Workbox.ts

/*
  Copyright 2019 Google LLC

  Use of this source code is governed by an MIT-style
  license that can be found in the LICENSE file or at
  https://opensource.org/licenses/MIT.
*/

import {Deferred} from 'workbox-core/_private/Deferred.js';
import {dontWaitFor} from 'workbox-core/_private/dontWaitFor.js';
import {logger} from 'workbox-core/_private/logger.js';

import {messageSW} from './messageSW.js';
import {WorkboxEventTarget} from './utils/WorkboxEventTarget.js';
import {urlsMatch} from './utils/urlsMatch.js';
import {WorkboxEvent, WorkboxLifecycleEventMap} from './utils/WorkboxEvent.js';

import './_version.js';


// The time a SW must be in the waiting phase before we can conclude
// `skipWaiting()` wasn't called. This 200 amount wasn't scientifically
// chosen, but it seems to avoid false positives in my testing.
const WAITING_TIMEOUT_DURATION = 200;

// The amount of time after a registration that we can reasonably conclude
// that the registration didn't trigger an update.
const REGISTRATION_TIMEOUT_DURATION = 60000;

// The de facto standard message that a service worker should be listening for
// to trigger a call to skipWaiting().
const SKIP_WAITING_MESSAGE = {type: 'SKIP_WAITING'};

/**
 * A class to aid in handling service worker registration, updates, and
 * reacting to service worker lifecycle events.
 *
 * @fires [message]{@link module:workbox-window.Workbox#message}
 * @fires [installed]{@link module:workbox-window.Workbox#installed}
 * @fires [waiting]{@link module:workbox-window.Workbox#waiting}
 * @fires [controlling]{@link module:workbox-window.Workbox#controlling}
 * @fires [activated]{@link module:workbox-window.Workbox#activated}
 * @fires [redundant]{@link module:workbox-window.Workbox#redundant}
 * @memberof module:workbox-window
 */
class Workbox extends WorkboxEventTarget {
  private readonly _scriptURL: string;
  private readonly _registerOptions: RegistrationOptions = {};
  private _updateFoundCount = 0;

  // Deferreds we can resolve later.
  private readonly _swDeferred: Deferred<ServiceWorker> = new Deferred();
  private readonly _activeDeferred: Deferred<ServiceWorker> = new Deferred();
  private readonly _controllingDeferred: Deferred<ServiceWorker> = new Deferred();

  private _registrationTime: DOMHighResTimeStamp = 0;
  private _isUpdate?: boolean;
  private _compatibleControllingSW?: ServiceWorker;
  private _registration?: ServiceWorkerRegistration;
  private _sw?: ServiceWorker;
  private readonly _ownSWs: Set<ServiceWorker> = new Set();
  private _externalSW?: ServiceWorker;
  private _waitingTimeout?: number;

  /**
   * Creates a new Workbox instance with a script URL and service worker
   * options. The script URL and options are the same as those used when
   * calling `navigator.serviceWorker.register(scriptURL, options)`. See:
   * https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/register
   *
   * @param {string} scriptURL The service worker script associated with this
   *     instance.
   * @param {Object} [registerOptions] The service worker options associated
   *     with this instance.
   */
  constructor(scriptURL: string, registerOptions: {} = {}) {
    super();

    this._scriptURL = scriptURL;
    this._registerOptions = registerOptions;

    // Add a message listener immediately since messages received during
    // page load are buffered only until the DOMContentLoaded event:
    // https://github.com/GoogleChrome/workbox/issues/2202
    navigator.serviceWorker.addEventListener('message', this._onMessage);
  }

  /**
   * Registers a service worker for this instances script URL and service
   * worker options. By default this method delays registration until after
   * the window has loaded.
   *
   * @param {Object} [options]
   * @param {Function} [options.immediate=false] Setting this to true will
   *     register the service worker immediately, even if the window has
   *     not loaded (not recommended).
   */
  async register({immediate = false} = {}) {
    if (process.env.NODE_ENV !== 'production') {
      if (this._registrationTime) {
        logger.error('Cannot re-register a Workbox instance after it has ' +
            'been registered. Create a new instance instead.');
        return;
      }
    }

    if (!immediate && document.readyState !== 'complete') {
      await new Promise((res) => window.addEventListener('load', res));
    }

