HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>insertBefore null</title>
</head>
<body>
  <ul id="parent"><li>One</li></ul>
  <pre id="out"></pre>
  <script>
    var parent = document.getElementById("parent");
    var item = document.createElement("li");
    item.textContent = "Last";
    parent.insertBefore(item, null);
    document.getElementById("out").textContent =
      parent.lastElementChild.textContent;
  </script>
</body>
</html>

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Node.insertBefore — null appends (Try It)