Awesome Open Source
Awesome Open Source


Proxy fetch requests through the Background Sync API

Made with ❤ at @outlandish

npm version js-standard-style

Fetch Sync allows you to proxy fetch requests through the Background Sync API so that they are honoured if made when the UA is offline! Hooray!

Check out a live demo here.

  • Make requests offline that will be sent when the UA regains connectivity (even if the web page is no longer open).
  • Responses are forwarded back to the client as soon as they are received.
  • Implements a familiar fetch-like API: similar function signature and the same return type (a Response).
  • Make named requests that have their response stored in an IDBStore which you can collect in subsequent user sessions.
  • Manage sync operations with fetchSync.{get,getAll,cancel,cancelAll}().
  • Can be used with existing Service Worker infrastructures with importScripts, or handles SW registration for you.
  • If the browser does not support Background Sync, the library will fall back on normal fetch requests.


npm install fetch-sync --save

Table of Contents


The library utilises some new technologies so currently only works in some browsers. It definitely works in Chrome Canary with the experimental-web-platform-features flag enabled.

The browser must support:


Chrome Canary Chrome Firefox IE Opera Safari



// ES6
import fetchSync from 'fetch-sync'
// CommonJS
var fetchSync = require('fetch-sync')
<!-- Script, using minified dist -->
<script src="/node_modules/fetch-sync/dist/fetch-sync.min.js"></script>


See Initialise for details on importing and registering the service worker.


Existing Service Worker

If your application already uses a Service Worker, you can import the fetch-sync worker using importScripts:


And then call fetchSync.init() somewhere in your application's initialisation procedure.

No Service Worker

fetch-sync can handle registration if you don't use a SW already...

Either serve the fetch-sync worker file with a header "Service-Worker-Allowed : /", or to avoid configuring headers, create a Service Worker script in the root of your project and use the method above for 'Existing Service Worker'.

Then see the example under Usage for the fetchSync.init() method.


fetchSync.init([options]) : Promise

Initialise fetchSync.

  • options {Object} (optional) options object

Look at the documentation for sw-register available options and for more details on Service Worker registration.


// Import client lib...

// ES6
import fetchSync from 'fetch-sync'

// ES5
var fetchSync = require('fetch-sync')
<!-- Script, using bundled dist -->
<script src="/node_modules/fetch-sync/dist/fetch-sync.min.js"></script>
// Initialise, passing in worker lib location...

  url: 'node_modules/fetch-sync/dist/fetch-sync.sw.js',
  scope: '<website address>' // e.g. 'http://localhost:8000'

fetchSync([name, ]request[, options]) : Sync

Perform a sync Background Sync operation.

  • [name] {String} (optional) name of the sync operation
  • request {String|Request} URL or an instance of fetch Request
  • [options] {Object} (optional) fetch options object

Returns a Promise that resolves on success of the fetch request. Rejects if a sync exists with this name already.

There are also some properties/methods on the Promise. See the Sync API for more details.

If called with a name:

  • the response will be stored and can be retrieved later using e.g. fetchSync.get('name').then(sync => sync.response).
  • the response will not automatically be removed from the IDBStore in the worker. You should request that a named sync be removed manually by using sync.remove().
  • see the Sync API for more details.


  • named GET

    fetchSync('GetMessages', '/messages')
      .then((response) => response.json())
      .then((json) => console.log(
  • unnamed POST

    const post = fetchSync('/update-profile', {
      method: 'POST',
      body: { name: '' }
    // cancel the sync...
  • unnamed with options

    const headers = new Headers();
    headers.append('Authorization', 'Basic abcdefghijklmnopqrstuvwxyz');
    // `fetchSync` accepts the same args as `fetch`...
    fetchSync('/send-message', { headers })
  • named with options

    fetchSync('/get-messages', { headers })
  • unnamed with Request

      new Request('/messages')

fetchSync.get(name) : Sync

Get a sync by its name.

  • name {String} name of the sync operation to get

Returns a Promise that resolves with success of the sync operation or reject if sync operation is not found.

There are also some properties/methods on the Promise. See the Sync API for more details.


fetchSync('SendMessage', '/message', { body: 'Hello, World!' })

const sync = fetchSync.get('SendMessage')

sync.then((response) => {
  if (response.ok) {
    alert(`Your message was received at ${new Date(sync.syncedOn).toDateString()}.`
  } else {
    alert('Message failed to send.')

fetchSync.getAll() : Array<Sync>

Get all sync operations.

Returns an array of all sync operations (named and unnamed).


  .then((syncs) => syncs.forEach(sync => sync.cancel()))

fetchSync.cancel(name) : Promise

Cancel the sync with the given name.

  • name {String} name of the sync operation to cancel


fetchSync('Update', '/update', { body })

fetchSync.cancelAll() : Promise

Cancel all syncs, named and unnamed.

Sync API

sync.cancel() : Promise

Cancels the sync operation.

Returns a Promise of success of the cancellation.


const sync = fetchSync.get('Update')

The unique ID of the sync operation. This will be its name if it has one.

The name of the sync operation if it has one.


The time that the sync operation was created.


The time that the sync operation was completed.



As the library depends on Service Workers and no headless browser has (good enough) support for Service Workers that would allow tests to be executed within the console, tests are ran through the browser using Mocha and Chai.

On running npm test an Express server will be started at localhost:8000.

Run the tests:

$ cd fetch-sync
$ npm test


The library is bundled by Webpack and transpiled by Babel.

  • Install dependencies: npm install
  • Start Webpack in a console: npm run watch
  • Start the test server in another: npm test
  • Navigate to http://localhost:8000


All pull requests and issues welcome!

If you're not sure how, check out Kent C. Dodds' great video tutorials on!

Author & License

fetch-sync was created by Sam Gluck and is released under the MIT license.

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