Change downloaded javascript using ServiceWorker

ServiceWorker.js

/*
  Using ServiceWorker, every request (except from iframes) can be intercepted and the javascript 
  could be changed before returning the result. Here is an example that if "*foo.js" is
  requested, downloads it, but then returns 'console.log("hi there");'
  
  Related: https://github.com/bahmutov/service-turtle/issues/7
*/
var allowJavaScriptFromAnywhere = {
  'Access-Control-Allow-Origin': '*',
  'Content-Type': 'text/javascript; charset=utf-8'
};

function javascriptResponse(src) {
  var responseOptions = {
    status: 200,
    headers: allowJavaScriptFromAnywhere
  };
  return new Response(src, responseOptions);
}

self.addEventListener('fetch', function (event) {
  // if page tries to download foo.js
  if (/foo\.js$/.test(event.request.url)) {
    event.respondWith(
      fetch(event.request.clone())
        .then(function (response) {
          // response has possible text / json / blob as promises
          return response.text().then(function (src) {
            // src - original source code
            var transformedSource = 'console.log("hi there, from ServiceWorker!");';
            // construct new response with changed JavaScript source
            return javascriptResponse(transformedSource);
          });
        })
    );
    return;
  }
});