How to hash CSS module class names in Nextjs 13?

Question

How can I edit/minify/hash/hide/obfuscate css class names in Next JS?

I tried many ways including this thread

Getting the following errors when trying this solution.

yarn build
yarn run v1.22.19
$ next build
warn  - You have enabled experimental feature (appDir) in next.config.js.
warn  - Experimental features are not covered by semver, and may cause unexpected or broken application behavior. Use at your own risk.
info  - Thank you for testing `appDir` please leave your feedback at https://nextjs.link/app-feedback

warn  - The @next/font/google font Inter has no selected subsets. Please specify subsets in the function call or in your next.config.js, otherwise no fonts will be preloaded. Read more: https://nextjs.org/docs/messages/google-fonts-missing-subsets
info  - Creating an optimized production build
Failed to compile.

HookWebpackError: Unexpected '/'. Escaping special characters with \ may help.
    at makeWebpackError (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\webpack\bundle5.js:28:308185)   
    at C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\webpack\bundle5.js:28:105236
    at eval (eval at create (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\webpack\bundle5.js:13:28771), <anonymous>:44:1)
-- inner error --
Error: Unexpected '/'. Escaping special characters with \ may help.
    at C:\k\vercel\static\css\9db6a345a2f242fe.css:1:817
    at Root._error (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:78465)   
    at Root.error (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:124360)   
    at Parser.error (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:86811)  
    at Parser.unexpected (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:87297)
    at Parser.combinator (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:85544)
    at new Parser (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:78322)    
    at Processor._root (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:95242)
    at Processor._runSync (C:\k\vercel\node_modules\.pnpm\next@13.0.6_m5sxuueb27gk6ddc5gums6vtgq\node_modules\next\dist\compiled\cssnano-simple\index.js:190:95749)


> Build failed because of webpack errors
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Answer 1

Thanks to @brc-dd for updating his answer here.

The following config worked for me.

const path = require("path");
const loaderUtils = require("loader-utils");

const hashOnlyIdent = (context, _, exportName) =>
  loaderUtils
    .getHashDigest(
      Buffer.from(
        `filePath:${path
          .relative(context.rootContext, context.resourcePath)
          .replace(/\\+/g, "/")}#className:${exportName}`
      ),
      "md4",
      "base64",
      6
    )
    .replace(/[^a-zA-Z0-9-_]/g, "_")
    .replace(/^(-?\d|--)/, "_$1");

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

  webpack(config, { dev }) {
    const rules = config.module.rules
      .find((rule) => typeof rule.oneOf === "object")
      .oneOf.filter((rule) => Array.isArray(rule.use));

    if (!dev)
      rules.forEach((rule) => {
        rule.use.forEach((moduleLoader) => {
          if (
            moduleLoader.loader?.includes("css-loader") &&
            !moduleLoader.loader?.includes("postcss-loader")
          )
            moduleLoader.options.modules.getLocalIdent = hashOnlyIdent;
        });
      });

    return config;
  },
};

module.exports = nextConfig;