js___webpack_fundamentals/webpack.config.js

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

const defaultEnv = { env: "prod" };

/**
 *
 * @param {string|Array} addonsArg
 * @return {unknown[]}
 */
const addons = addonsArg => {
  // Normalize array of addons (flatten).
  let addons = [].concat
    .apply([], [addonsArg])
    // If addons is undefined, filter it out.
    .filter(Boolean);

  return addons.map(addonName =>
    require(`./build-utils/addons/webpack.${addonName}.js`)
  );
};

// Export a function returning config instead of a static config.
// The purpose is to allow use of the environment.
module.exports = (env = defaultEnv) => {
  if (!env || !env.env) {
    throw new Error(
      "You must pass an -env.env flag into your build for webpack to work"
    );
  }

  console.log("Env: ", env);
  const envConfig = require(`./build-utils/webpack.${env.env}`);

  // The rightmost overwrites (merges into) its left sibling.
  const mergedConfig = webpackMerge(
    commonConfig,
    envConfig,
    ...addons(env.addons)
  );

  console.log(mergedConfig);
  return mergedConfig;
};