    // Set this flag to true if any service worker was controlling the page
    // at registration time.
    this._isUpdate = Boolean(navigator.serviceWorker.controller);

    // Before registering, attempt to determine if a SW is already controlling
    // the page, and if that SW script (and version, if specified) matches this
    // instance's script.
    this._compatibleControllingSW = this._getControllingSWIfCompatible();

    this._registration = await this._registerScript();

    // If we have a compatible controller, store the controller as the "own"
    // SW, resolve active/controlling deferreds and add necessary listeners.
    if (this._compatibleControllingSW) {
      this._sw = this._compatibleControllingSW;
      this._activeDeferred.resolve(this._compatibleControllingSW);
      this._controllingDeferred.resolve(this._compatibleControllingSW);

      this._compatibleControllingSW.addEventListener(
          'statechange', this._onStateChange, {once: true});
    }

    // If there's a waiting service worker with a matching URL before the
    // `updatefound` event fires, it likely means that this site is open
    // in another tab, or the user refreshed the page (and thus the previous
    // page wasn't fully unloaded before this page started loading).
    // https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle#waiting
    const waitingSW = this._registration.waiting;
    if (waitingSW && urlsMatch(waitingSW.scriptURL, this._scriptURL)) {
      // Store the waiting SW as the "own" Sw, even if it means overwriting
      // a compatible controller.
      this._sw = waitingSW;

      // Run this in the next microtask, so any code that adds an event
      // listener after awaiting `register()` will get this event.
      dontWaitFor(Promise.resolve().then(() => {
        this.dispatchEvent(new WorkboxEvent('waiting', {
          sw: waitingSW,
          wasWaitingBeforeRegister: true,
        }));
        if (process.env.NODE_ENV !== 'production') {
          logger.warn('A service worker was already waiting to activate ' +
              'before this script was registered...');
        }
      }));
    }

    // If an "own" SW is already set, resolve the deferred.
    if (this._sw) {
      this._swDeferred.resolve(this._sw);
      this._ownSWs.add(this._sw);
    }

    if (process.env.NODE_ENV !== 'production') {
      logger.log('Successfully registered service worker.', this._scriptURL);

      if (navigator.serviceWorker.controller) {
        if (this._compatibleControllingSW) {
          logger.debug('A service worker with the same script URL ' +
              'is already controlling this page.');
        } else {
          logger.debug('A service worker with a different script URL is ' +
              'currently controlling the page. The browser is now fetching ' +
              'the new script now...');
        }
      }

      const currentPageIsOutOfScope = () => {
        const scopeURL = new URL(
            this._registerOptions.scope || this._scriptURL, document.baseURI);
        const scopeURLBasePath = new URL('./', scopeURL.href).pathname;
        return !location.pathname.startsWith(scopeURLBasePath);
      };
      if (currentPageIsOutOfScope()) {
        logger.warn('The current page is not in scope for the registered ' +
            'service worker. Was this a mistake?');
      }
    }

    this._registration.addEventListener('updatefound', this._onUpdateFound);
    navigator.serviceWorker.addEventListener(
        'controllerchange', this._onControllerChange, {once: true});

    return this._registration;
  }

  /**
   * Checks for updates of the registered service worker.
   */
  async update() {
    if (!this._registration) {
      if (process.env.NODE_ENV !== 'production') {
        logger.error('Cannot update a Workbox instance without ' +
          'being registered. Register the Workbox instance first.');
      }
      return;
    }

    // Try to update registration
    await this._registration.update();
  }

  /**
   * Resolves to the service worker registered by this instance as soon as it
   * is active. If a service worker was already controlling at registration
   * time then it will resolve to that if the script URLs (and optionally
   * script versions) match, otherwise it will wait until an update is found
   * and activates.
   *
   * @return {Promise<ServiceWorker>}
   */
  get active() {
    return this._activeDeferred.promise;
  }

