[Material-UI] Using custom server in production mode does not attach MUI production classes

Bug report

Describe the bug

when I was working with Material UI. the development build is working good. but when I was running production build with custom server in next js. It is not rendering the jss class correctly in production mode. It is working correctly when I was doing next build and next start(without custom server).

To Reproduce

Clone Material UI with NextJS example from repo: https://github.com/mui-org/material-ui/tree/master/examples/nextjs

Create custom server.js

const express = require('express');
const next = require('next');
const dev = process.env.NEXT_ENV.trim() !== 'production';
const app = next({ dev });

app
  .prepare()
  .then(() => {
    const server = express();
    const handle = app.getRequestHandler();

    const port = process.env.PORT || 3000;

    server.get('*', (req, res) => handle(req, res));
    server.listen(port, err => {
      if (err) throw err;
      console.log(`Server is running on port ${port}`);
    });
  })
  .catch(ex => {
    console.error(ex.stack);
    process.exit(1);
  });

Run npm script: set NODE_ENV=production && node server.js

Expected behavior

Next js with custom server should load the correct JSS class name in production mode(set NODE_ENV=production && node server.js ).

[Yzrsah]

@CalvinSundar

Next and Material-UI (Branch Next) is tested and working with JSS and custom server.js with NODE_ENV=production and no build.

To Reproduce:

• Clone the Next branch version of the Material-UI Next.js example: Here
• Add custom server.js
• Start with NODE_ENV=production && node server.js script

Outcome:

• JSS rendered correctly

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@CalvinSundar Please provide a visual screenshot from DevTools showing incorrect JSS classes so we can understand if there’s any issues.

@timneutkens The issue seems closable.

[oliviertassinari]

@timneutkens You can close the issue: NEXT_ENV !== NODE_ENV.

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Do you plan on having a plugin system? I wish people could install a simple package to make Material-UI works.