HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>event + Promise</title>
  <style>
    #game { width: 320px; height: 140px; border: 2px solid #333; background: #eee; display: flex; align-items: center; justify-content: center; }
  </style>
</head>
<body>
  <div id="game">
    <button type="button" id="go">Request pointer lock</button>
  </div>
  <pre id="out">(waiting)</pre>
  <script>
    var el = document.getElementById("game");
    var out = document.getElementById("out");
    document.addEventListener("pointerlockerror", function () {
      out.textContent = "Event: pointerlockerror (on document)";
    });
    document.getElementById("go").addEventListener("click", function () {
      var maybe = el.requestPointerLock();
      if (maybe && typeof maybe.catch === "function") {
        maybe.catch(function (err) {
          out.textContent =
            "Promise rejected: " + (err && err.message ? err.message : String(err));
        });
      }
    });
  </script>
</body>
</html>

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Document pointerlockerror — event + Promise (Try It)