Annotated webpack.config file from Tyler McG's React Course including HMR

webpack.config.babel.js

/**
 * putting babel into the filename means the file runs through babel
 * so we can use ES6 to merge configs at the bottom
 * */

import webpack from 'webpack'
import path from 'path'
import HtmlWebpackPlugin from 'html-webpack-plugin'

// references which npm command we are running - start or production
const LAUNCH_COMMAND = process.env.npm_lifecycle_event

const isProduction = LAUNCH_COMMAND === 'production'

// so we can use env property inside .babelrc
process.env.BABEL_ENV = LAUNCH_COMMAND

// dev and production directories
const PATHS = {
  // assumes 'app/index.js'
  app: path.join(__dirname, 'app'),
  build: path.join(__dirname, 'dist')
}

const HTMLWebpackPluginConfig = new HtmlWebpackPlugin({
  template: PATHS.app + '/index.html',
  filename: 'index.html',
  inject: 'body'
})

// when this new webpack plugin is enabled in the config it tells react
// we are in production mode
const productionPlugin = new webpack.DefinePlugin({
  'process.env': {
    NODE_ENV: JSON.stringify('production')
  }
})

// base is what production and dev have in common
const base = {
  entry: [
    // path now depends on prod vs dev
    PATHS.app
  ],
  output: {
    path: PATHS.build,
    filename: 'index_bundle.js'
  },
  module: {
    loaders: [
      {test: /\.js$/, exclude: /node_modules/, loader: 'babel-loader'},
      // enable css modules
      {test: /\.css$/, loader: 'style!css?sourceMap&modules&localIdentName=[name]__[local]___[hash:base64:5]'}
    ]
  },
  resolve: {
    // import statements automaticall start inside ./app to save ../ etc
    root: path.resolve('./app')
  }
}

const developmentConfig = {
  // source maps for debugging
  devtool: 'cheap-module-inline-source-map',
  // react-hmre settings
  devServer: {
    contentBase: PATHS.build,
    hot: true,
    inline: true,
    progress: true
  },
  plugins: [HTMLWebpackPluginConfig, new webpack.HotModuleReplacementPlugin()]
}

const productionConfig = {
  devtool: 'cheap-module-source-map',
  plugins: [HTMLWebpackPluginConfig, productionPlugin]
}

// merge base config object with relevant build / dev config
export default Object.assign({}, base, isProduction === true ? productionConfig : developmentConfig)