Inténtalo tú mismo - JavaScript Reference nodelist_length

<!DOCTYPE html>
<html>
<body>

<p>Click the button get the node names of the body element's child nodes.</p>

<button onclick="myFunction()">Try it</button>

<p><strong>Note:</strong> Whitespace inside elements is considered as text, and text
is considered as nodes.</p>

<p id="demo"></p>

<script>
function myFunction() {
   var nodelist = document.body.childNodes;

   var txt = "";
   var i;
   for (i = 0; i < nodelist.length; i++) {
   txt = txt + nodelist[i].nodeName + "<br>";
   }

   document.getElementById("demo").innerHTML = txt;
}
</script>

</body>
</html>