sw.js

this.addEventListener('install', function(event) {
  event.waitUntil(preLoad())
})

var preLoad = function() {
  console.log('Installing web app')
  return caches.open('offline').then(function(cache) {
    console.log('caching index and important routes')
    return cache.addAll(['/burner/'])
  })
}

this.addEventListener('fetch', function(event) {
  event.respondWith(
    checkResponse(event.request).catch(function() {
      return returnFromCache(event.request)
    }),
  )
  event.waitUntil(addToCache(event.request))
})

var checkResponse = function(request) {
  return new Promise(function(fulfill, reject) {
    fetch(request).then(function(response) {
      if (response.status !== 404) {
        fulfill(response)
      } else {
        reject()
      }
    }, reject)
  })
}

var addToCache = function(request) {
  return caches.open('offline').then(function(cache) {
    return fetch(request).then(function(response) {
      console.log(response.url + ' was cached')
      return cache.put(request, response)
    })
  })
}

var returnFromCache = function(request) {
  return caches.open('offline').then(function(cache) {
    return cache.match(request).then(function(matching) {
      if (!matching || matching.status == 404) {
        return cache.match('index.html')
      } else {
        return matching
      }
    })
  })
}