1024">Kilobits</option>
                        <option value="1048576">Megabits</option>
                        <option value="1073741824">Gigabits</option>
                    </select>
                    <br>
                    <input type="text" name="to" id="to" readonly="readonly">
                    <select id="toWhat" name="toWhat">
                        <option>Convert to</option>
                        <option value="8192">Kilobytes</option>
                        <option value="8388608">Megabytes</option>
                        <option value="8589934592">Gigabytes</option>
                    </select>
                </div>
                <br>
                <input class="bttn" id="calc" type="button" value="Convert">
            </form>
        </div>

JavaScript

var converter = function() {
                var from = document.units.elements.from,
                fromWhat = document.units.elements.fromWhat,
                to = document.units.elements.to,
                toWhat = document.units.elements.toWhat;
 
                if (fromWhat.value === "Convert from" && toWhat.value === "Convert to") {
                        alert("Please choose what you want to convert, and what you want it be converted to.");
                        } else if (fromWhat.value === "Convert from") {
                                alert("Please choose what you want to convert.");
                        } else if (toWhat.value === "Convert to") {
                                alert("Please choose what you want to convert to.");
                        } else if ((isNaN(from.value) === true) || (isNaN(to.value) === true) || (from === null) || (from.value === undefined)) {
                                alert("Please enter a number.");
                        } else {
                        to.value = from.value*fromWhat.value/toWhat.value;   
        
    }
    return false;
};

$("#calc").click(converter);