[PWA] Cache Third Party Resources from a CDN in a React PWA

Our service worker caches our static assets - but only those assets that are included in our React App. This means that assets like Bootstrap, which we're including from a third-party CDN, won't be included in the cache, because they're not available at webpack compile time.

We'll change the service worker to add caching for those assets by using workbox's registerRoutemethod, and a regular expression to match the entire URL of the asset. We'll pick the staleWhileRevalidate cache strategy, though cacheFirst, or networkFirst would also be valid choices.

Finally, we'll change the name of the cache they are stored by supplying a cacheName parameter.

For some CDN files we also want to cache them:

    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">

    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>

To do that, we can update our sw.js config:

workbox.skipWaiting();
workbox.clientsClaim();

// Cache the files from CDN
workbox.routing.registerRoute(
    new RegExp('https:.*min.(css|js)'),
    workbox.strategies.staleWhileRevalidate()
  )

// Cache all static files
workbox.precaching.precacheAndRoute(self.__precacheManifest || [])

It's very important when registering a route that it matches the entire URL of the resource that we're trying to cache, otherwise it won't work.

The second argument to register a route tells workbox what strategy to use when caching that resource. Common strategies could be cache first or network first. For Bootstrap we'll use staleWhileRevalidate. This will serve Bootstrap as first as possible from the cache first. Then update the cache in the background by making the network call also. We can also give a cache name so that we can indentify from the Dev tool.

workbox.skipWaiting();
workbox.clientsClaim();

workbox.routing.registerRoute(
    new RegExp('https:.*min.(css|js)'),
    workbox.strategies.staleWhileRevalidate({
        cacheName: 'cdn-cache'
    })
  )

workbox.precaching.precacheAndRoute(self.__precacheManifest || [])