gistfile1.js

function assertOkResponse(response) {
  if (response.type === 'opaque' || response.status === 200) {
    return response;
  }
  throw Error("Bad response: " + response.status);
}

fetch(url, {'mode': 'cors'}).then(assertOkResponse).then(function(response) {
  // cache
}).catch(function(err) {
  console.error('Fetch Error :-S', err);
  throw err;
});