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React.js + Flux server-side rendering with Koa

2015-07-15This post is over 2 years old and may now be out of date

(2 minute read)

I recently had to get server-side rendering working on a React.js client project. We had augmented our front-end React app with flummox and react-router and I wanted to be sure that we could re-use these components on the server-side without much extra work. I'm going to document our final file structure and code in this article.

## Front-end

Our front-end code is laid out as such:


The frontend folder contains our React app. The ui subfolder contains our React components and the data subfolder contains our Flux stores. The app.js file gets run on the client-side (but not the server-side):

var React = require('react'),
  Router = require('react-router'),
  appRoutes = require('./routes');
Router.run(appRoutes, Router.HistoryLocation, (Handler, state) => {
    <Handler routes={state.routes} params={state.params} query={state.query} />,

As you can see it bootstraps react-router and renders the page. The routes file contains our routes and our root App component:

var React = require('react'),
    Router = require('react-router');
var { Route, DefaultRoute, RouteHandler } = Router;
var Layout = require('./ui/pages/layout'),
    HomePage = require('./ui/pages/home');
var FluxManager = require('./data/index'),
  FluxComponent = require('./ui/components/fluxComponent');
var App = React.createClass({
  getInitialState: function() {
    return {
      flux: new FluxManager(),
  render: function() {
    return (
      <FluxComponent flux={this.state.flux}  connectToStores={['catalogue']}>
        <Layout {...this.state}>
          <RouteHandler {...this.props} {...this.state}/>
  componentDidMount: function() {
module.exports = (
  <Route handler={App}>
    <DefaultRoute name="home" handler={HomePage} />

We have a FluxManager class which inherits from the Flux class within the flummox package. You don't need to use a Flux implementation when building your React app and you certainly don't need one for server-side rendering. However if you do decide you need one then flummox is a pretty good choice as it's isomorphic.


In the back-end we replicate what was in the front-end app.js but instead render to a string which we then return to the client:

var frontendRoutes = require('../frontend/routes'),
  koa = require('koa'),
  Q = require('bluebird'),
  Router = require('react-router'),
  React = require('react');
var app = koa();
app.use(function *(){
  var reactOutput = yield new Q((resolve, reject) => {
    Router.run(frontendRoutes, this.request.path, (Handler, state) => {
        <Handler routes={state.routes} params={state.params} query={state.query} />
  this.body = '<!DOCTYPE html><html><body><div id="main">'
     + reactOutput 
     + '</div></body></html>';

Note: For simplicity sake I wrap the React output in a HTML string in the above code. In a practical implementation I would use a template engine like Jade instead.

Final thoughts

This setup works pretty well. One caveat to note is that server-side rendering just means your component's initial UI is rendered server-side. The componentDidMount() method and other related lifecycle methods only get executed on the client. Which means that if you're initiating a data fetch from within these methods (like I do above) then you'll still have the same behaviour you had before (in that data isn't fetched until client-side rendering has finished).

There are some solutions out there which attempt to make data-loading itself isomorphic. I have yet to investigate these fully.

One thing to keep in mind when shifting data fetches to be done server-side (i.e. prior to page rendering) is that your initial page load time will increase - which means your user will have to wait longer for your page to load from your server. If you fetch data once the app has rendered in the browser at least you can then display a loading indicator or a similar mechanism to the user to make your app feel responsive.

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