Custom loader for Webpack (in Next.js)

loader.js

module.exports = function (source) {
  console.log("The original file was here:", this.resourcePath);
  console.log(source);

  return source;
};

next.config.js

const withLinaria = require("next-linaria");
const path = require("path");

module.exports = withLinaria({
  reactStrictMode: true,
  webpack: (config) => {
    const mutatedConfig = config;
  
    // Position 0
    const loaders = {
      "custom-loader": path.resolve("./loader.js"),
      ...mutatedConfig.resolveLoader.alias,
    };

    mutatedConfig.resolveLoader.alias = loaders;

    console.log(mutatedConfig.resolveLoader.alias);

    mutatedConfig.module.rules.push({
      test: /\.js$/i,
      use: [
        {
          loader: "custom-loader",
        },
      ],
    });

    return mutatedConfig;
  },
});