HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>isExtended detect</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var lines;
    if ("isExtended" in window.screen) {
      lines = [String(window.screen.isExtended), typeof screen.isExtended];
    } else {
      lines = ["isExtended not supported"];
    }
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Screen.isExtended — detect (Try It)