Access localStorage from service worker

You cannot access localStorage (and also sessionStorage) from a webworker process, they result will be undefined, this is for security reasons.

You need to use postMessage() back to the Worker's originating code, and have that code store the data in localStorage.

You should use localStorage.setItem() and localStorage.getItem() to save and get data from local storage.

More info:

Worker.postMessage()

Window.localStorage

Pseudo code below, hoping it gets you started:

 // include your worker
 var myWorker = new Worker('YourWorker.js'),
   data,
   changeData = function() {
     // save data to local storage
     localStorage.setItem('data', (new Date).getTime().toString());
     // get data from local storage
     data = localStorage.getItem('data');
     sendToWorker();
   },
   sendToWorker = function() {
     // send data to your worker
     myWorker.postMessage({
       data: data
     });
   };
 setInterval(changeData, 1000)

Broadcast Channel API is easier

There are several ways to communicate between the client and the controlling service worker, but localStorage is not one of them. IndexedDB is, but this might be an overkill for a PWA that by all means should remain slim.

Of all means, the Broadcast Channel API results the easiest. It is by far much easier to implement than above-mentioned postMessage() with the MessageChannel API.

Here is how broadcasting works

Define a new broadcasting channel in both the service worker and the client.

const channel4Broadcast = new BroadcastChannel('channel4');

To send a broadcast message in either the worker or the client:

channel4Broadcast.postMessage({key: value});

To receive a broadcast message in either the worker or the client:

channel4Broadcast.onmessage = (event) => {
    value = event.data.key;
}

I've been using this package called localforage that provides a localStorage-like interface that wraps around IndexedDB. https://github.com/localForage/localForage

You can then import it by placing it in your public directory, so it is served by your webserver, and then calling: self.importScripts('localforage.js'); within your service worker.