webpack.base.config.js

const { resolve } = require('path');

module.exports = {
  context: resolve(__dirname, '../'),
  entry: ['babel-polyfill', './app/js/main.js'],
  resolve: {
    extensions: [
      '*', '.js', '.jsx',
    ],
    modules: [
      resolve(__dirname, '../app'),
      'node_modules',
    ],
  },
  output: {
    filename: '[name]-[hash:6].js',
    path: resolve(__dirname, '../build'),
    publicPath: '/',
  },

  module: {
    rules: [
    ...
    ],
  },

  plugins: [
    new webpack.DefinePlugin({
      'process.env': {
        NODE_ENV: JSON.stringify(process.env.NODE_ENV),
        API_URL: JSON.stringify(process.env.API_URL),
      },
    }),
  ],
};