HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>contains null</title>
</head>
<body>
  <div id="host">Host</div>
  <pre id="out"></pre>
  <script>
    var host = document.getElementById("host");
    var missing = document.getElementById("nope");
    document.getElementById("out").textContent =
      host.contains(null) + "\n" +
      host.contains(missing);
  </script>
</body>
</html>

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Node.contains — null is false (Try It)