Using JS serviceWorker as identity proxy - trying to detect when `access_token` is lost

I am evaluating using a JS ServiceWorkers as an identity proxy, injecting the access_token on fetch() calls using the javascript below:

const addAuthHeader = function (event) {
    destURL = new URL(event.request.url);
    if (whitelistedOrigins.includes(destURL.origin) && whitelistedPathRegex.test(destURL.pathname)) {
        const modifiedHeaders = new Headers(event.request.headers);
        if (token) {
           modifiedHeaders.append('Authorization', token) //< Injection
        const authReq = new Request(event.request, {headers: modifiedHeaders, mode: 'cors' });
        event.respondWith((async () => fetch(authReq))());

// Intercept all fetch requests and add the auth header
self.addEventListener('fetch', addAuthHeader);

In this example the token is stored in a variable within the serviceWorker class. Click here for more information about this approach.

One problem I am running into is that seemingly randomly something happens to the serviceWorker and the token variable loses its value, and the access_token is lost.

Is there a way to detect that the serviceWorker has been updated? Or, to protect the token variable? Is there a design pattern/standard you can point me towards related to using serviceWorker as an identity proxy as I have done?