webpack.config.js

const webpack = require('webpack');
const HtmlWebPackPlugin = require('html-webpack-plugin');

// checks if it is script prod or dev
// looks for the corresponding env file
// return environment variables
const checkEnv = require('./checkEnv.setup');

const HtmlPlugin = new HtmlWebPackPlugin({
  template: './src/index.html',
  filename: './index.html',
});

// the env object is the one pass in the command script
module.exports = env => {
  const envKeys = checkEnv(env); // corresponding env variables
  const EnvironmentPlugin = new webpack.DefinePlugin(envKeys); // set enviromental variables in the compile time

  return {
    entry: './src/index.js', //default value
    module: {
      rules: [
        {
          test: /\.js$/,
          exclude: /node_modules/,
          use: {
            loader: 'babel-loader',
          },
        },
        {
          test: /\.(s*)css$/,
          use: ['style-loader', 'css-loader', 'sass-loader'],
        },
        {
          test: /\.(png|jpe?g|gif)$/i,
          use: [
            {
              loader: 'file-loader',
            },
          ],
        },
      ],
    },
    plugins: [HtmlPlugin, EnvironmentPlugin],
  };
};