Service worker to save form data when browser is offline

Question

I am new to Service Workers, and have had a look through the various bits of documentation (Google, Mozilla, serviceworke.rs, Github, StackOverflow questions). The most helpful is the ServiceWorkers cookbook.

Most of the documentation seems to point to caching entire pages so that the app works completely offline, or redirecting the user to an offline page until the browser can redirect to the internet.

What I want to do, however, is store my form data locally so my web app can upload it to the server when the user's connection is restored. Which "recipe" should I use? I think it is Request Deferrer. Do I need anything else to ensure that Request Deferrer will work (apart from the service worker detector script in my web page)? Any hints and tips much appreciated.

Console errors

The Request Deferrer recipe and code doesn't seem to work on its own as it doesn't include file caching. I have added some caching for the service worker library files, but I am still getting this error when I submit the form while offline:

Console: {"lineNumber":0,"message":
"The FetchEvent for [the form URL] resulted in a network error response: 
the promise was rejected.","message_level":2,"sourceIdentifier":1,"sourceURL":""}

My Service Worker

/* eslint-env es6 */
/* eslint no-unused-vars: 0 */
/* global importScripts, ServiceWorkerWare, localforage */
importScripts('/js/lib/ServiceWorkerWare.js');
importScripts('/js/lib/localforage.js');

//Determine the root for the routes. I.e, if the Service Worker URL is http://example.com/path/to/sw.js, then the root is http://example.com/path/to/


var root = (function() {
  var tokens = (self.location + '').split('/');
  tokens[tokens.length - 1] = '';
  return tokens.join('/');
})();

//By using Mozilla’s ServiceWorkerWare we can quickly setup some routes for a virtual server. It is convenient you review the virtual server recipe before seeing this.


var worker = new ServiceWorkerWare();

//So here is the idea. We will check if we are online or not. In case we are not online, enqueue the request and provide a fake response. 
//Else, flush the queue and let the new request to reach the network.


//This function factory does exactly that.


function tryOrFallback(fakeResponse) {

//Return a handler that…


  return function(req, res) {

//If offline, enqueue and answer with the fake response.


    if (!navigator.onLine) {
      console.log('No network availability, enqueuing');
      return enqueue(req).then(function() {

//As the fake response will be reused but Response objects are one use only, we need to clone it each time we use it.


        return fakeResponse.clone();
});
}

//If online, flush the queue and answer from network.


    console.log('Network available! Flushing queue.');
    return flushQueue().then(function() {
      return fetch(req);
});
};
}

//A fake response with a joke for when there is no connection. A real implementation could have cached the last collection of updates and keep a local model. For simplicity, not implemented here.


worker.get(root + 'api/updates?*', tryOrFallback(new Response(
  JSON.stringify([{
    text: 'You are offline.',
    author: 'Oxford Brookes University',
    id: 1,
    isSticky: true
}]),
  { headers: { 'Content-Type': 'application/json' } }
)));

//For deletion, let’s simulate that all went OK. Notice we are omitting the body of the response. Trying to add a body with a 204, deleted, as status throws an error.


worker.delete(root + 'api/updates/:id?*', tryOrFallback(new Response({
    status: 204
})));

//Creation is another story. We can not reach the server so we can not get the id for the new updates. 
//No problem, just say we accept the creation and we will process it later, as soon as we recover connectivity.


worker.post(root + 'api/updates?*', tryOrFallback(new Response(null, {
    status: 202
})));

//Start the service worker.


worker.init();

//By using Mozilla’s localforage db wrapper, we can count on a fast setup for a versatile key-value database. We use it to store queue of deferred requests.


//Enqueue consists of adding a request to the list. Due to the limitations of IndexedDB, Request and Response objects can not be saved so we need an alternative representations. 
//This is why we call to serialize().`


function enqueue(request) {
  return serialize(request).then(function(serialized) {
    localforage.getItem('queue').then(function(queue) {
      /* eslint no-param-reassign: 0 */
      queue = queue || [];
      queue.push(serialized);
      return localforage.setItem('queue', queue).then(function() {
        console.log(serialized.method, serialized.url, 'enqueued!');
      });
    });
  });
}

//Flush is a little more complicated. It consists of getting the elements of the queue in order and sending each one, keeping track of not yet sent request. 
//Before sending a request we need to recreate it from the alternative representation stored in IndexedDB.


