workbox offline response from IDB instead of cache

I am building an vueJs application with a service worker. I decided to use Workbox with an InjestManifest method to had my own routes.

on fetch when online : 1- answer with the network 2- wrtting body to IDB (through localforage) 3- send back the response

here everything is working perfectly, the sw intercepts the fetch and come back with an appropirate response, IDB contains rigth details. response sent back to fecth when online: Response {type: "cors", url: "http://localhost:3005/api/events", redirected: false, status: 200, ok: true, …}

the issue is when I go offline. my intention id to connect to Locaforage and retrieve the content and build a response. The issue is that this response is not considered as appropriate by Fetch who then reject it. Console.log confirms that the .catch in sw is working but it looks like the response it sends is rejected. here is the console.log of the response I am sending back to fetch when offline; Response {type: "default", url: "", redirected: false, status: 200, ok: true, …}

I do not know if fetch is not happy becasue the url of the repsonse is not the same as on the request but workbox is supposed to allow responding with other resposnes than the ones coming from cache or fetch.

here is the code

importScripts('localforage.min.js')

localforage.config({
  name: 'Asso-corse'
})
workbox.skipWaiting()
workbox.clientsClaim()

workbox.routing.registerRoute(
  new RegExp('https://fonts.(?:googleapis|gstatic).com/(.*)'),
  workbox.strategies.cacheFirst({
    cacheName: 'googleapis',
    plugins: [
      new workbox.expiration.Plugin({
        maxEntries: 30
      })
    ]
  })
)
workbox.routing.registerRoute( new RegExp('http://localhost:3005/api/'), function (event) {
  fetch(event.url)
    .then((response) => {
      var cloneRes = response.clone()
      console.log(cloneRes)
      cloneRes.json()
      .then((body) => {
        localforage.setItem(event.url.pathname, body)
      })
      return response
    })
    .catch(function (error) {
      console.warn(`Constructing a fallback response, due to an error while fetching the real response:, ${error}`)
        localforage.getItem(event.url.pathname)
        .then((res) => {
         let payload = new Response(JSON.stringify(res), { "status" : 200 , 
    "statusText" : "MyCustomResponse!" })
         console.log(payload)
        return payload
        })
  })
    })
workbox.precaching.precacheAndRoute(self.__precacheManifest || [])

I am really stuck there as all documentation on workbox relates to leveraging cache. I am leveraging localforage as it supports promises which is what is required to make offline capability working.

Thanks

Your catch() handler needs to return either a Response object, or a promise for a Response object.

Adjusting the formatting of your sample code a bit, you're currently doing:

.catch(function (error) {
  console.warn(`Constructing a fallback response, due to an error while fetching the real response:, ${error}`)
  localforage.getItem(event.url.pathname).then((res) => {
    let payload = new Response(JSON.stringify(res), { "status" : 200 , "statusText" : "MyCustomResponse!" })
    console.log(payload)
    return payload
  })
})

Based on that formatting, I think it's clearer that you're not returning either a Response or a promise for a Response from within your catch() handler—you're not returning anything at all.

Adding in a return before your localforage.getItem(...) statement should take care of that:

.catch(function (error) {
  console.warn(`Constructing a fallback response, due to an error while fetching the real response:, ${error}`)
  return localforage.getItem(event.url.pathname).then((res) => {
    let payload = new Response(JSON.stringify(res), { "status" : 200 , "statusText" : "MyCustomResponse!" })
    console.log(payload)
    return payload
  })
})

But, as mentioned in the comments to your original question, I don't think that using IndexedDB to store this type of URL-addressable data is necessary. You can just rely on the Cache Storage API, which Workbox will happily use by default, when storing and retrieving JSON data obtained from an HTTP API.