Request Deferrer Recipe - Beyond Offline

service-worker.js

/* eslint-env es6 */
/* eslint no-unused-vars: 0 */
/* global importScripts, ServiceWorkerWare, localforage */
importScripts('./lib/ServiceWorkerWare.js');
importScripts('./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 quotations and keep a local model. For simplicity, not implemented here.

worker.get(root + 'api/quotations?*', tryOrFallback(new Response(
  JSON.stringify([{
    text: 'You are offline and I know it well.',
    author: 'The Service Worker Cookbook',
    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/quotations/: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 quotations. No problem, just say we accept the creation and we will process it later, as soon as we recover connectivity.
```
worker.post(root + 'api/quotations?*', 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));
}

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service-worker.js

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