HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>exitFullscreen promise</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} button{margin-right:0.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <button id="enter" type="button">Enter fullscreen</button>
  <button id="exit" type="button">Exit now</button>
  <pre id="out">Ready</pre>
  <script>
    var out = document.getElementById("out");
    function exitNow() {
      if (!document.fullscreenElement) {
        out.textContent = "skip — already windowed";
        return;
      }
      document
        .exitFullscreen()
        .then(function () {
          out.textContent = "exit finished";
        })
        .catch(function (err) {
          out.textContent = "exit failed: " + err;
        });
    }
    document.getElementById("enter").addEventListener("click", function () {
      document.documentElement.requestFullscreen().catch(function (err) {
        out.textContent = "enter failed: " + err.message;
      });
    });
    document.getElementById("exit").addEventListener("click", exitNow);
  </script>
</body>
</html>

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Document.exitFullscreen — then / catch (Try It)