cache-query.js

function queryAPI(path) {
  const cacheEntry = window.__data[path];
  if (!cacheEntry) {
    // issue a normal XHR API request
    return fetch(path);
  } else if (cacheEntry.data) {
    // the server has pushed us the data already
    return Promise.resolve(cacheEntry.data);
  } else {
    // the server is still pushing the data
    // so we'll put ourselves in the queue to 
    // be notified when its ready
    const waiting = {};
    cacheEntry.waiting.push(waiting);
    return new Promise((resolve) => {
      waiting.resolve = resolve;
    });
  }
}