CORS Issue when deploying on Vercel

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https://vercel.com/guides/using-express-with-vercel

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Vercel Staff

Hi , welcome to the Vercel Community!

Thanks for sharing the details and the repository. Let me try and recreate this issue.

I also tried what VO suggested me, it is on this branch.

But the result is still the same...

Here are the main changes:

const corsOptions = {
  origin: config.frontEndUrl,
  methods: ['GET', 'POST', 'PUT', 'DELETE', 'PATCH', 'OPTIONS'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  credentials: true,
  optionsSuccessStatus: 200,
};

app.use(cors(corsOptions));

{
  "version": 2,
  "builds": [
    {
      "src": "api/index.ts",
      "use": "@vercel/node"
    }
  ],
  "routes": [
    {
      "src": "/(.*)",
      "dest": "/api/index.ts"
    }
  ]
}

And even add middleware...

import { Request, Response, NextFunction } from 'express';
import { config } from '../your-config-file-path'; // Adjust this import as needed

export function corsMiddleware(req: Request, res: Response, next: NextFunction) {
  res.setHeader('Access-Control-Allow-Origin', config.frontEndUrl);
  res.setHeader('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, PATCH, OPTIONS');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  res.setHeader('Access-Control-Allow-Credentials', 'true');

  if (req.method === 'OPTIONS') {
    return res.status(200).end();
  }

  next();
}

Vercel Staff

Hi , thanks for the update.

Can you share the URLs where I can confirm the behavior? For example, the frontend URL that connects to any of the endpoints which isn't working as you expect it to.

I have 2 methods for testing:

First

curl -v --request OPTIONS 'https://dev-web2-backend.vercel.app/api/v1/auth/status' -H 'Origin: https://dev-web2-backend.vercel.app/api/v1/auth/status' -H 'Access-Control-Request-Method: GET'

VS

curl -v --request OPTIONS 'http://localhost:3001/api/v1/auth/status' -H 'Origin: http://localhost:3001/api/v1/auth/status' -H 'Access-Control-Request-Method: GET'

And then, on vercel, I deployed my frontend that is calling the same end-point,

I think I spotted the issue.

On main deployment (in sync with dev branch on github), it works.

If I do:

curl -v --request OPTIONS 'https://api.open-source-economy.com/api/v1/auth/status' -H 'Origin: https://api.open-source-economy.com/api/v1/auth/status' -H 'Access-Control-Request-Method: GET'

It returns:

< access-control-allow-credentials: true
< access-control-allow-headers: Content-Type, Authorization
< access-control-allow-methods: GET,HEAD,PUT,PATCH,POST,DELETE
< access-control-allow-origin: http://localhost:3000/
< cache-control: public, max-age=0, must-revalidate

So it is working. But on my dev branch and dev vercel deployment, the access control headers are not included..

Do you know how to fix that?

Vercel Staff

In the last reply,

curl -v --request OPTIONS 'https://api.open-source-economy.com/api/v1/auth/status' -H 'Origin: https://api.open-source-economy.com/api/v1/auth/status' -H 'Access-Control-Request-Method: GET'

Both the URLs seem to be of the same origin, so I'm not sure if that's what you were aiming for.

Also, why does it return access-control-allow-origin: http://localhost:3000/ on the main deployment. Shouldn't this be the domain of the frontend?

When I do that (wich is a preview deployment)

curl -v --request OPTIONS 'https://dev-web2-backend.vercel.app/api/v1/auth/status' -H 'Origin: https://dev-web2-backend.vercel.app/api/v1/auth/status' -H 'Access-Control-Request-Method: GET'

I do not not see any access-control-allow-XXXX and that is my problem.

I have them in the production deployment (and that works, if I will change the http://localhost:3000/ to the correct one), but not on the preview, I do have the access-control-allow-XXXX headers.

Why is so?