store.js

function store(opts) {
  var list = [];
  var local  = opts.local;
  var remote = opts.remote;

  function replace(items) {
    if (items.length) {
      list = items;
    }
    return items;
  }

  function fetchNew() {
    return local.last()
    .then(function(last) {
      return last ? remote.after(last) : remote.latest();
    });
  }

  function latest() {
    local.latest()
    .then(replace)
    .then(fetchNew)
    .then(replace)
    .catch(function(e) {
      console.error(e.stack);
    });

    return function() {
      return list;
    };
  }

  return {
    latest: latest
  };
}

module.exports = store;