backgroundfetch.js

const swReg = await navigator.serviceWorker.ready
if('BackgroundFetchManager' in self) {
  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
  })
}