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Using Plugins

In a number of situations, it’s beneficial being able to manipulate a request and response as it’s being fetched and cached as it allows you to add additional behaviors to your service worker without writing substantial boilerplate code.

Workbox Plugins allow you to add additional behaviors by manipulating responses and requests during the lifecycle of a request.

Out of the box Workbox provides a number of plugins that you can use and you can implement your own plugins if you want to add custom logic.

Workbox Plugins

Workbox provides the following plugins:

  • BackgroundSyncPlugin: If a network request ever fails, add it to a background sync queue and retry the request when the next sync event is triggered.

  • BroadcastUpdatePlugin: Whenever a cache is updated, dispatch a message on a Broadcast Channel or via postMessage().

  • CacheableResponsePlugin: Only cache requests that meet a certain criteria.

  • ExpirationPlugin: Manage the number and maximum age of items in the cache.

  • RangeRequestsPlugin: Respond to requests that include a Range: header with partial content from a cache.

You can use these plugins with a Workbox strategy by adding an instance to the plugins property:

import {registerRoute} from 'workbox-routing';
import {CacheFirst} from 'workbox-strategies';
import {ExpirationPlugin} from 'workbox-expiration';

  ({request}) => request.destination === 'image',
  new CacheFirst({
    cacheName: 'images',
    plugins: [
      new ExpirationPlugin({
        maxEntries: 60,
        maxAgeSeconds: 30 * 24 * 60 * 60, // 30 Days

Custom Plugins

You can create your own plugins by passing in an object that has any of the following methods:

  • cacheWillUpdate: Called before a Response is used to update a cache. You can alter the response before it's added to the cache or return null to avoid updating the cache at all.

  • cacheDidUpdate: Called when a new entry is added to a cache or an existing entry is updated. Useful if you wish to perform an action after a cache update.

  • cacheKeyWillBeUsed: Called before a request is used as a cache key, for both cache lookups (when mode is 'read') and cache writes (when mode is 'write'). This can come in handy if you need to override or normalize your URLs prior to using them for cache access.

  • cachedResponseWillBeUsed: Called prior to a response from the cache being used, this callback allows you to examine that response, and potentially return null or a different response to be used instead.

  • requestWillFetch: This is called whenever a network request is about to be made. You can alter the Request in this callback.

  • fetchDidFail: Called when a network request fails, most likely due to a NetworkError. Note that this does not get called when a response with an error status, like 404 Not Found, is returned from the network.

  • fetchDidSucceed: Called when a network request is successful, regardless of what the HTTP status is of the response.

  • handlerWillStart: Called before any handler logic starts running. This callback can be used to set the initial handler state (e.g. record the start time).

  • handlerWillRespond: Called before the strategies handle() method returns a response. This callback can be used to modify that response before returning it to a route handler or other custom logic.

  • handlerDidRespond: Called after the strategy's handle() method returns a response. This callback can be used to record any final response details, e.g. after changes made by other plugins.

  • handlerDidComplete: Called after all extend lifetime promises added to the event from the invocation of this strategy have settled. This callback can be used to report on any data that needs to wait until the handler is done in order to calculate (e.g. cache hit status, cache latency, network latency).

  • handlerDidError: Called if the handler was unable to provide a valid response from any source. This callback can be used to provide "fallback" content as an alternative to a network error.

All of these functions will be called with await whenever a cache or fetch event reaches the relevant point for the callback.

A plugin using all of these callbacks would look like this:

const myPlugin = {
  cacheWillUpdate: async ({request, response, event, state}) => {
    // Return `response`, a different `Response` object, or `null`.
    return response;
  cacheDidUpdate: async ({cacheName, request, oldResponse, newResponse, event, state}) => {
    // No return expected
    // Note: `newResponse.bodyUsed` is `true` when this is called,
    // meaning the body has already been read. If you need access to
    // the body of the fresh response, use a technique like:
    // const freshResponse = await caches.match(request, {cacheName});
  cacheKeyWillBeUsed: async ({request, mode, params, event, state}) => {
    // `request` is the `Request` object that would otherwise be used as the cache key.
    // `mode` is either 'read' or 'write'.
    // Return either a string, or a `Request` whose `url` property will be used as the cache key.
    // Returning the original `request` will make this a no-op.
    return request;
  cachedResponseWillBeUsed: async ({cacheName, request, matchOptions, cachedResponse, event, state}) => {
    // Return `cachedResponse`, a different `Response` object, or null.
    return cachedResponse;
  requestWillFetch: async ({request, event, state}) => {
    // Return `request` or a different `Request` object.
    return request;
  fetchDidFail: async ({originalRequest, request, error, event, state}) => {
    // No return expected.
    // NOTE: `originalRequest` is the browser's request, `request` is the
    // request after being passed through plugins with
    // `requestWillFetch` callbacks, and `error` is the exception that caused
    // the underlying `fetch()` to fail.
  fetchDidSucceed: async ({request, response, event, state}) => {
    // Return `response` to use the network response as-is,
    // or alternatively create and return a new `Response` object.
    return response;
  handlerWillStart: async ({request, event, state}) => {
    // No return expected. 
    // Can set initial handler state here.
  handlerWillRespond: async ({request, response, event, state}) => {
    // Return `response` or a different `Response` object.
    return response;
  handlerDidRespond: async ({request, response, event, state}) => {
    // No return expected. 
    // Can record final response details here.
  handlerDidComplete: async ({request, response, error, event, state}) => {
    // No return expected. 
    // Can report any data here.
  handlerDidError: async ({request, event, error, state}) => {
    // Return `response`, a different `Response` object as a fallback, or `null`.
    return response;

The event object passed to each plugin callback above represents the original event that triggered the fetch or cache action. In some cases there will not be an original event, so your code should check for its existence before referencing it. Also, when invoking the handle() method of a strategy, the event you pass to handle() will be the event passed to the plugin callbacks.

All plugin callbacks are also passed a state object which is unique to the particular plugin object and strategy invocation (i.e. the call to handle()). This makes it possible to write plugins where one callback can conditionally do something based on what another callback in the same plugin did (e.g. compute the time delta between running requestWillFetch() and fetchDidSucceed() or fetchDidFail()).

Third-party Plugins

We encourage developers to hook into Workbox's lifecycle events in creative ways, and publish your custom plugins as a module.

The following third-party plugins are available:

You may be able to find more by searching in npm's repository.

If you've built a Workbox plugin that you'd like to share, add the 'workbox-plugin' keyword when you publish it. And let us know via @WorkboxJS!