Using react-hot-loader with webpack

May 14, 2015

I was working on a project running its own nodejs HTTP server, with expressjs. It had a ReactJS front-end, and was using webpack to create some bundles, all good. But it was lacking of Hot Reloading.

Not working with hot reloading nowadays in like being stuck in 2000. F5. Maj+F5. Ctrl+R.

You lose the current state of the page. You lose your time to get it back. You lose your productivity. You lose your patience. You start accusing people around, and the ambience quickly sucks.

HR is something we must work with nowadays, front-end side, and back-end side (you don’t want to restart the API every time you add something).

It’s even more true when you work with websites and web-applications with CSS and JS bundles. You don’t want to rebundle the whole things over and over just because you changed a background-color or because you fixed an undefined is not a function.

We’ll see how to enable HR from an existing application using webpack, expressjs and react-hot-loader. We’ll start with the V1, then we’ll update our code to use the latest react-hot-loader V3, still in development, but already working (with React functional components!).

If you want to know more about how Webpack does HR (low-level), how webpack-[hot|dev]-middleware work, I suggest you to read: Webpack Hot Reloading and React: how ?.

A previous version of this post was using the WebpackDevServer with the proxy option. I decided to remove it and present this other solution instead, being simpler.


The less, the better

Some background and constraints:

  • In production, the front static resources will be served by some nginx not expressjs.
  • The expressjs server (api) should not change and still be up on the same port :3000.
  • The public assets paths and all other things that my expressjs is providing should not changed.
  • The generated HTML should not changed.
  • The whole HR thing to be quasi-transparent and can be disable in a production build (behind some environment variable).

The existing code

Note that we won’t use babel, nor JSX, nor import/export. We keep it simple but we are still using ES2015 features (Chrome supports them now).

The expressjs server is running on localhost:3000, fairly simple:

const express = require('express')
const path = require('path')

const app = express()
// Read "public/" physical folder as "/". eg: http://localhost:3000/index.html
app.use(express.static(path.join(__dirname, 'public')))
app.use('/dist', express.static(path.join(__dirname, 'dist'))) // JS bundles in /dist/bundle.js
app.get('/api', (req, res) => res.send('Hello World!'))
app.listen(3000, () => console.log("Listening to localhost:3000"))

We have an index and some components defined:

var React = require('react')
var ReactDOM = require('react-dom')

    new React.createFactory(require('./Hello.js'))(),

For HR to work, render must refer to components defined in other files.

var React = require('react')

module.exports = class Hello extends React.Component {
    constructor() { super(); this.state = { message: null } }
    componentWillMount() { // we call our classic expressjs api
        fetch('./api').then(res => res.text())
            .then(message => this.setState({ message }))
    render() {
        return React.createElement("div", { style: { backgroundColor: 'orange' }},
                                          [ this.state.message || 'loading...' ])

A classic index.html SPA:

    <div id="app"></div>
    <script src="dist/bundle.js"></script>

And our initial and minimal webpack.config.js:

var path = require('path')
var webpack = require('webpack')

module.exports = {
    entry: [ './src/index.js' ],
    output: {
        path: path.join(__dirname, 'dist'),
        filename: 'bundle.js',
        publicPath: '/dist/'
    module: {
        loaders: [{
            test: /\.js$/,
            loaders: [],
            include: path.join(__dirname, 'src')


The addons


We’ll obviously start by adding the packages to handle HR:

  • react-hot-loader: the plugin that catch HR events in the front-end and update the view.
  • webpack-dev-middleware: recompiles the Javascript bundle when a source changes and serves the bundle up-to-date.
  • webpack-hot-middleware: notify the front-end through SSE when the bundle has been recompiled.
$ npm install --save-dev react-hot-loader webpack-dev-middleware webpack-hot-middleware
+ "react-hot-loader": "^1.3.1",
+ "webpack-dev-middleware": "^1.9.0",
+ "webpack-hot-middleware": "^2.13.2"

Webpack configuration

We tell webpack to:

