HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>document.open side effects</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} iframe{width:100%;height:120px;border:1px solid #ccc;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <button id="btn" type="button">Demo clear side effect</button>
  <iframe id="preview" title="Preview"></iframe>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var doc = document.getElementById("preview").contentDocument;
      doc.open();
      doc.write("<p id='old'>Old content</p>");
      doc.close();
      var before = !!doc.getElementById("old");
      doc.open();
      var afterOpen = !!doc.getElementById("old");
      doc.write("<p>New content</p>");
      doc.close();
      document.getElementById("out").textContent = JSON.stringify({
        before: before,
        afterOpen: afterOpen
      }, null, 2);
    });
  </script>
</body>
</html>

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Document.open — clears existing nodes (Try It)