HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>exitFullscreen change</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} button{margin-right:0.5rem;} #stage{border:1px solid #ccc;padding:1rem;margin:0.75rem 0;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <div id="stage">Fullscreen stage</div>
  <button id="enter" type="button">Enter</button>
  <button id="exit" type="button">Exit</button>
  <pre id="status">…</pre>
  <script>
    var status = document.getElementById("status");
    var stage = document.getElementById("stage");
    function updateStatus() {
      status.textContent = document.fullscreenElement
        ? "fullscreen: " + document.fullscreenElement.tagName
        : "windowed";
    }
    document.addEventListener("fullscreenchange", updateStatus);
    updateStatus();
    document.getElementById("enter").addEventListener("click", function () {
      stage.requestFullscreen().catch(function (err) {
        status.textContent = "enter failed: " + err.message;
      });
    });
    document.getElementById("exit").addEventListener("click", function () {
      if (document.fullscreenElement) {
        document.exitFullscreen().catch(console.error);
      }
    });
  </script>
</body>
</html>

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Document.exitFullscreen — fullscreenchange (Try It)