HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fullscreenchange</title>
  <style>
    #stage { padding: 1rem; background: #1e293b; color: #fff; }
  </style>
</head>
<body>
  <div id="stage">Stage</div>
  <p><button id="go" type="button">Enter / exit</button></p>
  <pre id="out">Waiting for fullscreenchange...</pre>
  <script>
    var stage = document.getElementById("stage");
    var out = document.getElementById("out");
    document.addEventListener("fullscreenchange", function () {
      out.textContent =
        "fullscreenchange → " + (document.fullscreenElement !== null);
    });
    document.getElementById("go").addEventListener("click", function () {
      if (!document.fullscreenElement) {
        stage.requestFullscreen().catch(function (e) { out.textContent = e; });
      } else {
        document.exitFullscreen().catch(function (e) { out.textContent = e; });
      }
    });
  </script>
</body>
</html>

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fullscreenchange events (Try It)