javascript how to modify the current response in service worker?

I use service worker to modify the fetch response with below code,

self.addEventListener('install', function(event) {
  console.log('install');
});

self.addEventListener('activate', function(event) {
  console.log('Claiming control');
  return self.clients.claim();
});

self.addEventListener('fetch', function(event) {
  console.log("fetch event")
  event.respondWith(
    fetch(event.request).then(function (response) {
      if(response.url.match('.mp4')){
        console.log(event);
        response.arrayBuffer().then(
          buffer =>{
            let length = 100
            view = new Uint8Array(buffer,0,length)
            for(i=0,j=length - 1; i<j; i++,j--){
                view[i] = view[i] ^ view[j]
                view[j] = view[j] ^ view[i]
                view[i] = view[i] ^ view[j]
            }
          }
        )
      }
      return response.clone();
    })
  );
});

Whichi will lead to this warning and error,

The FetchEvent for “” resulted in a network error response: the promise was rejected.

sw.js:60 Uncaught (in promise) TypeError: Failed to execute ‘clone’ on ‘Response’: Response body is already used at sw.js:60

---

If I put the clone() before arrayBuffer() like below,

self.addEventListener('install', function(event) {
  console.log('install');
});

self.addEventListener('activate', function(event) {
  console.log('Claiming control');
  return self.clients.claim();
});

self.addEventListener('fetch', function(event) {
  console.log("fetch event")
  event.respondWith(
    fetch(event.request).then(function (response) {
      if(response.url.match('.mp4')){
        console.log(event);
        responseCloned = response.clone();
        responseCloned.arrayBuffer().then(
          buffer =>{
            let length = 100
            view = new Uint8Array(buffer,0,length)
            for(i=0,j=length - 1; i<j; i++,j--){
                view[i] = view[i] ^ view[j]
                view[j] = view[j] ^ view[i]
                view[i] = view[i] ^ view[j]
            }
          }
        )
      }
      return responseCloned;
    })
  );
});

It will lead to below warning and error,

The FetchEvent for “” resulted in a network error response: a Response whose “body” is locked cannot be used to respond to a request.

GET http://localhost/web/357765_decrypted.mp4 net::ERR_FAILED

Answer

Try instantiating and returning a new Response() with the decrypted bytes:

self.addEventListener('fetch', event => {
  if (event.request.url.endsWith('.mp4') {
    event.respondWith(getDecryptedResponse(event.request))
  }
})

async function getDecryptedResponse(request) {
  const response = await fetch(request)
  const bytes = new Uint8Array(await response.arrayBuffer())

  return new Response(
    new Blob([ decryptMp4Bytes(bytes) ], { type: 'application/mp4' }),
    {
      headers: response.headers,  // possibly required for this case
    }
  )
}