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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>focus() — event</title>
</head>
<body>
  <h1>window focus / blur events</h1>
  <p>Switch to another tab, then come back.</p>
  <pre id="out">Listening...</pre>
  <script>
    var out = document.getElementById("out");
    window.addEventListener("focus", function () {
      out.textContent = "Window gained focus";
    });
    window.addEventListener("blur", function () {
      out.textContent = "Window lost focus";
    });
  </script>
</body>
</html>

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JavaScript Window focus() — focus event (Try It)