index.js

import React from "react";
import {render} from "react-dom";
import App from "./App";

const el = document.getElementById("app");

if (__DEV__) { // 
  import("react-hot-loader").then(rhl => {
    const {AppContainer} = rhl;
    render(<AppContainer><App /></AppContainer>, el);
    if (module.hot) {
      module.hot.accept(["./App.js"], () => {
        const NewApp = require("./App").default;
        render(<AppContainer><NewApp /></AppContainer>, el);
      });
    }
  });
} else {
  render(<App />, el);
}

webpack.config.js

module.exports = {
  entry: {
    index: [
      "react-hot-loader/patch",
      "webpack-dev-server/client?http://0.0.0.0:1337",
      "webpack/hot/only-dev-server",
      "src/index.js"
    ]
  },
  devServer: {
    hot: true
  },
  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    new webpack.DefinePlugin({
      __DEV__: true,
      __PROD__: false
    })
  ]
}