JSFiddle - React, Tailwind, and code Playground

HTML

<div class="asdf">Found.</div>

JavaScript

function getElementsByClassName(className) {
    var found = [];
    var elements = document.getElementsByTagName("*");
    for (var i = 0; i < elements.length; i++) {
        var names = elements[i].className.split(' ');
        for (var j = 0; j < names.length; j++) {
            if (names[j] == className) found.push(elements[i]);
        }
    }
    return found;
}

var e = getElementsByClassName("asdf");
alert(e[0].innerHTML);