code: function (mapName, index) {
              if (this.maps[mapName][index] === undefined) {
                  return [];
              }
              return this.maps[mapName][index].slice(); //slice() makes a copy of the array
          }
docs: Returns all the records stored in the `mapName` with the map value
  equal to `index`
  
  @public
  @param {String} mapName
  @param {String} index
  @return {Object[]}