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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?

Yaroslav Nekryach
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  • It's probably because your middleware in this example is calling ".end()" which is stopping the request there. Express middleware needs to perform its logic and then call "next()" in order for the request to continue to a route. – nerdy beast Oct 08 '20 at 15:27
  • It is an example. I don't want the middleware to be called at all. – Yaroslav Nekryach Oct 09 '20 at 07:41
  • It is not really clear what you want to achieve. If you don't want the globally registered middleware to be called, don't use `res.end()` use `next()` but I think that you generally need to restructure your application. Not sure if that is what you are looking for, but checkout the sections about Guards in NestJS documentation? Otherwise please clarify what you are trying to achieve – Mathias Maerker Oct 09 '20 at 09:50

1 Answers1

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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.

Joel V
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