JSFiddle - React, Tailwind, and code Playground

HTML

<table class="table table-striped" id="res">
        <tr>
            <th>Category</th>
            <th>Amount</th>
            <th>Type</th>
            <th>Description</th>
        </tr>

            <tr>
                <td>a</td>
                <td class="countable">
                111
                </td>
                <td>aaa</td>
            </tr>
            <tr>
                <td>b</td>
                <td class="countable">
                222
                </td>
                <td>bbb</td>
            </tr>

    </table>

JavaScript

var cls = document.getElementById("res").getElementsByTagName("td");
    var sum = 0;
    for (var i = 0; i < cls.length; i++){
        if(cls[i].className == "countable"){
            sum += isNaN(cls[i].innerHTML) ? 0 : parseInt(cls[i].innerHTML);
        }
    }
    alert('sum is ' + sum);