How to use Service Workers in Google Chrome extensions to modify an HTTP response body?

Question

Now that Google Chrome extensions can register Service Workers, how can I use them in order to modify HTTP responses from all hosts, e.g. by replacing all occurrences of cat with dog?

Below is a sample code from Craig Russell, but how to use it inside Chrome extensions and bind it to all hosts?

self.addEventListener('fetch', function(event) {
    event.respondWith(
        fetch(event.request).then(function(response) {
            var init = {
                status:     response.status,
                statusText: response.statusText,
                headers:    {'X-Foo': 'My Custom Header'}
            };
            response.headers.forEach(function(v,k) {
                init.headers[k] = v;
            });
            return response.text().then(function(body) {
                return new Response(body.replace(/cat/g, 'dog'), init);
            });
        })
    );
});

Answer 1

Solution

≪manifest.json≫:

{"manifest_version":2,"name":"","version":"0","background":{"scripts":["asd"]}}

≪asd≫:

navigator.serviceWorker.register('sdf.js').then(x=>console.log('done', x))

≪sdf.js≫:

addEventListener('fetch', e=> e.respondWith(fetch/*btw xhr is undefined*/(e.request).then(r=>{
  if(r.headers === undefined /*request not end with <.htm>, <.html>*/){}else{
    console.assert(r.headers.get('content-type')===null)/*for some odd reason this is empty*///[
      let h = new Headers()
      r.headers.forEach((v,k)=>h.append(k,v))
      Object.defineProperty(r,'headers',{'writable':true})
      r.headers = h
      r.headers.append('content-type','text/html'/*btw <htm> doesnt work*/)
    //]
  }
  return r.text().then(_=>new Response(_.replace(/cat/g,'dog'),r))
})))

Go to ≪page url≫ (≪chrome-extension://≪ext id≫/≪page path≫≫) and see the replacements.

---

Standalone response

≪manifest.json≫ and ≪asd≫ same as above.

≪sdf.js≫:

addEventListener('fetch', e=> e.respondWith(new Response('url: '+e.request.url,{headers:{'content-type':'text/html'/*, etc*/}})))

---

Btw

Serviceworker has other events that can be delved into eg:

addEventListener('message', e=>{
  console.log('onmessage', e)
})
addEventListener('activate', e=>{
  console.log('onactivate', e)
})
addEventListener('install', e=>{
  console.log('oninstall', e)
})

---

Alternatives

Note that currently serviceworker [–cf webNavigation, webRequest] is the only way to do this because some lamer in Google development team has decided that its "insecure" for response-modifying webNavigation and webRequest.

---

Note

Chrome bugs:

• Extension ≪Reload≫ will not reload serviceworkers. You need to remove the extension and load it back in.
• Page refresh does not refresh, even with chrome devtools disable cache hard reload. Use normal page visit.