JSFiddle - React, Tailwind, and code Playground

HTML

<table class="myTable">
    <tr class="myClass2">
        <td>Content</td>
        <td>Content</td>
    </tr>
    <tr class="myClass">
        <td>Content</td>
        <td>Content</td>
    </tr>
    <tr class="myClass">
        <td>Content</td>
        <td>Content</td>
    </tr>
    <table>

JavaScript

var c = document.getElementsByClassName('myTable')[0],
    x = c.getElementsByClassName('myClass');

while (x.length) {
    x[0].className = 'otherClass';
}
var y = c.getElementsByClassName('otherClass');
alert(y.length);