JSFiddle - React, Tailwind, and code Playground

HTML

<select class="box-select" name="level" id="level" onchange="update(this)">
    <option value="5">Level 5</option>
    <option value="10">Level 10</option>
</select>
<!-- a lot of code -->
 <span class="box-label" id="pricep">0 USD</span>

JavaScript

$(function () {
    function update(select)
    {
        // select comes from 'this'
        //Ali
        var a;
        if(select.value = "5")
        {
            a = 0.1;
        }

        if(select.value = "10")
        {
            a = 0.35;
        }

        var y;
        var x = data.value;
        y = a * x;
        document.getElementById("pricep").innerHTML = "$"+y;
    }
});