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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>attributes live</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var el = document.createElement("div");
    document.body.insertBefore(el, document.getElementById("out"));
    var map = el.attributes;
    var lines = [];
    lines.push(String(map.length));
    el.setAttribute("data-role", "card");
    lines.push(String(map.length));
    lines.push(map.getNamedItem("data-role").value);
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.attributes — live map (Try It)