service-worker-prefetch.js

const SITEMAP_CACHE_KEY = 'sitemap-cached-pages-v1'

self.addEventListener('install', event => {
  event.waitUntil(cacheSitemap())
})

const cacheSitemap = _ => caches.open(SITEMAP_CACHE_KEY)
  .then(cache => 
    cacheSitemap()
      .then(urls => urls.map(u => u.toString()))
      .then(urls => cache.addAll(urls))
  )

const fetchSitemap = _ => new Promise((resolve, reject) => {
  fetch('./sitemap.xml')
    .then(res => res.text())
    .then(sitemap => resolve(extractUrlsFromSitemap(sitemap)))
    .catch(reject)
})

const extractUrlsFromSitemap = sitemap => {
  const parser = new DOMParser().parseFromString(sitemap, "text/xml")
  return Array.prototype.slice.call(parser.querySelectorAll('url>loc'))
    .map(loc => new URL(loc.textContent))
}