JSFiddle - React, Tailwind, and code Playground

HTML

<table class='groups'>
    <tr class="odd"><td>1</td></tr>
    <tr class="odd"><td>2</td></tr>
    <tr class="odd"><td>3</td></tr>
    <tr class="even"><td>4</td></tr>
    <tr class="even"><td>5</td></tr>
    <tr class="even"><td>6</td></tr>
    <tr class="odd"><td>7</td></tr>
    <tr class="odd"><td>8</td></tr>
    <tr class="odd"><td>9</td></tr>
</table>
<button>Detect</button>

JavaScript

$('button').click(function() {
    var lastGroup = ''
    $('.groups tr').each(function() {
        if(lastGroup != this.className) {
            $(this).css('background-color', 'red');
            lastGroup = this.className;
        }
    });
});