  /**
   * Resolves to the service worker registered by this instance as soon as it
   * is controlling the page. If a service worker was already controlling at
   * registration time then it will resolve to that if the script URLs (and
   * optionally script versions) match, otherwise it will wait until an update
   * is found and starts controlling the page.
   * Note: the first time a service worker is installed it will active but
   * not start controlling the page unless `clients.claim()` is called in the
   * service worker.
   *
   * @return {Promise<ServiceWorker>}
   */
  get controlling() {
    return this._controllingDeferred.promise;
  }

  /**
   * Resolves with a reference to a service worker that matches the script URL
   * of this instance, as soon as it's available.
   *
   * If, at registration time, there's already an active or waiting service
   * worker with a matching script URL, it will be used (with the waiting
   * service worker taking precedence over the active service worker if both
   * match, since the waiting service worker would have been registered more
   * recently).
   * If there's no matching active or waiting service worker at registration
   * time then the promise will not resolve until an update is found and starts
   * installing, at which point the installing service worker is used.
   *
   * @return {Promise<ServiceWorker>}
   */
  getSW(): Promise<ServiceWorker> {
    // If `this._sw` is set, resolve with that as we want `getSW()` to
    // return the correct (new) service worker if an update is found.
    return this._sw !== undefined ?
      Promise.resolve(this._sw) :
      this._swDeferred.promise;
  }

  /**
   * Sends the passed data object to the service worker registered by this
   * instance (via [`getSW()`]{@link module:workbox-window.Workbox#getSW}) and resolves
   * with a response (if any).
   *
   * A response can be set in a message handler in the service worker by
   * calling `event.ports[0].postMessage(...)`, which will resolve the promise
   * returned by `messageSW()`. If no response is set, the promise will never
   * resolve.
   *
   * @param {Object} data An object to send to the service worker
   * @return {Promise<Object>}
   */
  async messageSW(data: object) {
    const sw = await this.getSW();
    return messageSW(sw, data);
  }

  /**
   * Sends a `{type: 'SKIP_WAITING'}` message to the service worker that's
   * currently in the `waiting` state associated with the current registration.
   * 
   * If there is no current registration or no service worker is `waiting`,
   * calling this will have no effect.
   */
  messageSkipWaiting() {
    if (this._registration && this._registration.waiting) {
      messageSW(this._registration.waiting, SKIP_WAITING_MESSAGE);
    }
  }

  /**
   * Checks for a service worker already controlling the page and returns
   * it if its script URL matches.
   *
   * @private
   * @return {ServiceWorker|undefined}
   */
  private _getControllingSWIfCompatible() {
    const controller = navigator.serviceWorker.controller;
    if (controller && urlsMatch(controller.scriptURL, this._scriptURL)) {
      return controller;
    } else {
      return undefined;
    }
  }

  /**
   * Registers a service worker for this instances script URL and register
   * options and tracks the time registration was complete.
   *
   * @private
   */
  private async _registerScript() {
    try {
      const reg = await navigator.serviceWorker.register(
          this._scriptURL, this._registerOptions);

      // Keep track of when registration happened, so it can be used in the
      // `this._onUpdateFound` heuristic. Also use the presence of this
      // property as a way to see if `.register()` has been called.
      this._registrationTime = performance.now();

      return reg;
    } catch (error) {
      if (process.env.NODE_ENV !== 'production') {
        logger.error(error);
      }
      // Re-throw the error.
      throw error;
    }
  }

  /**
   * @private
   */
  private readonly _onUpdateFound = () => {
    // `this._registration` will never be `undefined` after an update is found.
    const registration = this._registration!;
    const installingSW = registration.installing as ServiceWorker;

