HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>subscription UI</title>
</head>
<body>
  <button id="pushBtn" disabled>—</button>
  <button id="run">Check on load</button>
  <pre id="out">Click Check (needs HTTPS or localhost).</pre>
  <script>
    async function updatePushButton() {
      var btn = document.getElementById("pushBtn");
      var out = document.getElementById("out");
      if (!window.isSecureContext || !("serviceWorker" in navigator)) {
        out.textContent = "Need secure context + serviceWorker.";
        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" }));
      try {
        await navigator.serviceWorker.register(url);
        var registration = await navigator.serviceWorker.ready;
        var subscription = await registration.pushManager.getSubscription();
        if (subscription) {
          btn.textContent = "Push Enabled";
          btn.disabled = false;
          out.textContent = subscription.endpoint;
        } else {
          btn.textContent = "Enable Push";
          btn.disabled = false;
          out.textContent = "Not subscribed";
        }
      } catch (err) {
        out.textContent = "Error: " + err.message;
      }
      URL.revokeObjectURL(url);
    }
    document.getElementById("run").addEventListener("click", updatePushButton);
  </script>
</body>
</html>

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getSubscription() — page-load UI (Try It)