webpack.config.js

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

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'),
            historyApiFallback: true,
            publicPath: '/dist/'
        }
    };
};