    // If the script URL passed to `navigator.serviceWorker.register()` is
    // different from the current controlling SW's script URL, we know any
    // successful registration calls will trigger an `updatefound` event.
    // But if the registered script URL is the same as the current controlling
    // SW's script URL, we'll only get an `updatefound` event if the file
    // changed since it was last registered. This can be a problem if the user
    // opens up the same page in a different tab, and that page registers
    // a SW that triggers an update. It's a problem because this page has no
    // good way of knowing whether the `updatefound` event came from the SW
    // script it registered or from a registration attempt made by a newer
    // version of the page running in another tab.
    // To minimize the possibility of a false positive, we use the logic here:
    const updateLikelyTriggeredExternally =
        // Since we enforce only calling `register()` once, and since we don't
        // add the `updatefound` event listener until the `register()` call, if
        // `_updateFoundCount` is > 0 then it means this method has already
        // been called, thus this SW must be external
        this._updateFoundCount > 0 ||
        // If the script URL of the installing SW is different from this
        // instance's script URL, we know it's definitely not from our
        // registration.
        !urlsMatch(installingSW.scriptURL, this._scriptURL) ||
        // If all of the above are false, then we use a time-based heuristic:
        // Any `updatefound` event that occurs long after our registration is
        // assumed to be external.
        (performance.now() >
            this._registrationTime + REGISTRATION_TIMEOUT_DURATION) ?
                // If any of the above are not true, we assume the update was
                // triggered by this instance.
                true : false;

    if (updateLikelyTriggeredExternally) {
      this._externalSW = installingSW;
      registration.removeEventListener('updatefound', this._onUpdateFound);
    } else {
      // If the update was not triggered externally we know the installing
      // SW is the one we registered, so we set it.
      this._sw = installingSW;
      this._ownSWs.add(installingSW);
      this._swDeferred.resolve(installingSW);

      // The `installing` state isn't something we have a dedicated
      // callback for, but we do log messages for it in development.
      if (process.env.NODE_ENV !== 'production') {
        if (navigator.serviceWorker.controller) {
          logger.log('Updated service worker found. Installing now...');
        } else {
          logger.log('Service worker is installing...');
        }
      }
    }

    // Increment the `updatefound` count, so future invocations of this
    // method can be sure they were triggered externally.
    ++this._updateFoundCount;

    // Add a `statechange` listener regardless of whether this update was
    // triggered externally, since we have callbacks for both.
    installingSW.addEventListener('statechange', this._onStateChange);
  }

  /**
   * @private
   * @param {Event} originalEvent
   */
  private readonly _onStateChange = (originalEvent: Event) => {
    // `this._registration` will never be `undefined` after an update is found.
    const registration = this._registration!;
    const sw = originalEvent.target as ServiceWorker;
    const {state} = sw;
    const isExternal = sw === this._externalSW;

    const eventProps: {
      sw: ServiceWorker;
      originalEvent: Event;
      isUpdate?: boolean;
      isExternal: boolean;
    } = {
      sw,
      isExternal,
      originalEvent
    };
    if (!isExternal && this._isUpdate) {
      eventProps.isUpdate = true;
    }

    this.dispatchEvent(new WorkboxEvent(state as keyof WorkboxLifecycleEventMap, eventProps));

    if (state === 'installed') {
      // This timeout is used to ignore cases where the service worker calls
      // `skipWaiting()` in the install event, thus moving it directly in the
      // activating state. (Since all service workers *must* go through the
      // waiting phase, the only way to detect `skipWaiting()` called in the
      // install event is to observe that the time spent in the waiting phase
      // is very short.)
      // NOTE: we don't need separate timeouts for the own and external SWs
      // since they can't go through these phases at the same time.
      this._waitingTimeout = self.setTimeout(() => {
        // Ensure the SW is still waiting (it may now be redundant).
        if (state === 'installed' && registration.waiting === sw) {
          this.dispatchEvent(new WorkboxEvent('waiting', eventProps));

          if (process.env.NODE_ENV !== 'production') {
            if (isExternal) {
              logger.warn('An external service worker has installed but is ' +
                  'waiting for this client to close before activating...');
            } else {
              logger.warn('The service worker has installed but is waiting ' +
                  'for existing clients to close before activating...');
            }
          }
        }
      }, WAITING_TIMEOUT_DURATION);
    } else if (state === 'activating') {
      clearTimeout(this._waitingTimeout);
      if (!isExternal) {
        this._activeDeferred.resolve(sw);
      }
    }

