JSFiddle - React, Tailwind, and code Playground

HTML

<table id="myTable">
    <tr><th>A</th></tr>
    <tr><td>B</td></tr>
    <tr><td>C</td></tr>
    <tr><td>D</td></tr>
</table>

JavaScript

var rows = document.getElementById('myTable').getElementsByTagName('tr');
console.log(rows);
var count = 0;
for(var i = 0; i < rows.length; i++)
{    
    if (rows[i].getElementsByTagName('th').length == 0)
        count++;
}
alert(count);