self.addEventListener('fetch', async function(event) {
        if (event.request.headers.get("range")) {
          const response = await fetch(event.request.clone());
    
          return  event.respondWith(this.getPartialResponse(event.request, response));
        }
    
        return  event.respondWith(fetch(event.request));
    }
    
    async getPartialResponse(req, res) {
      const pos = Number(/^bytes\=(\d )\-$/g.exec(req.headers.get("range"))[1]);
      const ab = await res.arrayBuffer();
      const headers = new Headers(res.headers);
    
      headers.append("Content-Range", `bytes ${pos}-${ab.byteLength - 1}/${ab.byteLength}`);
      headers.append("Content-Length", ab.byteLength - pos   1);
    
      return new Response(ab.slice(pos), {
        status: 206,
        statusText: "Partial Content",
        headers
      });
    }