cors image request retry not work

[QingLeiLi]

I am investigating a usage scenario of ServiceWorker:

1. Our static resources are hosted on the CDN. We have two CDN domain names, https://cdna.com and https: //cdnb/.com, and both have set the access-control-allow-origin header to *
2. HTML loads an image on CDN https://cdna.com: <img src = "https://cdna.com/img.png">
3. Due to DNS pollution, https://cdna.com/img.png failed to load
4. Important: Can I switch the URL to https://cdnb.com/img.png and try again?

The ServiceWorker I run on chrome didn't give the resolved Response to the website, but the loading failed.

Could anyone tell me why this is, thanks in advance.

[QingLeiLi]

A sample code:

self.addEventListener('fetch', function(e) {
  if(e.request.url === 'https://cdna.com/img.png'){
    return fetch(new Request('https://cdnb.com/img.png'));
  }else{
    return fetch(e.request);
  }
});

[QingLeiLi]

Sorry, It is a mistake, I forgot to add e.respondWith