HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>id empty case</title>
</head>
<body>
  <p>Plain paragraph</p>
  <pre id="out"></pre>
  <script>
    var plain = document.querySelector("p");
    var lines = [];
    lines.push(JSON.stringify(plain.id));
    plain.id = "Note";
    lines.push(String(document.getElementById("note")));
    lines.push(String(document.getElementById("Note") === plain));
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.id — empty & case (Try It)