Heading with a span element.
A paragraph with one, two spans.

function byTagName(node, tagName){
	var found = [];
	tagName = tagName.toUpperCase();
	
function explore(node){
  for(var i = 0; i < node.childNodes.length; i++)
	{
	 var child = node.childNodes[i];
	  if(child.nodeType == document.ELEMENT_NODE)
		{
		  if(child.nodeName == tagName)
			  found.push(child);
			   explore(child);
		}
	}
}
explore(node);
return found;
}

console.log(byTagName(document.body, "h1").length);
// -> 1
console.log(byTagName(document.body, "span").length);
//->3
var para = document.querySelector("p");
console.log(byTagName(para, "span").length);
//->2