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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>open() — named window</title>
</head>
<body>
  <button type="button" id="helpBtn">Open help (click twice)</button>
  <pre id="out"></pre>
  <script>
    document.getElementById("helpBtn").onclick = function () {
      var help = window.open("", "AppHelp", "width=500,height=400");
      if (help) {
        if (help.document.body && help.document.body.innerHTML === "") {
          help.document.write("<h1>Help</h1><p>Singleton help window.</p>");
          help.document.close();
          document.getElementById("out").textContent = "Help window opened.";
        } else {
          help.focus();
          document.getElementById("out").textContent = "Help window focused (reused).";
        }
      } else {
        document.getElementById("out").textContent = "Popup blocked.";
      }
    };
  </script>
</body>
</html>

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JavaScript open() — named window reuse (Try It)