index.js

const CACHE_NAME = '$$$toolbox-cache$$$http://localhost:5000/$$$'

function linkIsAvailableOffline (links) {
  links.forEach(link => {
    const HREF = link.getAttribute('href')

    caches.open(CACHE_NAME).then(cache => {
      cache.match(HREF).then(response => {
        link.setAttribute('data-offline', response ? true : false)
      })
    })
  })
}