JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<tbody>
   <tr><td class="classone">1234</td></tr>
   <tr><td class="classtwo">1234</td></tr>
   <tr><td class="classthree">1234</td></tr>
</tbody>
</table>


<table>
<tbody>
   <tr><td class="classone">1234</td></tr>
   <tr><td class="classtwo">1234</td></tr>
   <tr><td class="classthree">1234</td></tr>
</tbody>
</table>

JavaScript

alert(sumByClass('classone'));

function sumByClass(classname){
    var els;
    var i, sum = 0;

    if(document.getElementsByClassName) // Modern
        els = document.getElementsByClassName(classname);
    else if(document.querySelectorAll) // IE 8
        els = document.querySelectorAll('.' + classname);

    for(i = els.length; i--;)
        sum += parseInt(els[i].innerText || els.textContent, 10)

    return sum;
}