webpack.config.js

const path = require('path');

module.exports = {
  entry: ['@babel/polyfill', './src/index.js'],
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'bundle.js'
  },
  devServer: {
    contentBase: path.resolve(__dirname, 'dist'),
    watchContentBase: true,
    stats: 'errors-only',
    port: 3000,
    open: true
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: ['@babel/preset-env', '@babel/preset-react'],
            plugins: ['@babel/plugin-proposal-class-properties']
          }
        }
      },
      {
        test: /\.(sa|sc|c)ss$/,
        use: [
          'style-loader',
          'css-loader',
          {
            loader: 'sass-loader',
            options: {
              implementation: require('sass')
            }
          }
        ]
      }
    ]
  }
};