webpack.config.js

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

const extractSass = new ExtractTextPlugin({
  filename: "app.[contenthash].css",
  disable: process.env.NODE_ENV === "development"
});

module.exports = {
  entry: ['./src/index.js', './src/assets/scss/app.scss'],
  output: {
    path: path.resolve(__dirname, './build'),
    filename: 'app.[hash].js'
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: ['babel-loader']
      },
      {
        test: /\.scss$/,
        use: extractSass.extract({
          use: [{
            loader: "css-loader"
          }, {
            loader: "sass-loader"
          }],
          fallback: "style-loader"
        })
      }
    ]
  },
  devServer: {
    port: 3000,
    historyApiFallback: {
      index: 'index.html'
    }
  },
  plugins: [
    new HtmlWebpackPlugin({
      filename: 'index.html',
      template: './src/index.html'
    }),
    extractSass
  ]
}