HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>whenActivated</title>
</head>
<body>
  <pre id="out">(waiting)</pre>
  <script>
    function whenActivated(fn) {
      if ("prerendering" in document && document.prerendering) {
        document.addEventListener("prerenderingchange", fn, { once: true });
      } else {
        fn();
      }
    }
    whenActivated(function () {
      document.getElementById("out").textContent =
        "Safe to start analytics / ads now";
    });
  </script>
</body>
</html>

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Document prerenderingchange — whenActivated helper (Try It)