code: function (mapName, callback) {
              this.mapFunctions[mapName] = callback;
              this.maps[mapName] = {};
              this.each(function (object) {
                  this.addObjectToMap(mapName, object);
              });
              return this;
          }
docs: Specify the map function.
  Map function takes two arguments: the object and emit function
  emit function takes 2 arguments: the index and the object to store on that index;
  
  @public
  @param {String} mapName
  @param {Function(object, emit)} callback
  @return {Com}