recursiveGetElementByClassName.js

function getElementsByClassName(className) {
    const results = [];

    const checkRecursive = (element, cName) => {
        const cn = element.getAttribute("class");
        if (cn !== undefined && cn !== null && cn.includes(cName)) {
            results.push(element);
        }
				// element.children MDN
        if (element.children.length > 0) {
            for (const e of element.children) {
                checkRecursive(e, cName);
            }
        }
    };

  	checkRecursive(document.body, className);
    return results;
}

console.log(getElementsByClassName("pikachu").length); // 14