HTML Code

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

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Screen.availWidth — with availHeight (Try It)