Brendan Sudol
June 2016

Intro to Isomorphic React

An isomorphic JavaScript app is one that runs the same code on both the server and client. Maybe not quite as cool as the fancy name may suggest, but it’s still pretty freaking cool, especially when paired with React. In essence, you can render a React component on the server and ship the HTML down to the client which then hooks into it and handles any future updates.

There are a number of isomorphic tutorials out there, but they all seemed rather complicated and overwhelming to me (and included things more relevant to larger apps, like additional routing and state management libraries). I wanted to make a simple “hello world” like example of an isomorphic React app. So here goes:

0. Initial set-up

To get started, you should have Node.js installed and have a basic understanding of using npm and JavaScript modules.

$ git clone
$ cd isomorphic-react-demo
$ npm install

1. The shared, React component:

import React from 'react';

class App extends React.Component {
  constructor(props) {
    this.state = {count: 0};
    this.handleClick = this.handleClick.bind(this);

  handleClick() {
    this.setState({count: this.state.count + 1});

  render() {
    return (
        <h1>Hi {}!</h1>
        <button onClick={this.handleClick}>
          Clicks: {this.state.count}

export default App

Nothing fancy here. All this component does is say ‘hi’ (to a name passed in as a prop) and counts the number of button clicks.

2. The server:

import express from 'express';
import React from 'react';
import ReactDOMServer from 'react-dom/server';

import App from './components/App';

let app = express();

app.use(express.static(__dirname + '/static'));
app.set('port', (process.env.PORT || 5000));
app.set('views', __dirname + '/views');
app.set('view engine', 'ejs');

app.get('/', (req, res) => {
  const data = {name: || "pal"};
  const content = ReactDOMServer.renderToString(<App {} />);

  res.render("index", {
    content: content,
    state: JSON.stringify(data)

app.listen(app.get('port'), () => {
  console.log(`Server running on port ${app.get('port')}...`);

And the view (which uses EJS for templating):

<!DOCTYPE html>
    <meta charset="utf-8">
      window.__STATE__ = <%- state %>    
    <div id="app"><%- content %></div>
    <script src="/bundle.js"></script>

I’m using Express, a minimal and popular Node.js web application framework.

There’s only one route. It checks for a name URL parameter and passes it to the React component which gets rendered to a string and shipped down to the view (and added within <div id="app">). Even if a user has JavaScript disabled, they’ll see the UI elements — woohoo!

3. The client:

import React from 'react';
import ReactDOM from 'react-dom';

import App from './components/App';

const data = window.__STATE__;
const div = document.getElementById('app');

ReactDOM.render(<App {} />, div);

The client-side JS picks up where the server left off. React notices the component already on the page and simply attaches event handlers to the relevant elements and handles future UI changes.

I’m using Webpack to transpile, bundle, and minify the client-side code (see config for more details).

4. Sharing state

One potential thing to keep in mind is sharing the state (i.e., React props) between the server and client. In this example, we pass the state down into the view (alongside the React component string), store it as a JavaScript global variable (window.__STATE__), and then consume this variable on the client.

5. Build

npm start

I added a few npm scripts to kick off the web server (as well as watch for changes and run tests).

After running npm start or npm run serve, open http://localhost:3000/?name=Bob and marvel at your new isomorphic app 😊