HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fullscreenchange toggle</title>
  <style>
    #fs { padding: 24px; border: 2px solid #333; background: #f5f5f5; }
    #fs:fullscreen { background: #222; color: #fff; }
  </style>
</head>
<body>
  <div id="fs">
    <p>Fullscreen demo box</p>
    <button type="button" id="toggle-fullscreen">Toggle Fullscreen Mode</button>
  </div>
  <pre id="log">(click the button)</pre>
  <script>
    var el = document.getElementById("fs");
    var log = document.getElementById("log");
    function fullscreenchangeHandler() {
      if (document.fullscreenElement) {
        log.textContent =
          "Element: " + document.fullscreenElement.id + " entered fullscreen mode.";
      } else {
        log.textContent = "Leaving fullscreen mode.";
      }
    }
    el.addEventListener("fullscreenchange", fullscreenchangeHandler);
    document.getElementById("toggle-fullscreen").addEventListener("click", function () {
      if (document.fullscreenElement) {
        document.exitFullscreen();
      } else {
        el.requestFullscreen();
      }
    });
  </script>
</body>
</html>

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Element fullscreenchange — toggle (Try It)