webpack.dev.config.js

const webpack = require('webpack')

module.exports = config => Object.assign({}, {
  module: {
    loaders: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: 'babel-loader'
      },
      {
        test: /\.scss$/,
        use: [
          'style-loader',
          'css-loader',
          'sass-loader'
        ]
      }
    ]
  },
  plugins: [
    new webpack.SourceMapDevToolPlugin({
      filename: 'bundle.js.map'
    }),
    new webpack.DefinePlugin({
      'process.env.NODE_ENV': JSON.stringify('development')
    })
  ]
}, config)