Migrating Express application to NestJS

Question

I have an existing express application, and it is large enough. I want to migrate it to NestJS application, but do it step by step. So I have created Nest app with an existed Express instance, by faced a problem - all nest handlers always add after Express handlers. But I have a root Express middleware I want to apply only for some handlers that go after it. How can I apply Nest handlers before Express handlers?

For example. I created a new NestJS project, and changed main.ts

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { ExpressAdapter } from '@nestjs/platform-express';
import * as express from 'express';

const expressApp = express();
const authMiddleware = (req, res) => res.end('Auth middleware');
expressApp.use(authMiddleware);
expressApp.get('/test', ((req, res) => res.end('Test')))

async function bootstrap() {
  const app = await NestFactory.create(AppModule, new ExpressAdapter(expressApp));

  await app.listen(3000);
}
bootstrap();

On the root path I expect to see Hello World! but I see Auth middleware

I understand that I can do something like

expressApp.get('/test', authMiddleware, ((req, res) => res.end('Test')))

But current project is too large with sub routers and not only one middleware. Maybe there is another way to do it right?

Answer 1

Your auth middleware is always ending the request and not behaving the way middleware usually does. Typically middleware has a signature like this...

const authMiddleware = (req, res, next) => {
  // do auth things like verify a JWT and attach it to req.user

  next()
}
expressApp.use(authMiddleware)

const loggingMiddleware = (req, res, next) => {
  const { headers, body, user, route } = req
  console.log({ headers, body, user, route })
  next()
}
expressApp.use(loggingMiddleware)

In the above code, the authMiddleware ALWAYS executes before the loggingMiddleware because it was attached first. If, say, we reversed the order and attached loggingMiddleware first, req.user would always be undefined. The Express documentation is very descriptive in how the middleware control flow operates, and NestJS follows the same patterns under the hood.