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/*
  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 } 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 {@link workbox-window.Workbox#message}
 * @fires {@link workbox-window.Workbox#installed}
 * @fires {@link workbox-window.Workbox#waiting}
 * @fires {@link workbox-window.Workbox#controlling}
 * @fires {@link workbox-window.Workbox#activated}
 * @fires {@link workbox-window.Workbox#redundant}
 * @memberof workbox-window
 */
class Workbox extends WorkboxEventTarget {
    /**
     * 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)](https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/register).
     *
     * @param {string|TrustedScriptURL} scriptURL The service worker script
     *     associated with this instance. Using a
     *     [`TrustedScriptURL`](https://web.dev/trusted-types/) is supported.
     * @param {Object} [registerOptions] The service worker options associated
     *     with this instance.
     */
    // eslint-disable-next-line @typescript-eslint/ban-types
    constructor(scriptURL, registerOptions = {}) {
        super();
        this._registerOptions = {};
        this._updateFoundCount = 0;
        // Deferreds we can resolve later.
        this._swDeferred = new Deferred();
        this._activeDeferred = new Deferred();
        this._controllingDeferred = new Deferred();
        this._registrationTime = 0;
        this._ownSWs = new Set();
        /**
         * @private
         */
        this._onUpdateFound = () => {
            // `this._registration` will never be `undefined` after an update is found.
            const registration = this._registration;
            const installingSW = registration.installing;
            // 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.toString()) ||
                // 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
         */
        this._onStateChange = (originalEvent) => {
            // `this._registration` will never be `undefined` after an update is found.
            const registration = this._registration;
            const sw = originalEvent.target;
            const { state } = sw;
            const isExternal = sw === this._externalSW;
            const eventProps = {
                sw,
                isExternal,
                originalEvent,
            };
            if (!isExternal && this._isUpdate) {
                eventProps.isUpdate = true;
            }
            this.dispatchEvent(new WorkboxEvent(state, 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
         */
        this._onControllerChange = (originalEvent) => {
            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
         */
        this._onMessage = async (originalEvent) => {
            // Can't change type 'any' of data.
            // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
            const { data, ports, 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)) {
                this.dispatchEvent(new WorkboxEvent('message', {
                    // Can't change type 'any' of data.
                    // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
                    data,
                    originalEvent,
                    ports,
                    sw: source,
                }));
            }
        };
        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.toString())) {
            // 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.toString());
            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.toString(), 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);
        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() {
        // 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 {@link 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>}
     */
    // We might be able to change the 'data' type to Record<string, unknown> in the future.
    // eslint-disable-next-line @typescript-eslint/ban-types
    async messageSW(data) {
        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) {
            void 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}
     */
    _getControllingSWIfCompatible() {
        const controller = navigator.serviceWorker.controller;
        if (controller &&
            urlsMatch(controller.scriptURL, this._scriptURL.toString())) {
            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
     */
    async _registerScript() {
        try {
            // this._scriptURL may be a TrustedScriptURL, but there's no support for
            // passing that to register() in lib.dom right now.
            // https://github.com/GoogleChrome/workbox/issues/2855
            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;
        }
    }
}
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 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 {MessagePort[]} ports The `ports` value from `originalEvent`.
 * @property {Workbox} target The `Workbox` instance.
 */
/**
 * The `installed` event is dispatched if the state of a
 * {@link workbox-window.Workbox} instance's
 * {@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw|registered service worker}
 * 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 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
 * {@link 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 {@link workbox-window.Workbox#register}
 * method was called.
 *
 * @event 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 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
 * {@link workbox-window.Workbox} instance's
 * {@link https://developers.google.com/web/tools/workbox/modules/workbox-precaching#def-registered-sw|registered service worker}
 * changes to `activated`.
 *
 * @event 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
 * {@link 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 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.
 */