webpack.common.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.(js|jsx)$/,
        exclude: /(node_modules|bower_components)/,
        use: {
          loader: 'babel-loader'
        }
      },
      {
        test: /\.css$/i,
        use: ['style-loader', 'css-loader']
      },
      {
        test: /\.s[ac]ss$/i,
        use: [
          {
            loader: 'style-loader' // inject CSS to page
          },
          {
            loader: 'css-loader' // translates CSS into CommonJS modules
          },
          {
            loader: 'postcss-loader', // Run postcss actions
            options: {
              plugins: function () { // postcss plugins, can be exported to postcss.config.js
                return [
                  require('autoprefixer')
                ]
              }
            }
          },
          {
            loader: 'sass-loader' // compiles Sass to CSS
          }
        ]
      }
    ]
  }
}