webpack.config.js

const buildValidations = require('./build-utils/build-validations');
const commonConfig = require('./build-utils/webpack.common');

const webpackMerge = require('webpack-merge');

// We can include Webpack plugins, through addons, that do 
// not need to run every time we are developing.
// We will see an example when we set up 'Bundle Analyzer'
const addons = (/* string | string[] */ addonsArg) => {
  
  // Normalize array of addons (flatten)
  let addons = [...[addonsArg]] 
    .filter(Boolean); // If addons is undefined, filter it out

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

// 'env' will contain the environment variable from 'scripts' 
// section in 'package.json'.
// console.log(env); => { env: 'dev' }
module.exports = env => {

  // We use 'buildValidations' to check for the 'env' flag
  if (!env) {
    throw new Error(buildValidations.ERR_NO_ENV_FLAG);
  }

  // Select which Webpack configuration to use; development 
  // or production
  // console.log(env.env); => dev
  const envConfig = require(`./build-utils/webpack.${env.env}.js`);
  
  // 'webpack-merge' will combine our shared configurations, the 
  // environment specific configurations and any addons we are
  // including
  const mergedConfig = webpackMerge(
    commonConfig,
    envConfig,
    ...addons(env.addons)
  );

  // Then return the final configuration for Webpack
  return mergedConfig;
};