old_contents: /**
   * @description Reducer for changing music state.
   */
  
  import * as ActionNames from '../actions/action-types';
  
  /**
   * @description Change state of music.
   * @param {Object} state
   * @param {Object} action
   * @return {Object} state
   */
  const music = (state, action) => {
    switch (action.type) {
      case ActionNames.ADD_MUSIC:
        return {
          id: action.id,
          songName: action.songName,
          albumName: action.albumName,
          artistName: action.artistName,
          filePath: action.filePath,
        };
      default:
        return state;
    }
  };
  
  /**
   * @description Make state of music list.
   * @param {Array} state
   * @param {Object} action
   * @return {Array}
   */
  const musics = (state = [], action) => {
    switch (action.type) {
      case ActionNames.ADD_MUSIC:
        return [
          ...state,
          music(undefined, action),
        ];
      default:
        return state;
    }
  };
  
  export default musics;
  
new_contents: /**
   * @description Reducer for changing music state.
   */
  
  import * as ActionNames from '../actions/action-types';
  
  /**
   * @description Change state of music.
   * @param {Object} state
   * @param {Object} action
   * @return {Object} state
   */
  const music = (state, action) => ({
      id: action.id,
      songName: action.songName,
      albumName: action.albumName,
      artistName: action.artistName,
      filePath: action.filePath,
  });
  
  /**
   * @description Make state of music list.
   * @param {Array} state
   * @param {Object} action
   * @return {Array}
   */
  const musics = (state = [], action) => {
    switch (action.type) {
      case ActionNames.ADD_MUSIC:
        return [
          ...state,
          music(undefined, action),
        ];
      default:
        return state;
    }
  };
  
  export default musics;
  
subject: remove not needed logic