webpack.config.js

const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const Dotenv = require('dotenv-webpack');

module.exports = {
  mode: 'development',

  entry: {
    KintoneCustomization: path.join(__dirname, 'src', 'index.js'),
  },

  output: {
    path: path.resolve(__dirname, 'dist'),
  },

  resolve: {
    extensions: ['.js', '.json']
  },

  module: {
    rules: [
      {
        test: /\.?js$/,
        exclude: /(node_modules|bower_components)/,
        use: {
          loader: 'babel-loader', // https://webpack.js.org/loaders/babel-loader/#root
          options: {
            presets: [
              '@babel/preset-env',
              ['@babel/preset-react', { 'runtime': 'automatic' }]
            ],  // https://babeljs.io/docs/en/presets/
            plugins: ['@babel/plugin-transform-runtime'],
          }
        }
      },
      {
        test: /\.css$/i,
        use: ['style-loader', 'css-loader'],
      }
    ]
  },

  plugins: [
    new HtmlWebpackPlugin({
      template: path.join(__dirname, 'src', 'index.html'),
    }),
    new Dotenv({
      safe: true, // load '.env.example' to verify the '.env' variables are all set. Can also be a string to a different file.
    })
  ]
};