HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createAttributeNS viewBox</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} svg{border:1px solid #ccc;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <svg id="svg" width="120" height="120"></svg>
  <button id="btn">Set viewBox via createAttributeNS</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var svg = document.getElementById("svg");
      var attr = document.createAttributeNS("", "viewBox");
      attr.value = "0 0 100 100";
      svg.setAttributeNode(attr);
      document.getElementById("out").textContent = svg.getAttribute("viewBox");
    });
  </script>
</body>
</html>

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Document.createAttributeNS — SVG viewBox (Try It)