get first element

a.js

document.body.getElementsByTagName("*")[0]

b.js

document.body.querySelector("*")

c.js

var it = document.createNodeIterator(document.body, NodeFilter.SHOW_ELEMENT);
it.nextNode();
it.nextNode();

common.html

<p>first</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>
<p>aaa</p>

d.js

function getFirstChild (root) {
  var nodes = root.childNodes;
  for (var i = 0, it; it = nodes[i]; i++) {
    if (it.nodeType === Node.ELEMENT_NODE) return it;
  }
}
getFirstChild(document.body);

e.js

document.body.firstElementChild;