js___webpack_fundamentals/webpack.config.js

const commonConfig = require("./build-utils/webpack.common");
const webpackMerge = require("webpack-merge");

const defaultEnv = { env: "prod" };

// Export a function returning config instead of a static config.
// The purpose is to allow use of the environment.
module.exports = (env = defaultEnv) => {
  const envConfig = require(`./build-utils/webpack.${env.env}`);

  // The latter overwrites (merges into) the former.
  const config = webpackMerge(commonConfig, envConfig);
  return config;
};