HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>tagName compare</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var btn = document.createElement("button");
    var lines = [];
    lines.push("=== \"BUTTON\": " + (btn.tagName === "BUTTON"));
    lines.push("=== \"button\": " + (btn.tagName === "button"));
    lines.push("localName === \"button\": " + (btn.localName === "button"));
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.tagName — safe compare (Try It)