HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>width reserved</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var reserved = screen.width - screen.availWidth;
    document.getElementById("out").textContent = [
      "full: " + screen.width,
      "avail: " + screen.availWidth,
      "reserved: " + reserved
    ].join("\n");
  </script>
</body>
</html>

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Screen.width — reserved space (Try It)