JSFiddle - React, Tailwind, and code Playground

HTML

<div class="itemRelated">
    <ul>
        <li class="even">Hi There</li>
        <li class="odd">Whats'up</li>
        <li class="even">BugsBunny</li>

CSS

.cf {
    color:red;
}

JavaScript

var list = [];
var elements = document.getElementsByTagName('li');
for (var i = 0; i < elements.length; ++i) {
    if (elements[i].className == "even" || elements[i].className == "odd") {
        list.push(elements[i]);
    };
}
for (var i = 0; i < list.length; ++i) {
    if (list[i].className.split(' ').indexOf('cf') < 0) {
        list[i].className = list[i].className + ' cf';
    }
}