onmessage = function(e) {
  clients.matchAll().then(function(c) {
    if (c.length === 0) {
      // We cannot proceed.
      return;
    }

    registration
      .unregister()
      .then(
        function() {
          c[0].postMessage("DONE");
        },
        function() {
          c[0].postMessage("ERROR");
        }
      )
      .then(function() {
        c[0].postMessage("FINISH");
      });
  });
};