json-cache.js

<script type="text/javascript">
  // the server will write out the paths of any API calls it plans to 
  // run server-side so the client knows to wait for the server, rather
  // than doing its own XHR request for the data
  window.__data = {
    '/my/api/path': {
        waiting: [],
    }
  };

  window.__dataLoaded = function(path, data) {
    const cacheEntry = window.__data[path];
    if (cacheEntry) {
      cacheEntry.data = data;
      for (var i = 0;i < cacheEntry.waiting.length; ++i) {
        cacheEntry.waiting[i].resolve(cacheEntry.data);
      }
      cacheEntry.waiting = [];
    }
  };
</script>