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Awesome Open Source

Meteor Service Worker - An universal Service Worker for meteor apps.

Long story short : if you want this to be a package upvote @mitar's proposal

The problem

By default, Meteor has mechanisms to keep working once offline :

  • Methods : If the client is disconnected, methods will resume once logged again (if the tab is not closed) :

If a client calls a method and is disconnected before it receives a response, it will re-call the method when it reconnects

  • Each client has a offline database (minimongo) synced with online's one when the client is connected to the server.
  • Pages can be generated offline with a client-side router as FlowRouter.

So you can be disconnected of the server and your application is working perfectly and synchronise to the server as long as you don't close your tab. But if the client isn't connected, for whatever reasons (server down, no connection), he cannot use your website.

The solution

To make Meteor working offline we just need to cache javascript and CSS files so we can return them from the cache if the user cannot connect to the server.

Service Worker is the solution to make the website working offline. Due to specificities of Meteor, usual snippets of Service Worker (SW) don't work with Meteor. Service Worker caches JS files (mandatory for any Meteor app) but also the CSS, fonts, images or whatever asset that is requested from your server or another one at least once by your webiste. The Service Worker caches assets not databases, in order to cache databases you need ground:db

The SW returns cached version even when online, so you're app is faster. This cache is more powerful than simply caching with proper headers (leverage browser caching) as SW has an higher priority. If one or multiple CSS/JS files have changed the SW detect it and replace the old JS/CSS by the new one.

How the Service Worker functions

The SW is basically a network proxy so every request made by the website go through the SW. So here's how this Service Worker is dealing with Meteor :

  1. The Service Worker asks the server the HTML which with Meteor is really the same whatever the URL is and is only composed of <link> and <script> requesting CSS and Javascript files.
  • If the server didn't give the HTML, the SW returns the cached version of it.
  1. The browser has the HTML, so it ask for CSS/JS which are in the format file.js?hash=XXX where XXX is the hash. The Service Worker extracts the hash so it knows if it has the latest version of the file so if it the case the SW returns the cached version otherwise it asks for a new version to the server and caches it.

Once the browser has the CSS/JS files he can start rendering your app.

For other assets than CSS/JS (e.g : the SW caches the asset and returns the cached version each time it's asked by the website.

How to get the Service Worker on my Meteor App ?

  1. Download sw.js and put it in the root of the /public folder.
  2. Register your Service Worker to the client in your client : Meteor.startup(() => { navigator.serviceWorker.register('/sw.js') .then() .catch(error => console.log('ServiceWorker registration failed: ', err)); }); Service Workers are only available to secure origins. So be sure your server has https (localhost is considered as secure). And that the website you made request to are considered as secure. So your subdomain or your CDN must have HTTPS enabled.

Special information

Debugging Information (Chrome)

All instructions below require you to to launch your Developer Tools (Ctrl-Shift-I in Linux)

In your Developer Tools, you will notice that assets are loaded from (Service Worker) To see what is cached click on Application -> Cache Storate To debug the SW script, go to chrome://serviceworker-internals/ and check Open Dev Tools ... which will start the Service Worker in its own Developer Tools


Service Worker is pretty new so the support may not be full. All major browsers either support Service Worker or states that it's in their plan. Support can be check on

Request from an external website URL

  • If you want a request from a third-party URL to be cached, you must make a cross-origin request and and the resource you're requesting must returns proper headers to allow requests.

More explanation

  • HTMLToCache is the URL the Service Worker will first ask to the server to get the HTML, which is the same in any page of your website, it just contains the <script> and the <head> to make the browser loads required CSS/JS files. If for whatever reasons your website is not accessible by the default value of HTMLToCache (which is the URL of your website with / at the end). You can change the value of the variable to another relative url (as '/index').

  • The SW caches only one HTML for all the website because it's always the same. It does not contain the DOM, JS files create the DOM. Also the SW never returns cached HTML if the client is online because otherwise it will create an infinite loop if hot reload is enabled : if the HTML the SW serves is different than the one on the server (e.g : you changed a CSS file so the hash of the href of the <link> tag changes), old JS/CSS are asked to the server and hot reload package is going to refresh your browser so you can get the new HTML with <script> and <link> with href targeting URLs to new JS/CSS files. But if we carry on serving an old cache we create an infinite loop.


  • 0.1 First version.
  • 0.1.1 Very light refactoring to better match ES6 and Airbnb's guideline.
  • 0.2
    • Add
    • Service worker now caches one HTML file and update it if needed.
  • 0.3 Major update.
    • Full refactoring (e.g : properer and more separated code).
    • When variable 'version' changes, the SW cleans old caches so it keeps only the new cache.
    • If the SW has downloaded a new version from the server of an asset with ?hash in its URL (e.g CSS/JS) it deletes old versions of the file.
    • Service worker now directly takes control of all the clients and skip its installation (basically, by default a SW doesn't takes control before user closes all its tabs with the website open in it).
    • If the SW doesn't have a cached requested file and user isn't connected, it now returns a new Response() with a 503 code.
    • Update, including by noticing that the SW delivers requests from cache even when there's connection with the server (if name, including hash= didn't changed).
    • Add a license (please do not delete it).
  • 0.3.1 - Update
  • 0.3.2 - Updated with new @mitar's proposal, instead of (previous proposal).

About the repository itself

I would be glad to have any contribution : PR, open issues etc.

I would like to make this SW a package so it could be dramatically improved (see todo list for some ideas I could implement) but as for now Meteor don't allow to make a package that register a worker. @Mitar has proposed to make a PR but Meteor's team refuses it because there's no enough support for this features. So if you want to have Meteor packages that can register workers so, for instance, I could make a package that would make the SW easier to install with a constructor which accept option so the SW can be modular please upvote Mitar's post (

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