ServiceWorkerContainer: message event

// in the service worker
async function messageClient(clientId) {
    const client = await clients.get(clientId);
    client.postMessage('Hi client!');
}

addEventListener('fetch', (event) => {
    messageClient(event.clientId);
    event.respondWith(() => {
      // ...
    });
});

// in the page being controlled
navigator.serviceWorker.addEventListener('message', (message) => {
    console.log(message);
});