HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fullscreenElement toggle</title>
  <style>
    #stage { padding: 1rem; background: #0f172a; color: #f8fafc; }
  </style>
</head>
<body>
  <div id="stage">Fullscreen stage</div>
  <p><button id="toggle" type="button">Toggle</button></p>
  <pre id="out">Click Toggle...</pre>
  <script>
    var stage = document.getElementById("stage");
    var out = document.getElementById("out");
    document.getElementById("toggle").addEventListener("click", function () {
      var p = !document.fullscreenElement
        ? stage.requestFullscreen()
        : document.exitFullscreen();
      Promise.resolve(p).then(function () {
        out.textContent = "now: " + (document.fullscreenElement
          ? document.fullscreenElement.id
          : "null");
      }).catch(function (err) {
        out.textContent = String(err);
      });
    });
  </script>
</body>
</html>

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Toggle via fullscreenElement (Try It)