Promise helper which works like mapM

promise-util.js

/**
 * @template T, S
 * @param  {[T]} items
 * @param  {function(T):Promise<S>} f
 * @return {Promise<[S]>}
 */
function traverse(items, f) {
   if (items.length == 0) {
    return Promise.resolve([]);
  }
  var item = items.shift();
  return f(item).then(v => traverse(items, f).then(rest => [v].concat(rest)));
}