bgfetch.js

const swReg = await navigator.serviceWorker.ready
const bgFetchRegistration = await swReg.backgroundFetch.fetch('postr-download', ['image.png'], {
  title: 'Postr.me is capturing',
  icons: [{
    sizes: '512x512',
    src: '/assets/img/icon-512.png',
    type: 'image/png'
  }],
  downloadTotal: 60 * 1024 * 1024
})

bgFetchRegistration.addEventListener('progress', event => {
  fetchProgress = event.currentTarget;
  progressStatus.innerHTML = `Progress: downloaded ${bytesToSize(
    fetchProgress.downloaded
  )}  from ${bytesToSize(fetchProgress.downloadTotal)} (${Math.round(
    (fetchProgress.downloaded * 100) / fetchProgress.downloadTotal
  )}%)`;
});