Add versioning to API routes

routeVersioning.ts

import {
  RequestHandler,
  Request,
  Response,
  NextFunction,
  Router,
} from 'express';

export enum APIVersion {
  v0 = 'v0',
  v1 = 'v1',
}

function getNewestByKeySort<K extends string, V>(o: { [key in K]?: V }) {
  const sortedKeys = (Object.keys(o) as K[]).sort();
  const newestKey = sortedKeys[sortedKeys.length - 1];
  return o[newestKey];
}

type VersionMap<T> = { [key in APIVersion]?: T };

export function isAPIVersion(version: string): version is APIVersion {
  return Object.values(APIVersion).some(v => v === version);
}

export function switchVersions(versions: VersionMap<RequestHandler | RequestHandler[]>) {
  const versionsRouters = (Object.keys(versions) as APIVersion[])
    .sort()
    .reduce<VersionMap<Router>>((prev, version, i, keys) => {
      const handlerOrHandlers = versions[version];

      if (handlerOrHandlers) {
        prev[version] = Router()
          .use(handlerOrHandlers);

        const previousRouter = i > 0 && prev[keys[i - 1]];
        if (previousRouter) {
          prev[version]?.use(previousRouter);
        }
      }
      return prev;
    }, {});

  return (req: Request, res: Response, next: NextFunction) => {
    // If the version is not specified in the URL then it defaults to the 'v0'.
    const version = req.version || APIVersion.v0;
    // If the version handlers were not provided the newest API version handlers will be used.
    const versionRouter = versionsRouters[version] || getNewestByKeySort(versionsRouters);
    if (!versionRouter) {
      return res.status(400).send({
        error: {
          message: `API version "${version}" for the endpoint "${req.path}" is not supported.`,
        },
      });
    }
    return versionRouter(req, res, next);
  }
}

// Middleware extracting the API version from the URL.
export function withVersion(handler: RequestHandler) {
  return (req: Request, res: Response, next: NextFunction) => {
    const { version: versionOrSegment }: { version?: string } = req.params;

    if (!versionOrSegment) return res.status(400).send({
      error: {
        message: 'Invalid URL path.',
      },
    });

    if (isAPIVersion(versionOrSegment)) {
      req.version = versionOrSegment;
    } else {
      req.url = `/${versionOrSegment}${req.url}`;
    }
    return handler(req, res, next);
  }
}