HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>hasFocus events</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;min-height:4rem;}</style>
</head>
<body>
  <p>Click away from this window, then come back. Watch the log update.</p>
  <pre id="out">Waiting for focus/blur...</pre>
  <script>
    var out = document.getElementById("out");
    var lines = [];
    function report(reason) {
      lines.push(reason + " -> hasFocus: " + document.hasFocus());
      if (lines.length > 8) lines.shift();
      out.textContent = lines.join("\n");
    }
    window.addEventListener("focus", function () { report("focus"); });
    window.addEventListener("blur", function () { report("blur"); });
    report("start");
  </script>
</body>
</html>

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Document.hasFocus — focus and blur events (Try It)