Given a list of URLs, check to see whether each of them are already stored in a cache

index.js

const CACHE_NAME = 'my-cache'; // Or whatever your cache is.
let urls = []; // An array of full, absolute URLs that may or may not be cached.

caches.open(CACHE_NAME)
  .then(cache => cache.keys())
  .then(responses => responses.map(response => response.url))
  .then(urlsInCache => Promise.all(urls.map(url => {
    return {
      url: url,
      cached: urlsInCache.includes(url)
    };
  }))).then(urlCacheStatuses => {
    // Do something with each item in the urlCacheStatuses array,
    // i.e. change the display of the UI element corresponding to
    // item.url if item.cached is true.
  });