    if (process.env.NODE_ENV !== 'production') {
      switch (state) {
        case 'installed':
          if (isExternal) {
            logger.warn('An external service worker has installed. ' +
                'You may want to suggest users reload this page.');
          } else {
            logger.log('Registered service worker installed.');
          }
          break;
        case 'activated':
          if (isExternal) {
            logger.warn('An external service worker has activated.');
          } else {
            logger.log('Registered service worker activated.');
            if (sw !== navigator.serviceWorker.controller) {
              logger.warn('The registered service worker is active but ' +
                  'not yet controlling the page. Reload or run ' +
                  '`clients.claim()` in the service worker.');
            }
          }
          break;
        case 'redundant':
          if (sw === this._compatibleControllingSW) {
            logger.log('Previously controlling service worker now redundant!');
          } else if (!isExternal) {
            logger.log('Registered service worker now redundant!');
          }
          break;
      }
    }
  }

  /**
   * @private
   * @param {Event} originalEvent
   */
  private readonly _onControllerChange = (originalEvent: Event) => {
    const sw = this._sw;
    const isExternal = sw !== navigator.serviceWorker.controller;

    // Unconditionally dispatch the controlling event, with isExternal set
    // to distinguish between controller changes due to the initial registration
    // vs. an update-check or other tab's registration.
    // See https://github.com/GoogleChrome/workbox/issues/2786
    this.dispatchEvent(new WorkboxEvent('controlling', {
      isExternal,
      originalEvent,
      sw,
      isUpdate: this._isUpdate,
    }));

    if (!isExternal) {
      if (process.env.NODE_ENV !== 'production') {
        logger.log('Registered service worker now controlling this page.');
      }
      this._controllingDeferred.resolve(sw);
    }
  }

  /**
   * @private
   * @param {Event} originalEvent
   */
  private readonly _onMessage = async (originalEvent: MessageEvent) => {
    const {data, source} = originalEvent;

    // Wait until there's an "own" service worker. This is used to buffer
    // `message` events that may be received prior to calling `register()`.
    await this.getSW();

    // If the service worker that sent the message is in the list of own
    // service workers for this instance, dispatch a `message` event.
    // NOTE: we check for all previously owned service workers rather than
    // just the current one because some messages (e.g. cache updates) use
    // a timeout when sent and may be delayed long enough for a service worker
    // update to be found.
    if (this._ownSWs.has(source as ServiceWorker)) {
      this.dispatchEvent(new WorkboxEvent('message', {
        data,
        sw: source as ServiceWorker,
        originalEvent,
      }));
    }
  }
}

export {Workbox};

// The jsdoc comments below outline the events this instance may dispatch:
// -----------------------------------------------------------------------

/**
 * The `message` event is dispatched any time a `postMessage` is received.
 *
 * @event module:workbox-window.Workbox#message
 * @type {WorkboxEvent}
 * @property {*} data The `data` property from the original `message` event.
 * @property {Event} originalEvent The original [`message`]{@link https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent}
 *     event.
 * @property {string} type `message`.
 * @property {Workbox} target The `Workbox` instance.
 */

/**
 * The `installed` event is dispatched if the state of a
 * [`Workbox`]{@link module:workbox-window.Workbox} instance's
 * [registered service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw}
 * changes to `installed`.
 *
 * Then can happen either the very first time a service worker is installed,
 * or after an update to the current service worker is found. In the case
 * of an update being found, the event's `isUpdate` property will be `true`.
 *
 * @event module:workbox-window.Workbox#installed
 * @type {WorkboxEvent}
 * @property {ServiceWorker} sw The service worker instance.
 * @property {Event} originalEvent The original [`statechange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/onstatechange}
 *     event.
 * @property {boolean|undefined} isUpdate True if a service worker was already
 *     controlling when this `Workbox` instance called `register()`.
 * @property {boolean|undefined} isExternal True if this event is associated
 *     with an [external service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-window#when_an_unexpected_version_of_the_service_worker_is_found}.
 * @property {string} type `installed`.
 * @property {Workbox} target The `Workbox` instance.
 */

