docs: function _eachSeries(arr, iterator, callback) {
    callback = callback || function() {};
    if (!arr.length) {
      return callback();
    }
    var completed = 0;
    var iterate = function() {
      iterator(arr[completed], function(err) {
        if (err) {
          callback(err);
          callback = function() {};
        }
        else {
          completed += 1;
          if (completed >= arr.length) {
            callback();
          }
          else {
            iterate();
          }
        }
      });
    };
    iterate();
  }