HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>registrations detect</title>
</head>
<body>
  <button id="run">Detect registrations()</button>
  <pre id="out">Click Detect.</pre>
  <script>
    function legacyRegistrations(pushManager) {
      if (pushManager && typeof pushManager.registrations === "function") {
        return pushManager.registrations();
      }
      if (navigator.push && typeof navigator.push.registrations === "function") {
        return navigator.push.registrations();
      }
      return null;
    }
    document.getElementById("run").addEventListener("click", function () {
      var out = document.getElementById("out");
      if (!window.isSecureContext || !("serviceWorker" in navigator)) {
        out.textContent = legacyRegistrations(null) ? "found" : "null — use getSubscription()";
        return;
      }
      var sw = "self.addEventListener('install',function(e){self.skipWaiting();});self.addEventListener('activate',function(e){e.waitUntil(self.clients.claim());});";
      var url = URL.createObjectURL(new Blob([sw], { type: "application/javascript" }));
      navigator.serviceWorker.register(url).then(function () {
        return navigator.serviceWorker.ready;
      }).then(function (registration) {
        var req = legacyRegistrations(registration.pushManager);
        out.textContent = req ? "registrations() available (legacy)" : "null — use getSubscription()";
        URL.revokeObjectURL(url);
      }).catch(function (err) {
        out.textContent = "Error: " + err.message;
        URL.revokeObjectURL(url);
      });
    });
  </script>
</body>
</html>

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registrations() — feature detect (Try It)