webpack.config.js

const path = require('path');
const webpack = require('webpack');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const detect = require('detect-port');
const portfinder = require('portfinder');

const DEFAULT_PORT = 3000;

const config = port => ({
  devtool: 'cheap-module-source-map',
  entry: {
    app: './src/index'
  },
  output: {
    path: path.join(__dirname, 'dist'),
    filename: 'bundle.[hash].js',
    publicPath: '/'
  },
  devServer: {
    port,
    hot: true,
    historyApiFallback: true,
    stats: 'errors-only',
    clientLogLevel: 'error'
  },
  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    new webpack.NamedModulesPlugin(),
    new webpack.DefinePlugin({
      'process.env': {
        NODE_ENV: JSON.stringify('development')
      }
    }),
    new HtmlWebpackPlugin({
      template: './src/index.html',
      filename: 'index.html'
    })
  ],
  module: {
    rules: [
      {
        test: /\.js$/,
        use: ['babel-loader'],
        exclude: /node_modules/
      },
      {
        test: /\.scss$/,
        use: [
          'style-loader?sourceMap',
          'css-loader?sourceMap&modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]!sass-loader'
        ]
      }
    ]
  },
});

module.exports = detect(DEFAULT_PORT).then(port => {
  if (port === DEFAULT_PORT) {
    return config(DEFAULT_PORT);
  }

  return portfinder.getPortPromise().then(port => config(port));
});