webpack.config.js

const path = require("path");
const webpack = require('webpack');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const dotEnv = require('dotenv')

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

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

module.exports = (env) => {
  const isProduction = env.production === true;
  return {
    mode: isProduction ? "production": "development",
    entry: ['babel-polyfill' ,"./src/app.js"],
    output: {
      path: path.join(__dirname, "public", "dist"),
      filename: "bundle.js"
    },
    plugins: [new MiniCssExtractPlugin({
        filename: "styles.css"
      }),
      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),
        'process.env.FIREBASE_APP_ID': JSON.stringify(process.env.FIREBASE_APP_ID)
      })
    ],
    module: {
      rules: [{
        test: /\.m?js$/,
        exclude: /node_modules/,
        loader: 'babel-loader',
      }, {
        test: /\.s?css$/,
        use: [
          MiniCssExtractPlugin.loader,
          {
            loader: "css-loader",
            options: {
              sourceMap: true
            }
          },
          {
            loader: "sass-loader",
            options: {
              sourceMap: true
            }
          }
        ]
      }]
    },
    devtool: isProduction ? "source-map": "inline-source-map",
    devServer: {
      contentBase: path.join(__dirname, "public"),
      historyApiFallback: true,
      publicPath: "/dist/"
    }
  }
}