webpack.config.js

const path = require('path');
const webpack = require('webpack');
const ExtractTextPlugin = require('extract-text-webpack-plugin');

process.env.NODE_ENV = process.env.NODE_ENV || 'development';

if (process.env.NODE_ENV === 'test'){
    require('dotenv').config({ path: '.env.test' });
} else if(process.env.NODE_ENV === 'development'){
    require('dotenv').config({ path: '.env.development' });
}

module.exports = (env) => {
  const isProduction = env === 'production';
  const CSSExtract = new ExtractTextPlugin('styles.css');

  return {
      entry: ['babel-polyfill', './src/app.js'],
      output: {
          path: path.join(__dirname, 'public', 'dist'),
          filename: 'bundle.js'
      },
      module: {
          rules: [{
              loader: 'babel-loader',
              test: /\.js$/,
              exclude: /node_modules/
          }, {
              test: /\.s?css$/,
              use: CSSExtract.extract({
                  use: [
                      {
                          loader: 'css-loader',
                          options: {
                              sourceMap: true
                          }
                      },
                      {
                          loader: 'sass-loader',
                          options: {
                              sourceMap: true
                          }
                      },
                  ]
              })
          }]
      },
      plugins: [
            CSSExtract,
            new webpack.DefinePlugin({
                'process.env.FIREBASE_API_KEY': JSON.stringify(process.env.FIREBASE_API_KEY),
                'process.env.FIREBASE_AUTH_DOMAIN': JSON.stringify(process.env.FIREBASE_AUTH_DOMAIN),
                'process.env.FIREBASE_DATABASE_URL': JSON.stringify(process.env.FIREBASE_DATABASE_URL),
                'process.env.FIREBASE_PROJECT_ID': JSON.stringify(process.env.FIREBASE_PROJECT_ID),
                'process.env.FIREBASE_STORAGE_BUCKET': JSON.stringify(process.env.FIREBASE_STORAGE_BUCKET),
                'process.env.FIREBASE_MESSAGING_SENDER_ID': JSON.stringify(process.env.FIREBASE_MESSAGING_SENDER_ID)
            })
      ],
      devtool: isProduction ? 'source-map' : 'inline-source-map',
      devServer: {
          contentBase: path.join(__dirname, 'public'),
          hot: true,
          historyApiFallback: true, //For routing
          publicPath: "/dist"
      }
  }
};