postcss-loader dependency does not support ESM

[Gamzee]

Bug report

while compiling css with postcss-loader it uses the dependency cosmiconfig but this does not support ESM type
and will give the error

Error [ERR_REQUIRE_ESM]: require() of ES Module ~\_frontend\postcss.config.js from ~\_frontend\node_modules\postcss-loader\node_modules\cosmiconfig\dist\loaders.js not supported.

Actual Behavior

when postcss loader uses cosmiconfig it uses non esm compatible code and gives an error and fails to compile the css

Expected Behavior

postcss loader should compile the css while being ESM compatible

How Do We Reproduce?

heres my testing repository:
https://github.com/LithRakoon/npm
npm i
npm run build
see error

Please paste the results of npx webpack info here, and mention other relevant information

  System:
    OS: Windows 10 10.0.22621
    CPU: (8) x64 Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz
    Memory: 5.70 GB / 15.89 GB
  Binaries:
    Node: 18.12.1 - C:\Program Files\nodejs\node.EXE
    npm: 9.1.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.22621.819.0), Chromium (107.0.1418.56)
    Internet Explorer: 11.0.22621.1
  Packages:
    babel-loader: ^8.1.0 => 8.3.0
    css-loader: ^6.7.2 => 6.7.2
    postcss-loader: ^7.0.1 => 7.0.1
    sass-loader: ^13.2.0 => 13.2.0
    style-loader: ^0.23.1 => 0.23.1
    webpack: ^5.75.0 => 5.75.0
    webpack-cli: ^5.0.0 => 5.0.0
    webpack-dev-server: ^3.10.3 => 3.11.3

[Gamzee]

forgot i made my repo private, its public now sorry for the inconvience

[Alexander Akait]

You can't use:

import autoprefixer from 'autoprefixer',
        import cssnano from 'cssnano' ({
            reduceIdents:  {keyframes: false},
            discardUnused: {keyframes: false}
        })

it is invalid syntax

[Gamzee]

while this is true using

import autoprefixer from 'autoprefixer';
import cssnano from 'cssnano';
		
export default {
    plugins: [
		cssnano,
		autoprefixer
    ]
};

still gives the same error of a postcss-loader dependency not supporting ESM

[Alexander Akait]

Please use:

import autoprefixer from 'autoprefixer';
import cssnano from 'cssnano';

[Gamzee]

still gives the error that a dependencie of postcss loader doesnt support esm and fails

[László Károlyi]

Same issue here.

[László Károlyi]

Related: cosmiconfig/cosmiconfig#224

[Alexander Akait]

Okay, let's fix it on our side, firstly, please fix your configuration:

import path from 'path';
import url from 'url';
import webpack from 'webpack';
import * as MiniCssExtractPlugin from 'mini-css-extract-plugin';

const extractSASS = new MiniCssExtractPlugin.default({filename: "app.min.css"});
const jQuery = new webpack.ProvidePlugin({
    $: "jquery",
    jQuery: "jquery"
});

const __filename = url.fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

export default {
    mode: "production",
    entry: {
        main: [
            "./src/js/index.js"
        ],
    },
    output: {
        filename: "app.min.js",
        path:     path.resolve(__dirname, "dist")
    },
    module: {
        rules: [
            {
                test:    /\.js$/,
                exclude: /node_modules/,
                use: {
					loader: "babel-loader"
				}
            },
            {
				test: /\.scss$/,
				use:  [
                    {
                        loader: MiniCssExtractPlugin.default.loader
                    },
                    {
                        loader: "css-loader"
                    },
                    {
                        loader:  "postcss-loader",
                        options: {
                            postcssOptions: {
                                path: path.resolve(__dirname, "postcss.config.js")
                            }
                        }
                    },
                    {
                        loader: "sass-loader"
                    }
                ]
            }
        ]
    },
        plugins: [
            extractSASS,
            jQuery
        ]
};

the second please fix your postcss.config.js

import autoprefixer from 'autoprefixer';
import cssnano from 'cssnano';

export default {
    plugins: [
        autoprefixer(),
        cssnano({
            reduceIdents:  {keyframes: false},
            discardUnused: {keyframes: false}
        })
    ]
};

and then we will aply this patch on our code:

const moduleName = "postcss";
  const explorer = (0, _cosmiconfig.cosmiconfig)(moduleName, {
    searchPlaces: [
      'package.json',
      `.${moduleName}rc`,
      `.${moduleName}rc.json`,
      `.${moduleName}rc.yaml`,
      `.${moduleName}rc.yml`,
      `.${moduleName}rc.js`,
      `.${moduleName}rc.mjs`,
      `.${moduleName}rc.cjs`,
      `.config/${moduleName}rc`,
      `.config/${moduleName}rc.json`,
      `.config/${moduleName}rc.yaml`,
      `.config/${moduleName}rc.yml`,
      `.config/${moduleName}rc.js`,
      `.config/${moduleName}rc.mjs`,
      `.config/${moduleName}rc.cjs`,
      `${moduleName}.config.js`,
      `${moduleName}.config.mjs`,
      `${moduleName}.config.cjs`,
    ],
    loaders: {
      '.js': async (...args) => {
        let result;

        try {
          result = _cosmiconfig.defaultLoaders['.js'](...args);
        } catch (error) {
          let importESM;

          try {
            // eslint-disable-next-line no-new-func
            importESM = new Function("id", "return import(id);");
          } catch (e) {
            importESM = null;
          }

          if (
              error.code === "ERR_REQUIRE_ESM" &&
              require("url").pathToFileURL &&
              importESM
          ) {
            const urlForConfig = require("url").pathToFileURL(args[0]);

            result = await importESM(urlForConfig);
          } else {
            throw requireError;
          }

        }

        return result;
      },
      '.mjs': async (...args) => {
        let importESM;

        try {
          // eslint-disable-next-line no-new-func
          importESM = new Function("id", "return import(id);");
        } catch (e) {
          importESM = null;
        }

        if (
            require("url").pathToFileURL &&
            importESM
        ) {
          const urlForConfig = require("url").pathToFileURL(args[0]);

          result = await importESM(urlForConfig);
        }

        return result;
      }
    }
  });

And now ESM configuration will work