code: function (layer) {
                  if (!layer) {
                      throw new ArgumentError(
                          Logger.logMessage(Logger.LEVEL_SEVERE, "GeoJSONExporter", "exportLayer",
                              "missingLayer"));
                  }
  
                  return this.exportRenderables(layer.renderables);
              }
docs: Exports a [Layer]{@link Layer} in GeoJSON format of type GeometryCollection.
  @param {Layer} layer The layer to export.
  @throws {ArgumentError} If the specified argument is null or undefined.
  @returns {String} GeoJSON format.