npm install --save-dev @babel/plugin-proposal-class-properties code example

Example: '@babel/plugin-proposal-class-properties' webpack

const path = require('path');
const webpack = require('webpack');

module.exports = {
  target: 'node',
  mode: process.env.NODE_ENV === 'production' ? 'production' : 'development',
  entry: [ './src/index.js' ],
  output: {
    path: path.resolve(__dirname, 'build'),
    filename: 'index.js',
    library: 'index',
    libraryTarget: 'commonjs2'
  },
  plugins: [
    new webpack.IgnorePlugin(/^pg-native$/),
    new webpack.DefinePlugin({
      'process.env.BROWSER': false,
      __DEV__: process.env.NODE_ENV !== 'production',
    }),
  ],
  module: {
    rules: [
      {
        test: /\.(mjs|js|jsx)$/,
        exclude: /node_modules/,
        loader: 'babel-loader',


        options: {
          presets: [
            '@babel/preset-env',
            {
              plugins: [
                '@babel/plugin-proposal-class-properties'
              ]
            }
          ]
        },


      }
    ],
  }
};