HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createAttribute MDN</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} #div1{border:1px solid #ccc;padding:1rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <div id="div1">Target div</div>
  <button id="btn">Create and attach attribute</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var node = document.getElementById("div1");
      var a = document.createAttribute("my_attrib");
      a.value = "newVal";
      node.setAttributeNode(a);
      document.getElementById("out").textContent = node.getAttribute("my_attrib");
    });
  </script>
</body>
</html>

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Document.createAttribute — MDN basic (Try It)