https://eloquentjavascript.net/14_dom.html#i_VSftnyRTsV

Eloquent JavaScript 1402 Elements by tag name.html

<h1>Heading with a <span>span</span> element.</h1>
<p>A paragraph with <span>one</span>, <span>two</span>
  spans.</p>

<script>
  function byTagName(node, tagName) {
    const arr = [];
    Array.from(node.children).forEach((child) => {
      if (child.nodeName.toLowerCase() === tagName) {
      	arr.push(child);
      }
      arr.push(...byTagName(child, tagName))
    });
    
    return arr;
  }

  console.log(byTagName(document.body, "h1").length);
  // → 1
  console.log(byTagName(document.body, "span").length);
  // → 3
  let para = document.querySelector("p");
  console.log(byTagName(para, "span").length);
  // → 2
</script>