function flushQueue() {

//Get the queue


  return localforage.getItem('queue').then(function(queue) {
    /* eslint no-param-reassign: 0 */
    queue = queue || [];

//If empty, nothing to do!


    if (!queue.length) {
      return Promise.resolve();
    }

//Else, send the requests in order…


    console.log('Sending ', queue.length, ' requests...');
    return sendInOrder(queue).then(function() {

        //Requires error handling. Actually, this is assuming all the requests in queue are a success when reaching the Network. 
        //    So it should empty the queue step by step, only popping from the queue if the request completes with success.


      return localforage.setItem('queue', []);
    });
  });
}

//Send the requests inside the queue in order. Waiting for the current before sending the next one.


function sendInOrder(requests) {

//The reduce() chains one promise per serialized request, not allowing to progress to the next one until completing the current.


  var sending = requests.reduce(function(prevPromise, serialized) {
    console.log('Sending', serialized.method, serialized.url);
    return prevPromise.then(function() {
      return deserialize(serialized).then(function(request) {
        return fetch(request);
      });
    });
  }, Promise.resolve());
  return sending;
}

//Serialize is a little bit convolved due to headers is not a simple object.


function serialize(request) {
  var headers = {};

//for(... of ...) is ES6 notation but current browsers supporting SW, support this notation as well and this is the only way of retrieving all the headers.


  for (var entry of request.headers.entries()) {
    headers[entry[0]] = entry[1];
  }
  var serialized = {
    url: request.url,
    headers: headers,
    method: request.method,
    mode: request.mode,
    credentials: request.credentials,
    cache: request.cache,
    redirect: request.redirect,
    referrer: request.referrer
  };

//Only if method is not GET or HEAD is the request allowed to have body.


  if (request.method !== 'GET' && request.method !== 'HEAD') {
    return request.clone().text().then(function(body) {
      serialized.body = body;
      return Promise.resolve(serialized);
    });
  }
  return Promise.resolve(serialized);
}

//Compared, deserialize is pretty simple.


function deserialize(data) {
  return Promise.resolve(new Request(data.url, data));
}

var CACHE = 'cache-only';

// On install, cache some resources.
self.addEventListener('install', function(evt) {
    console.log('The service worker is being installed.');

    // Ask the service worker to keep installing until the returning promise
    // resolves.
    evt.waitUntil(precache());
});

// On fetch, use cache only strategy.
self.addEventListener('fetch', function(evt) {
    console.log('The service worker is serving the asset.');
    evt.respondWith(fromCache(evt.request));
});

// Open a cache and use `addAll()` with an array of assets to add all of them
// to the cache. Return a promise resolving when all the assets are added.
function precache() {
    return caches.open(CACHE).then(function (cache) {
        return cache.addAll([
          '/js/lib/ServiceWorkerWare.js',
          '/js/lib/localforage.js',
          '/js/settings.js'
        ]);
    });
}

// Open the cache where the assets were stored and search for the requested
// resource. Notice that in case of no matching, the promise still resolves
// but it does with `undefined` as value.
function fromCache(request) {
    return caches.open(CACHE).then(function (cache) {
        return cache.match(request).then(function (matching) {
            return matching || Promise.reject('no-match');
        });
    });
}

Here is the error message I am getting in Chrome when I go offline:

(A similar error occurred in Firefox - it falls over at line 409 of ServiceWorkerWare.js)

   ServiceWorkerWare.prototype.executeMiddleware = function (middleware, 
request) {
        var response = this.runMiddleware(middleware, 0, request, null);
        response.catch(function (error) { console.error(error); });
        return response;
    };

Answer 1

this is a little more advanced that a beginner level. But you will need to detect when you are offline or in a Li-Fi state. Instead of POSTing data to an API or end point you need to queue that data to be synched when you are back on line. This is what the Background Sync API should help with. However, it is not supported across the board just yet. Plus Safari......... So maybe a good strategy is to persist your data in IndexedDB and when you can connect (background sync fires an event for this) you would then POST the data. It gets a little more complex for browsers that don't support service workers (Safari) or don't yet have Background Sync (that will level out very soon). As always design your code to be a progressive enhancement, which can be tricky, but worth it in the end.