function defineNestedRoutes()

in packages/route-manifest/src/index.ts [194:250]
50 lines of code
11 McCabe index (conditional complexity)

  function defineNestedRoutes(
    defineRoute: DefineRouteFunction,
    options: DefineRoutesOptions,
    parentId?: string,
  ): void {
    const childFileIds = fileIds.filter((id) => {
      const parentFileId = findParentFileId(fileIds, id);
      return parentFileId === parentId;
    });

    for (let fileId of childFileIds) {
      const parentRoutePath = removeLastLayoutStrFromId(parentId) || '';
      const routePath: string | undefined = createRoutePath(
        // parentRoutePath = 'home', routeId = 'home/me', the new routeId is 'me'
        // in order to escape the child route path is absolute path
        fileId.slice(parentRoutePath.length + (parentRoutePath ? 1 : 0)),
      );
      const routeFilePath = normalizeSlashes(path.join('src', 'pages', files[fileId]));
      if (RegExp(`[^${validRouteChar.join('')}]+`).test(routePath)) {
        throw new Error(`invalid character in '${routeFilePath}'. Only support char: ${validRouteChar.join(', ')}`);
      }
      const isIndexRoute = isIndexFileId(fileId);
      const fullPath = createRoutePath(fileId);
      const uniqueRouteId = (fullPath || '') + (isIndexRoute ? '?index' : '');

      if (uniqueRouteId) {
        if (uniqueRoutes.has(uniqueRouteId)) {
          throw new Error(
            `Path ${JSON.stringify(fullPath)} defined by route ${JSON.stringify(routeFilePath)}
            conflicts with route ${JSON.stringify(uniqueRoutes.get(uniqueRouteId))}`,
          );
        } else {
          uniqueRoutes.set(uniqueRouteId, fileId);
        }
      }

      if (isIndexRoute) {
        let invalidChildRoutes = fileIds.filter(
          (id) => findParentFileId(fileIds, id) === fileId,
        );

        if (invalidChildRoutes.length > 0) {
          throw new Error(
            `Child routes are not allowed in index routes. Please remove child routes of ${fileId}`,
          );
        }

        defineRoute(routePath, files[fileId], {
          index: true,
        });
      } else {
        defineRoute(routePath, files[fileId], () => {
          defineNestedRoutes(defineRoute, options, fileId);
        });
      }
    }
  }