HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>onfullscreenchange</title>
  <style>
    #fs { padding: 24px; border: 2px solid #333; }
  </style>
</head>
<body>
  <div id="fs">
    <button type="button" id="toggle">Toggle</button>
  </div>
  <pre id="status">Fullscreen OFF</pre>
  <script>
    var el = document.getElementById("fs");
    var status = document.getElementById("status");
    el.onfullscreenchange = function () {
      status.textContent = document.fullscreenElement
        ? "Fullscreen ON"
        : "Fullscreen OFF";
    };
    document.getElementById("toggle").addEventListener("click", function () {
      if (document.fullscreenElement) {
        document.exitFullscreen();
      } else {
        el.requestFullscreen();
      }
    });
  </script>
</body>
</html>

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Element fullscreenchange — onfullscreenchange (Try It)