function each(ary, fn) {
  ary = Array.from(ary);
  return (function next() {
    if (ary.length) {
      return Promise.resolve(fn(ary.shift())).then(next);
    }
  })();
}
each(category.channels, channel => {
  return list(channel).then(items => {
    return each(items, item => {
      return process(item, category.id);
    });
  });
});