This snippet will show how to send a custom response to some requests, not all

selective-custom-request.js

// Offline Web Applications course on Udacity https://classroom.udacity.com/courses/ud899

//Using example from https://gist.github.com/luke-denton/2d0f00e76151d9e4bf412df8eb1bf354
// This example will replace all jpgs with superman.jpg
self.addEventListener('fetch', function(event) {
  if (event.request.url.endsWith('.jpg')) {
      event.respondWith(
        //Any request made will receive a superman jpg as a response
        fetch('/imgs/superman.jpg');
      );
  }
});