How to use the @esri/arcgis-rest-portal.updateGroup function in @esri/arcgis-rest-portal

authentication: UserSession,
  isGroup: boolean = false
): Promise {
  const updateOptions: any = {
    params: {
      // Pass image in directly because item object is serialized, which discards a blob
      thumbnail: blob
    },
    authentication: authentication
  };
  updateOptions[isGroup ? "group" : "item"] = {
    id: itemId
  };

  return isGroup
    ? portal.updateGroup(updateOptions)
    : portal.updateItem(updateOptions);
}

export function addThumbnailFromUrl(
  url: string,
  itemId: string,
  authentication: UserSession,
  isGroup: boolean = false
): Promise {
  const updateOptions: any = {
    authentication: authentication
  };
  updateOptions[isGroup ? "group" : "item"] = {
    id: itemId,
    thumbnailurl: url
  };
  return isGroup
    ? portal.updateGroup(updateOptions)
    : portal.updateItem(updateOptions);
}