HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>replace document.body</title>
</head>
<body id="oldBodyElement">
  <p>This content will be removed when body is replaced.</p>
  <pre id="out"></pre>
  <script>
    var newBodyElement = document.createElement("body");
    newBodyElement.id = "newBodyElement";
    document.body = newBodyElement;
    var note = document.createElement("p");
    note.textContent = "New body element — old children removed (MDN warning).";
    document.body.appendChild(note);
    var out = document.createElement("pre");
    out.textContent = "document.body.id: " + document.body.id;
    document.body.appendChild(out);
  </script>
</body>
</html>

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Document.body — replace body (Try It)