`getElementsByTagName()` is a method which exists on the Document and Element objects and returns an HTMLCollection of descendant elements within the Document/Element given a tag name.

getElementsByTagName.ts

export default function getElementsByTagName(
  el: Element,
  tagName: string,
): Array<Element> {
    const elements: Array<Element> = [];

  function traverseNode(el: Node) {
    const nodes = el.childNodes;
    nodes.forEach((node) => {
      if (node.nodeName === tagName.toUpperCase()) {
        elements.push(node as Element);
      }
      if (node.hasChildNodes()) {
        traverseNode(node);
      }
    });
  }
  traverseNode(el);
  return elements;
}

const document = new DOMParser().parseFromString(
  `<div id="foo">
    <span>Span</span>
    <p>Paragraph</p>
    <div id="bar">Div</div>
  </div>`,
  'text/html',
);

getElementsByTagName(document.body, 'div');
// [div#foo, div#bar] <-- This is an array of elements.