for...in

results.js

const seriesResults = [];

  // Original.
  for (const key in series.data) {
    const seriesID = parseInt(series.data[key].athlinks_series_id, 10);
    // Load Series information from the db.
    seriesResults.push(this._retrieveSerieInfoDynamodb(seriesID));
  }
  // Option 1
  series.data.map(seriesDataItem => {
    const seriesId = parseInt(seriesDataItem.athlinks_series_id, 10);
    return this._retrieveSerieInfoDynamodb(seriesId);
  })
    .forEach(seriesEntry => seriesResults.push(seriesEntry));

  // One liner does not require implicit return.
  // const seriesIds = series.data.map(seriesDataItem => parseInt(seriesDataItem.athlinks_series_id, 10));
  // Final
  const seriesDynamoIds = series.data.map(seriesDataItem => {
    const seriesId = parseInt(seriesDataItem.athlinks_series_id, 10);
    return this._retrieveSerieInfoDynamodb(seriesId);
  });

  seriesResults.push(...seriesDynamoIds);