Migrating from Custom Express Server with `express-validator` to Next API Routes

server.ts

import express, { Request, Response, NextFunction } from 'express';
import { check, validationResult, ValidationChain } from 'express-validator';
import bodyParser from 'body-parser';
import next from 'next';

const postMail = async (req: Request, res: Response): Promise<void> => {
  // Use nodemailer to post the email. Setup mailer() with nodemailer
  // and call it to Post an Email and return response
  const mailResponse = await mailer(req.body);
  res.json(mailResponse);
};

const postValidationRules = (): ValidationChain[] => {
  return [check('email').isEmail(), check('message').escape()];
};

const validate = (
  req: Request,
  res: Response,
  nextFn: NextFunction
): Response<string> | void => {
  const errors = validationResult(req);
  if (errors.isEmpty()) {
    return nextFn();
  }

  return res.status(422).json({
    errors,
  });
};

const port = parseInt(process.env.PORT || '3000', 10);
const clientUrl = process.env.CLIENT_URL || 'http://localhost';
const dev = process.env.NODE_ENV !== 'production';

const app = next({ dev });

const handle = app.getRequestHandler();

(async (): Promise<void> => {
  try {
    await app.prepare().then(() => {
      const server = express();

      server.use(bodyParser.json());
      server.use(bodyParser.urlencoded({ extended: true }));

      server.post(
        '/api/post-contact-form',
        postValidationRules(),
        validate,
        postMail
      );

      server.get('*', (req: Request, res: Response) => {
        return handle(req, res);
      });

      server.post('*', (req: Request, res: Response) => {
        return handle(req, res);
      });

      server.listen(port, (error: Error) => {
        if (error) throw error;
        console.info(`> ✔ Ready on Server: ${clientUrl}:${port}`);
      });
    });
  } catch (error) {
    console.error(`❌ Error`, error);
    process.exit(1);
  }
})();

I have created a repro and you can test it out here.

I left some inline-comments based on your previous recommendations and removed the input fields for brevity, created two separate form data formats for comparison, expecting the invalid one to fail. However both pass and return the final should be sanitized and emailed data.

I am currently looking into running the validation in an imperative way using run()
but the approach is too repetitive.