HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>hasFocus MDN UI</title>
  <style>body{padding:1rem;background:gray;text-align:center;font:1.25rem sans-serif;} button{margin-top:0.75rem;}</style>
</head>
<body>
  <p id="log">Focus check results are shown here.</p>
  <button id="newWindow" type="button">Open new window</button>
  <script>
    var body = document.querySelector("body");
    var log = document.getElementById("log");
    function checkDocumentFocus() {
      if (document.hasFocus()) {
        log.textContent = "This document has focus.";
        body.style.background = "white";
      } else {
        log.textContent = "This document does not have focus.";
        body.style.background = "gray";
      }
    }
    function openWindow() {
      window.open("about:blank", "demo", "width=420,height=240,left=150,top=150");
    }
    document.getElementById("newWindow").addEventListener("click", openWindow);
    setInterval(checkDocumentFocus, 300);
    checkDocumentFocus();
  </script>
</body>
</html>

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