HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fullscreen no-op</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    "use strict";
    var before = document.fullscreen;
    document.fullscreen = true;
    var after = document.fullscreen;
    document.getElementById("out").textContent =
      "before: " + before + "\n" +
      "after: " + after + "\n" +
      "changed: " + (before !== after);
  </script>
</body>
</html>

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document.fullscreen no-op setter (Try It)