/**
 * The `waiting` event is dispatched if the state of a
 * [`Workbox`]{@link module:workbox-window.Workbox} instance's
 * [registered service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw}
 * changes to `installed` and then doesn't immediately change to `activating`.
 * It may also be dispatched if a service worker with the same
 * [`scriptURL`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/scriptURL}
 * was already waiting when the [`register()`]{@link module:workbox-window.Workbox#register}
 * method was called.
 *
 * @event module:workbox-window.Workbox#waiting
 * @type {WorkboxEvent}
 * @property {ServiceWorker} sw The service worker instance.
 * @property {Event|undefined} originalEvent The original
 *    [`statechange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/onstatechange}
 *     event, or `undefined` in the case where the service worker was waiting
 *     to before `.register()` was called.
 * @property {boolean|undefined} isUpdate True if a service worker was already
 *     controlling when this `Workbox` instance called `register()`.
 * @property {boolean|undefined} isExternal True if this event is associated
 *     with an [external service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-window#when_an_unexpected_version_of_the_service_worker_is_found}.
 * @property {boolean|undefined} wasWaitingBeforeRegister True if a service worker with
 *     a matching `scriptURL` was already waiting when this `Workbox`
 *     instance called `register()`.
 * @property {string} type `waiting`.
 * @property {Workbox} target The `Workbox` instance.
 */

/**
 * The `controlling` event is dispatched if a
 * [`controllerchange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/oncontrollerchange}
 * fires on the service worker [container]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer}
 * and the [`scriptURL`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/scriptURL}
 * of the new [controller]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/controller}
 * matches the `scriptURL` of the `Workbox` instance's
 * [registered service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw}.
 *
 * @event module:workbox-window.Workbox#controlling
 * @type {WorkboxEvent}
 * @property {ServiceWorker} sw The service worker instance.
 * @property {Event} originalEvent The original [`controllerchange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/oncontrollerchange}
 *     event.
 * @property {boolean|undefined} isUpdate True if a service worker was already
 *     controlling when this service worker was registered.
 * @property {boolean|undefined} isExternal True if this event is associated
 *     with an [external service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-window#when_an_unexpected_version_of_the_service_worker_is_found}.
 * @property {string} type `controlling`.
 * @property {Workbox} target The `Workbox` instance.
 */

/**
 * The `activated` event is dispatched if the state of a
 * [`Workbox`]{@link module:workbox-window.Workbox} instance's
 * [registered service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw}
 * changes to `activated`.
 *
 * @event module:workbox-window.Workbox#activated
 * @type {WorkboxEvent}
 * @property {ServiceWorker} sw The service worker instance.
 * @property {Event} originalEvent The original [`statechange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/onstatechange}
 *     event.
 * @property {boolean|undefined} isUpdate True if a service worker was already
 *     controlling when this `Workbox` instance called `register()`.
 * @property {boolean|undefined} isExternal True if this event is associated
 *     with an [external service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-window#when_an_unexpected_version_of_the_service_worker_is_found}.
 * @property {string} type `activated`.
 * @property {Workbox} target The `Workbox` instance.
 */

/**
 * The `redundant` event is dispatched if the state of a
 * [`Workbox`]{@link module:workbox-window.Workbox} instance's
 * [registered service worker]{@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw}
 * changes to `redundant`.
 *
 * @event module:workbox-window.Workbox#redundant
 * @type {WorkboxEvent}
 * @property {ServiceWorker} sw The service worker instance.
 * @property {Event} originalEvent The original [`statechange`]{@link https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker/onstatechange}
 *     event.
 * @property {boolean|undefined} isUpdate True if a service worker was already
 *     controlling when this `Workbox` instance called `register()`.
 * @property {string} type `redundant`.
 * @property {Workbox} target The `Workbox` instance.
 */