Service workers — web workers in general — don't have direct access to the DOM at all. Instead, have the worker post the information to the main thread, and have code in the main thread update the DOM as appropriate. The theading model for JavaScript on browsers is that there is only one main UI thread (the default one your in-page code runs on), which can access the DOM. The others are walled off from it.
This page and this page both talk about messaging between service workers and clients. Here's a really simple example:
Script in the page loading the service worker:
(function() {
"use strict";
if (!navigator.serviceWorker || !navigator.serviceWorker.register) {
console.log("This browser doesn't support service workers");
return;
}
// Listen to messages from service workers.
navigator.serviceWorker.addEventListener('message', function(event) {
console.log("Got reply from service worker: " + event.data);
});
// Are we being controlled?
if (navigator.serviceWorker.controller) {
// Yes, send our controller a message.
console.log("Sending 'hi' to controller");
navigator.serviceWorker.controller.postMessage("hi");
} else {
// No, register a service worker to control pages like us.
// Note that it won't control this instance of this page, it only takes effect
// for pages in its scope loaded *after* it's installed.
navigator.serviceWorker.register("service-worker.js")
.then(function(registration) {
console.log("Service worker registered, scope: " + registration.scope);
console.log("Refresh the page to talk to it.");
// If we want to, we might do `location.reload();` so that we'd be controlled by it
})
.catch(function(error) {
console.log("Service worker registration failed: " + error.message);
});
}
})();
And in service-worker.js:
self.addEventListener("message", function(event) {
event.source.postMessage("Responding to " + event.data);
});
That relies on event.source, which is supported by current versions of Chrome and Firefox.
Alternately, instead of using event.source, you can send a message to multiple clients of a service worker using self.clients.matchAll; again in service-worker.js:
self.addEventListener("message", function(event) {
self.clients.matchAll().then(all => all.forEach(client => {
client.postMessage("Responding to " + event.data);
}));
});
matchAll accepts some filtering options.
You've said you're having trouble getting it to work. Here's a complete version of what's working for me in Chrome and Firefox:
service-worker.html:
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Service Worker</title>
</head>
<body>
(Look in the console.)
<script>
(function() {
"use strict";
if (!navigator.serviceWorker || !navigator.serviceWorker.register) {
console.log("This browser doesn't support service workers");
return;
}
// Listen to messages from service workers.
navigator.serviceWorker.addEventListener('message', function(event) {
console.log("Got reply from service worker: " + event.data);
});
// Are we being controlled?
if (navigator.serviceWorker.controller) {
// Yes, send our controller a message.
console.log("Sending 'hi' to controller");
navigator.serviceWorker.controller.postMessage("hi");
} else {
// No, register a service worker to control pages like us.
// Note that it won't control this instance of this page, it only takes effect
// for pages in its scope loaded *after* it's installed.
navigator.serviceWorker.register("service-worker.js")
.then(function(registration) {
console.log("Service worker registered, scope: " + registration.scope);
console.log("Refresh the page to talk to it.");
// If we want to, we might do `location.reload();` so that we'd be controlled by it
})
.catch(function(error) {
console.log("Service worker registration failed: " + error.message);
});
}
})();
</script>
</body>
</html>
service-worker.js:
self.addEventListener("message", function(event) {
//event.source.postMessage("Responding to " + event.data);
self.clients.matchAll().then(all => all.forEach(client => {
client.postMessage("Responding to " + event.data);
}));
});
As you can see, that's the version using self.clients.matchAll, with the commented out event.source version above it.
If I run that in two windows, each refresh of each window sends a message to the other windows (because I'm using self.clients.matchAll...).
