←  offliner-fetcher-urls.js
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How to write a fetcher
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Remember including offliner will export the off module with a namespace reserved for fetchers. Put your fetcher inside.

 
self.off.fetchers.urls = {
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Provide a type for your fetcher. All resources with the specified type will be handled by your fetcher.

 
  type: 'url',
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Provide a normalize() function if you want to provide a shortcut for resources. This case we allow to simply pass a string and we normalize it to a resource that can be handled by the prefetch() method.

 
  normalize: function (resource) {
    return { type: this.type, url: resource };
  },
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The prefetch() is passed with a list of resources of the fetcher's type and the cache to be populated. It must return a Promise resolving when prefetching is done.

 
  prefetch: function (resources, cache) {
    return Promise.all(resources.map(function (resource) {
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The HTTP cache is still working when leavin the service worker via fetch() so it's neccessary to bust the URL to recover a fresh copy.

 
      var bustedUrl = resource.url + '?__b=' + Date.now();
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The request is for the busted url in no-cors mode to allow resources from other origins.

 
      var request = new Request(bustedUrl, { mode: 'no-cors' });
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But when caching, the cache is for the original URL.

 
      return fetch(request).then(cache.put.bind(cache, resource.url));
    }));
  }
};