Function handleFetch

packages/service-worker/worker/src/assets.ts:121–176

* Process a request for a given resource and return it, or return null if it's not available.

(req: Request, _event: ExtendableEvent)

   * Process a request for a given resource and return it, or return null if it's not available.
   */
  async handleFetch(req: Request, _event: ExtendableEvent): Promise<Response | null> {
    const url = this.adapter.normalizeUrl(req.url);
    // Either the request matches one of the known resource URLs, one of the patterns for
    // dynamically matched URLs, or neither. Determine which is the case for this request in
    // order to decide how to handle it.
    if (this.urls.indexOf(url) !== -1 || this.patterns.some((pattern) => pattern.test(url))) {
      // This URL matches a known resource. Either it's been cached already or it's missing, in
      // which case it needs to be loaded from the network.

      // Open the cache to check whether this resource is present.
      const cache = await this.cache;

      // Look for a cached response. If one exists, it can be used to resolve the fetch
      // operation.
      let cachedResponse: Response | undefined;
      try {
        // Safari 16.4/17 is known to sometimes throw an unexpected internal error on cache access
        // This try/catch is here as a workaround to prevent a failure of the handleFetch
        // as the Driver falls back to safeFetch on critical errors.
        // See #50378
        cachedResponse = await cache.match(req, this.config.cacheQueryOptions);
      } catch (error) {
        throw new SwCriticalError(`Cache is throwing while looking for a match: ${error}`);
      }

      if (cachedResponse !== undefined) {
        // A response has already been cached (which presumably matches the hash for this
        // resource). Check whether it's safe to serve this resource from cache.
        if (this.hashes.has(url)) {
          // This resource has a hash, and thus is versioned by the manifest. It's safe to return
          // the response.
          return cachedResponse;
        } else {
          // This resource has no hash, and yet exists in the cache. Check how old this request is
          // to make sure it's still usable.
          if (await this.needToRevalidate(req, cachedResponse)) {
            this.idle.schedule(`revalidate(${cache.name}): ${req.url}`, async () => {
              await this.fetchAndCacheOnce(req);
            });
          }

          // In either case (revalidation or not), the cached response must be good.
          return cachedResponse;
        }
      }

      // No already-cached response exists, so attempt a fetch/cache operation.
      const res = await this.fetchAndCacheOnce(this.newRequestWithMetadata(req.url, req));

      // If this is successful, the response needs to be cloned as it might be used to respond to
      // multiple fetch operations at the same time.
      return res.clone();
    } else {
      return null;
    }
  }

  /**