Client.postMessage()

client.postMessage(message[, transfer]);
client.postMessage(message[, { transfer }]);

addEventListener('fetch', event => {
  event.waitUntil(async function() {
    // Exit early if we don't have access to the client.
    // Eg, if it's cross-origin.
    if (!event.clientId) return;

    // Get the client.
    const client = await clients.get(event.clientId);
    // Exit early if we don't get the client.
    // Eg, if it closed.
    if (!client) return;

    // Send a message to the client.
    client.postMessage({
      msg: "Hey I just got a fetch from you!",
      url: event.request.url
    });

  }());
});

navigator.serviceWorker.addEventListener('message', event => {
  console.log(event.data.msg, event.data.url);
});