<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <script>
      window.onmessage = function (event) {
        if (event.data !== "register") {
          return;
        }
        var promise = navigator.serviceWorker.register("worker.js");
        window.onmessage = function (event1) {
          if (event1.data !== "register") {
            return;
          }
          promise = promise.then(function (registration) {
            return navigator.serviceWorker.register("worker2.js");
          });
          window.onmessage = function (event2) {
            if (event2.data !== "unregister") {
              return;
            }
            promise.then(function (registration) {
              registration.unregister();
            });
            window.onmessage = null;
          };
        };
      };
    </script>
  </head>
  <body>
    This is a test page.
  </body>
<html>
