HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>exitFullscreen await</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">Stage — enter fullscreen, then Exit.</div>
  <button id="enter" type="button">Enter</button>
  <button id="exit" type="button">Exit</button>
  <pre id="out">Ready</pre>
  <script>
    var out = document.getElementById("out");
    var stage = document.getElementById("stage");
    async function leaveFullscreen() {
      if (!document.fullscreenElement) {
        out.textContent = "not in fullscreen";
        return;
      }
      await document.exitFullscreen();
      out.textContent = "exited";
    }
    document.getElementById("enter").addEventListener("click", function () {
      if (!stage.requestFullscreen) {
        out.textContent = "requestFullscreen missing";
        return;
      }
      stage.requestFullscreen().then(function () {
        out.textContent = "entered";
      }).catch(function (err) {
        out.textContent = "enter failed: " + err.message;
      });
    });
    document.getElementById("exit").addEventListener("click", function () {
      leaveFullscreen().catch(function (err) {
        out.textContent = String(err);
      });
    });
  </script>
</body>
</html>

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Document.exitFullscreen — guard + await (Try It)