Custom server config not working

[BenjaminToby]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/next-js-custom-server-bug-6z8fhc

To Reproduce

1. Create a basic next js application by running npx create-next-app@latest.
2. cd into the new directory and add a basic server.js file in the root directory with these content:

   
   const { createServer } = require("http");
   const { parse } = require("url");
   const fs = require("fs");
   const path = require("path");

const next = require("next");

const dev = process.env.NODE_ENV !== "production"; const hostname = "localhost"; const port = 3600;

const app = next({ dev, hostname, port, conf: { distDir: ".custom_dist", } });

const handle = app.getRequestHandler();

app.prepare() .then(() => { const server = createServer(async (req, res) => { try { const parsedUrl = parse(req.url || "", true); const { pathname, query } = parsedUrl;

            await handle(req, res, parsedUrl);
        } catch (err) {
            console.error("Error occurred handling", req.url, err);
            res.statusCode = 500;
            res.end("internal server error");
        }
    });

    server.on("error", (err) => {
        console.error("⚠️ SERVER ERROR =>", err.message);
    });

    server.listen(port, () => {
        console.log(`> Ready on http://${hostname}:${port}`);
    });
})
.catch((error) => {
    console.log(error);
});

3. Run `node server.js` to start the development server.

### Current vs. Expected behavior

### Expectation
When the development server is started, the dist directory should be loaded from the `conf` property in the `next` function argument, and there should be a new folder in the root directory named `.custom_dist`.

### Current Behavior
The default `.next` directory is used instead of the specified `.custom_dist` directory. 

### Verify canary release

- [X] I verified that the issue exists in the latest Next.js canary release

### Provide environment information

```bash
Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 14.0.4
  eslint-config-next: 14.0.4
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Additional context

This bug appeared with Next 13. I'm currently running Next 12 on my apps to avoid this bug.

[Bahlaouane-Hamza]

You need to also add distDir: '.custom_dist'to your next.config.js This works for me

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  distDir: '.custom_dist',
}

module.exports = nextConfig

[BenjaminToby]

@Bahlaouane-Hamza thanks for the suggestion. But the bug still persists. The Next JS documentation provides the option for a conf object which doesn't work in Next 13+, and this should be fixed.

In addition, there are scenarios where the next.config.js file isn't sufficient. For example: in one of my projects I'm loading the config object from a JSON file: this JSON file is tweaked during every build to generate a new build directory(for versioning purposes), and the server.js file picks up the new build directory automatically on every new build: of course it can't pick up the new directory in Next 13+ because the conf variable is broken.

[molily]

Here's what happens as far as I understand:

createServer – which is what you get with require('next') – instantiates NextCustomServer. Its superclass NextServer does save the passed config in this.options.conf:

https://github.com/vercel/next.js/blob/c1f8d9317588e51a8a31240f6add36b5f2c9f9bf/packages/next/src/server/next.ts#L71

But does not pass it through to the router server. So it's not used eventually.

https://github.com/vercel/next.js/blob/c1f8d9317588e51a8a31240f6add36b5f2c9f9bf/packages/next/src/server/next.ts#L274 https://github.com/vercel/next.js/blob/c1f8d9317588e51a8a31240f6add36b5f2c9f9bf/packages/next/src/server/lib/start-server.ts#L49 https://github.com/vercel/next.js/blob/c1f8d9317588e51a8a31240f6add36b5f2c9f9bf/packages/next/src/server/lib/router-server.ts#L63

The router server calls loadConfig without passing a customConfig/rawConfig:

https://github.com/vercel/next.js/blob/c1f8d93/packages/next/src/server/lib/router-server.ts#L82

So loadConfig always reads the next.config.js file:

https://github.com/vercel/next.js/blob/c1f8d9317588e51a8a31240f6add36b5f2c9f9bf/packages/next/src/server/config.ts#L865

So currently, the passed config is not used and next.config.js is currently only the source of truth.