musicService.js

(function(module) {
  
  function getRecommendation(selectedGenre, limit) {
    ...

    return Promise.resolve()
      .then(getSpotifyAccessToken)
      .then(getRecommendation)

    function getRecommendation() {
      return spotify.getRecommendations({
        seed_genres: selectedGenre,
        limit: limit
      })
      .then((resp) => {
        return _parseForTrackList(resp.body.tracks, limit)
      })
    }
  }

  ...
}(exports));