service-worker.js

import {
  timestamp, files, shell 
} from '@sapper/service-worker'

const ASSETS = `cache${timestamp}`

// `shell` is an array of all the files generated by the bundler,
// `files` is an array of everything in the `static` directory
const to_cache = shell.concat(files)
const staticAssets = new Set(to_cache)

self.addEventListener('install', event => {
  event.waitUntil(
    caches
      .open(ASSETS)
      .then(cache => cache.addAll(to_cache))
      .then(() => {
        self.skipWaiting()
      })
  )
})

self.addEventListener('activate', event => {
  event.waitUntil(
    caches.keys().then(async keys => {
      // delete old caches
      for (const key of keys) {
        if (key !== ASSETS) await caches.delete(key)
      }

      self.clients.claim()
    })
  )
})


/**
 * Fetch the asset from the network and store it in the cache. 
 * Fall back to the cache if the user is offline.
 */
async function fetchAndCache(request) {
  const cache = await caches.open(`offline${timestamp}`)

  try {
    const response = await fetch(request)
    cache.put(request, response.clone())
    return response
  } catch (err) {
    const response = await cache.match(request)
    if (response) return response

    throw err
  }
}

self.addEventListener('fetch', event => {
  if (event.request.method !== 'GET' || event.request.headers.has('range')) return

  const url = new URL(event.request.url)

  // don't try to handle e.g. data: URIs
  const isHttp = url.protocol.startsWith('http')
  const isDevServerRequest = url.hostname === self.location.hostname && url.port !== self.location.port
  const isStaticAsset = url.host === self.location.host && staticAssets.has(url.pathname)
  const skipBecauseUncached = event.request.cache === 'only-if-cached' && !isStaticAsset

  if (isHttp && !isDevServerRequest && !skipBecauseUncached) {
    event.respondWith(
      (async () => {
        // always serve static files and bundler-generated assets from cache.
        // if your application has other URLs with data that will never change,
        // set this variable to true for them and they will only be fetched once.
        const cachedAsset = isStaticAsset && await caches.match(event.request)

        // for pages, you might want to serve a shell `service-worker-index.html` file,
        // which Sapper has generated for you. It's not right for every
        // app, but if it's right for yours then uncomment this section
        /*
				if (!cachedAsset && url.origin === self.origin && routes.find(route => route.pattern.test(url.pathname))) {
					return caches.match('/service-worker-index.html');
				}
				*/

        return cachedAsset || fetchAndCache(event.request)
      })()
    )
  }
})