The Polymer library is in maintenance mode. For new development, we recommend Lit.

A service worker is a script associated with a specific web site that acts as a client-side proxy for network requests. The service worker can intercept network requests, access the browser's cache, and serve requests out of the cache instead of accessing the network.

A service worker can improve your app's performance and allow it to work offline.

The first time someone opens your site, the browser installs the site's service worker, and the service worker caches your app's critical resources. On subsequent visits, the service worker can load resources directly from the cache. If the user is completely offline, the service worker can still load the site, and display cached data or an offline message, as appropriate.

The Polymer Starter Kit template uses the Service Worker Precache (sw-precache) module for offline support. This module takes a list of files to cache and generates a service worker at build time, so you don't need to write your own service worker code. Service worker works well with an app shell strategy, where the app's main views and logic (the app shell) are cached so that they can be served without accessing the network.

Polymer CLI uses the Service Worker Precache (sw-precache) library. All of the Polymer build presets, and the default build, include a service worker.

For background, gotchas and debugging tips on service workers, see Introduction to Service Workers on Web Fundamentals.

To work with a service worker, your application must be served over HTTPS. However, you can test your service worker locally without a SSL certificate, because localhost is considered a secure origin.

For information on browser support for service worker, see Is Service Worker Ready.

To add a service worker to your build:

  1. Configure polymer.json.
  2. Add code to your entrypoint to register a service worker.
  3. Run polymer build.

A service worker is added to your build.

To configure polymer.json, set the "entrypoint" and "shell" properties. The service worker will automatically precache these resources.

Polymer CLI generates a service worker by default. You can switch service worker generation on or off with the "addServiceWorker" option in your build configuration.

Example polymer.json configuration

  "entrypoint": "index.html",
  "shell": "src/my-app.js",
  "builds": [
      "name": "esm-unbundled",
      "browserCapabilities": [
      "js": {
        "minify": true
      "css": {
        "minify": true
      "html": {
        "minify": true
      "bundle": false,
      "addServiceWorker": true

To use the service worker, add code to your app's entrypoint to register it:

Register a service worker

  // Feature detect for service worker capability in the browser
  if ('serviceWorker' in navigator) {
    // Delay registering until page load
    window.addEventListener('load', function() {

Registering a service worker doesn't speed up the first load of your site. You can delay registering it until after your app has loaded.

To customize your service worker, create a configuration file called sw-precache-config.js in your top-level project folder.

sw-precache-config.js exports a set of configuration options supported by Service Worker Precache. See Options parameter in the sw-precache README for more information.

If you're writing a single-page app and you want it to work completely offline, specify a fallback document to be served when the requested URL is not in the cache. For a single—page app, this is typically the same as the entrypoint. Configure fallback using the navigateFallback and navigateFallbackWhitelist parameters.

Example sw-precache-config.js

module.exports = {
  staticFileGlobs: [
  navigateFallback: '/index.html',
  navigateFallbackWhitelist: [/^(?!.*\.js$|\/data\/).*/]

Only paths that match the whitelist fall back to /index.html. In this case, the whitelist includes all files except those that end in .js (for JavaScript imports) and ones with /data/ in the path (for dynamically-loaded data).

The sw-precache library supports a number of other features, including runtime caching of your app's dynamic content.

For more information on the library, see Service Worker Precache: Getting Started.

For background on service workers, see Introduction to Service Worker on Web Fundamentals.