Method handleFetch

packages/service-worker/worker/src/data.ts:303–344

* Process a fetch event and return a `Response` if the resource is covered by this group, * or `null` otherwise.

(req: Request, event: ExtendableEvent)

   * or `null` otherwise.
   */
  async handleFetch(req: Request, event: ExtendableEvent): Promise<Response | null> {
    // Do nothing
    if (!this.patterns.some((pattern) => pattern.test(req.url))) {
      return null;
    }

    // Lazily initialize the LRU cache.
    const lru = await this.lru();

    // The URL matches this cache. First, check whether this is a mutating request or not.
    switch (req.method) {
      case 'OPTIONS':
        // Don't try to cache this - it's non-mutating, but is part of a mutating request.
        // Most likely SWs don't even see this, but this guard is here just in case.
        return null;
      case 'GET':
      case 'HEAD':
        // Handle the request with whatever strategy was selected.
        switch (this.config.strategy) {
          case 'freshness':
            return this.handleFetchWithFreshness(req, event, lru);
          case 'performance':
            return this.handleFetchWithPerformance(req, event, lru);
          default:
            throw new Error(`Unknown strategy: ${this.config.strategy}`);
        }
      default:
        // This was a mutating request. Assume the cache for this URL is no longer valid.
        const wasCached = lru.remove(req.url);

        // If there was a cached entry, remove it.
        if (wasCached) {
          await this.clearCacheForUrl(req.url);
        }

        // Sync the LRU chain to non-volatile storage.
        await this.syncLru();

        // Finally, fall back on the network.
        return this.safeFetch(req);
    }
  }

  private async handleFetchWithPerformance(
    req: Request,