HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>importNode deep</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <button id="btn" type="button">Compare deep flag</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var otherDoc = document.implementation.createHTMLDocument("");
      var wrap = otherDoc.createElement("div");
      wrap.innerHTML = "<strong>Hi</strong> there";
      var shallow = document.importNode(wrap, false);
      var deep = document.importNode(wrap, true);
      document.getElementById("out").textContent =
        "shallow children: " + shallow.childNodes.length +
        "\ndeep text: " + deep.textContent;
    });
  </script>
</body>
</html>

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Document.importNode — deep true vs false (Try It)