Next.JS issue with build vs local dev

I was able to resolve this issue. In my case, it was due to the difference between how I had set up (dev) and next build && next start. Take note of a few things to get it sorted -

1. If you have used an external library, chances are it doesn't support server-side rendering. I was using react-carousel from Brainhubeu and it having some issues. Resolve it by importing the library via next/dynamic imports with ssr:false option.

2. Another issue was that I was using/following an outdated boilerplate code for Tailwind and NextJS. Hence the way postcss.config.js was configured was error-prone. Here is the crux of the problem. at least for me - During the dev, everything worked fine because postcss didn't purge any of the classes of third party plugins/libraries that I imported, however, they were being purged when I did npm run build and npm run start

Now let's quickly see how you can solve this issue on your part -

1. Use the inbuilt purge option provided by TailwindCSS. For this, use the official starter-code/boilerplate code from the NextJS team. As they mention there - To control the generated stylesheet's filesize, this example uses Tailwind CSS' purge option to remove unused CSS.

2. Or you could try patching the problem if you don't want to go the previous way. I would only suggest this method if you are almost done with the project and just want to get this working because this is in no way a good solution. You will have to whitelist a bunch of css files from being purged. This is what I did but you can probably whitelist a lost more CSS classes as well -

// postcss.config.js 
  
const purgecss = [
  "@fullhuman/postcss-purgecss",
  {
    content: [
      "./node_modules/@brainhubeu/react-carousel/lib/style.css",
      "./node_modules/@brainhubeu/react-carousel/lib/style.css.map",
      "./node_modules/react-toastify/dist/*.css",
      "./components/**/*.js",
      "./pages/**/*.js",
    ],
    defaultExtractor: (content) => {
      const broadMatches = content.match(/[^<>"'`\s]*[^<>"'`\s:]/g) || [];
      const innerMatches = content.match(/[^<>"'`\s.()]*[^<>"'`\s.():]/g) || [];
      return broadMatches.concat(innerMatches);
    },
  },
];
module.exports = {
  plugins: [
    "postcss-import",
    "tailwindcss",
    "autoprefixer",
    ...(process.env.NODE_ENV === "production" ? [purgecss] : []),
  ],
};

Still I would suggest go the first way and copy your code from old repo to new, use SSR:false to use client-side libraries that hate SSR, and you should be good to go.

You can reach out to my Twitter @toughyear if you need help with this.