Send data to the Service Worker

index.html

<button onclick="sendMessage()">Send message</button>
<script>
let se;
const registration = navigator.serviceWorker.register('/sw.js').then(data => {
  se = data.active;
  console.log('ready');
});


function sendMessage() {
  se.postMessage('message from client');
}
</script>

sw.js

self.addEventListener('message', ({data}) => {
  console.log(data);
});