  • Inject the webpack HR API code in the bundle using the plugin HotModuleReplacementPlugin.
  • Inject the webpack-hot-middleware/client code into the bundle (that is using the webpack HR API).
  • Wrap the ReactJS components with some react-hot-loader code.
     entry: [
+        'webpack-hot-middleware/client'

    loaders: [{
        test: /\.js$/,
-       loaders: [],
+       loaders: ['react-hot-loader'],
        include: path.join(__dirname, 'src'),

+   plugins: [
+       new webpack.HotModuleReplacementPlugin()
+   ],


Finally, we must inject the middlewares into expressjs, to allow webpack to expose some special routes used for the HR piece.

Things like __webpack_hmr, [guid].hot-update.js[on] that contains the updated code.

Note that the bundle (parts) is recompiled into memory, the physical bundle.js is not updated (for the sake of performance).

// server.js

+var webpack = require('webpack');
+var webpackDevMiddleware = require("webpack-dev-middleware");
+var webpackHotMiddleware = require("webpack-hot-middleware");
+var config = require('./webpack.config.js');
+var compiler = webpack(config);
+app.use(webpackDevMiddleware(compiler, { quiet: true, publicPath: config.output.publicPath }));

Be sure the /dist is removed, otherwise expressjs will serve bundle.js from it, and not the HR-enhanced version. Or comment out app.use('/dist', express.static(path.join(__dirname, 'dist'))) if you had something like this.

publicPath is necessary in the configuration of the webpackDevMiddleware, that indicates to the middleware what is the path of the HR bundle to serve by expressjs (/dist/bundle.js).

HR is ready!

Now, we can node server.js:

  • our expressjs API is working (/api).
  • we can edit our components and they will be updated in live.

You can checkout the code here:

react-hot-loader V3

RHL v3 is working a bit differently, it’s more explicit.

Here are what we need to do:

npm install --save-dev react-hot-loader@next
   "devDependencies": {
-    "react-hot-loader": "^1.3.1",
+    "react-hot-loader": "^3.0.0-beta.6",

Our webpack.config.js:

    entry: [
+       'react-hot-loader/patch',

    loaders: [{
        test: /\.js$/,
-       loaders: ['react-hot-loader'],
+       loaders: ['react-hot-loader/webpack'],
        include: path.join(__dirname, 'src'),

RHL needs to setup some code before all, so we can use entry to do that (but require/import the file in index.js would work too).

Most of the changes are in the index.js file:

 var React = require('react')
 var ReactDOM = require('react-dom')

+var AppContainer = require('react-hot-loader').AppContainer
 var Hello = require('./Hello.js')

-  new React.createElement(Hello, null),
+  new React.createElement(AppContainer, null, React.createElement(Hello, null)),
+  // ie: <AppContainer><Hello></AppContainer>
+  document.getElementById('app'));
+if ( {
+'./Hello', () => {
+    const NextApp = require('./Hello')
+    // you could use `require('./Hello').default` if you're using import/export syntax
+    ReactDOM.render(
+      new React.createElement(AppContainer, null, React.createElement(NextApp, null)),
+      document.getElementById('app')
+    )
+  })
  • As it is the way to do things now, we import a container to wrap our App (higher-order component, HoC).
  • We handle ourself webpack HR API by rerendering the container on changes.

And… it’s done! the HR is working.

One of the big advantage is that functional components are now Hot Reloadable too, ie:

const Title = (props) => React.createElement("div", null, props.text)
     render() {
-        return React.createElement("div", { style: { backgroundColor: 'orange' }},
-                                          [ this.state.message || 'loading...' ])
+        return React.createElement("div", null,
+                 React.createElement(Title, { text: this.state.message || 'loading...' }))
+        // <div><Title text={this.state.message || 'loading...'} /></div>

You can change the title component, the HR will follow, awesome.

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Me, Stéphane Derosiaux

Hi, I'm Stéphane, CTO of Conduktor, a major Apache Kafka Platform. I live in France. Enthousiast and eager to learn, I'm always seeking for challenges. Find out more about me.

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