App.jsx

import styles from './styles.scss'

class SomeComponent extends React.Component {
  render () {
    return (
      <div className={styles['some-component']}>
        SomeComponent
      </div>
    )
  }
}

export default SomeComponent

webpack.config.js

const path = require('path')
const webpack = require('webpack')
const ExtractTextPlugin = require('extract-text-webpack-plugin')

const config = require('./config')

module.exports = {
  entry: {
    app: ['babel-polyfill', './src/app.js'],
  },
  output: {
    path: path.resolve('./build'),
    filename: '[name]-[hash].js',
    publicPath: '/'
  },
  module: {
    rules: [
      {
        test: /\.css?$/,
        use: ExtractTextPlugin.extract({
          use: ['css-loader']
        })
      },
      {
        test: /\.scss$/,
        exclude: [path.resolve('node_modules')],
        use: ExtractTextPlugin.extract({
          fallback: 'style-loader',
          use: [
            {
              loader: 'css-loader',
              options: {
                modules: true,
                sourceMap: true,
                localIdentName: '[local]-[hash:base64:5]'
              }
            },
            'postcss-loader',
            'sass-loader'
          ]
        })
      }
    ]
  },
  plugins: [
    new ExtractTextPlugin({
      filename: process.env.NODE_ENV === 'development'
        ? 'bundle.css'
        : 'bundle-[contenthash:base64:10].css',
      disable: process.env.NODE_ENV === 'development',
      allChunks: true
    })
  ],
  resolve: {
    extensions: ['.js', '.json', '.jsx', '.css', '.scss']
  }
}