HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>exitFullscreen detect</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">Detect Fullscreen API</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var canExit = typeof document.exitFullscreen === "function";
      var canEnter =
        typeof Element !== "undefined" &&
        typeof Element.prototype.requestFullscreen === "function";
      var enabled = document.fullscreenEnabled === true;
      document.getElementById("out").textContent = JSON.stringify(
        { canExit: canExit, canEnter: canEnter, enabled: enabled },
        null,
        2
      );
    });
  </script>
</body>
</html>

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Document.exitFullscreen — feature-detect (Try It)