gistfile1.js

var networkDataReceived = false;

startSpinner();

// fetch fresh data
var networkUpdate = fetch('/data.json').then(function(response) {
  return response.body.asJSON();
}).then(function(data) {
  networkDataReceived = true;
  updatePage();
});

// fetch cached data
caches.match('/data.json').then(function(response) {
  return response.body.asJSON();
}).then(function(data) {
  if (!networkDataReceived) {
    updatePage(data);
  }
}).catch(function() {
  // we didn't get cached data, the network is our last hope:
  return networkUpdate;
}).catch(showErrorMessage).then(stopSpinner);