Tips for Building Universal JavaScript Apps

Following in the footsteps of some great JavaScript developers like Nicolás Bevacqua, I've built my own, over-engineered blog. In my case, I built it as a single-page application running on React and Redux client-side and Express server-side. While the endeavor hasn't been the most practical I've ever undertaken (a blog hardly needs to be a SPA), it's been extremely helpful to me in learning an awesome technology stack.

One challenge I wanted to tackle as part of this project was to get my blogging app to support both server-side and client-side rendering (make it a "universal" app). In my experience, server-side rendering is great for enabling a quick initial render and client-side rendering is great for quick subsequent page views. As part of achieving this goal, I discovered a few handy tips. I'm going to share these in the hopes they'll be of help to others.

Abstract API Calls

A universal app is easiest to understand when the amount of conditional logic creating separate code branches for the server-side and the client-side is kept to a minimum. But obviously some things have to be done differently in these different environments. On the client, we make Ajax requests and render DOM. On the server, we load data from a database and render a string of HTML to send to the client. For the view layer, a library like React is great for keeping conditional logic away. You pretty much write your components in a client-side mindset and React will take care of what not to do server-side (e.g. setting up event listeners). For other concerns, you have to come up with your own abstraction to limit environment-based conditionals.

An important concern is how you make API calls to fetch your data. As said, this requires an Ajax request client-side and database calls server-side. To keep this simple for my universal application, I found the best approach was to abstract API calls into a separate object that can be passed into the application as a dependency. The application defines a client-side version of this object as a set of functions that make Ajax requests to the server and return promises. If no other API object is provided to the application, it uses this one by default. The server-side code also defines an API object that returns promises, but it makes database calls instead of Ajax calls. Since the server-side version has the exact same interface as the client-side version, the server can simply pass it to the application when running it server-side.

Let's look at some code examples to illustrate this approach:

I've constructed the entry point of my application as a class that can be instantiated:

class App {
  constructor(options) {
    this.options = options;
    options.api = options.api || appApi; = createStore(options.api);
    // ...

  // ...

The App class is responsible for, among other things, instantiating the Redux store and making the API object available to it, which it then makes available to action creators through a custom async action middleware. An action creator will call a function from the API object and return its promise for the middleware to track. As an example, consider this action creator that loads posts:

export function loadPosts(options) {
  return (api) =>
      .then((posts) => ({type: 'POSTS_ADD', payload: posts}));

The action creator returns a function that the middleware then invokes with the API object. The function then calls an API function and returns its promise, chaining onto it to specify an action to dispatch after the posts are loaded.

Let's look at the client-side implementation of this API function:

export default {
  // ...

  getPosts(queryParams = null) {
    let url = '/api/posts';
    url = queryParams ? url + stringifyQueryParams(queryParams) : url;
    return getJSON(url);

  // ...

Besides dealing with some query parameters (which I use to load unpublished posts in the admin view), this API function simply delegates to a getJSON utility for making an Ajax request and returning parsed JSON.

Now let's consider the server-side implementation:

export default {
  // ...

  getPosts({includeUnpublished = false} = {}) {
    let conditions = includeUnpublished ? {} : {published: true};
    let cacheKey = conditions.published ? CACHE_KEY_POSTS_ALL : CACHE_KEY_POSTS;

    return cacheOrQuery(
      () => Post.model.findAll({where: conditions})

  // ...

Besides some additional caching logic to minimize database access, the server-side version of the API function is still pretty simple, and it adheres to the same interface as the client-side version.

With this setup, the action creator in the app will simply call the server-side version directly when the app is run server-side. When running client-side, it will make an Ajax request and the server-side routing code will use the API object to load data for a response:

app.get('/', asyncRoute(function* postsRouteIndex(req, res) {
  let options = {includeUnpublished: !!req.query.includeUnpublished};

  if (options.includeUnpublished && !req.currentUser) {

  res.json(yield api.getPosts(options));

Overall, I find this to be an elegant approach to managing data access for a universal app. The only downside I've found is that it's sometimes necessary to duplicate authorization logic between client and server. For example, I have a both a client-side authorization middleware and a server-side authorization middleware. An alternative to this would be to use something like isomorphic-fetch so that the app makes HTTP requests both client-side and server-side. This would allow all the authorization logic to be captured in the server-side request handling code. Even when taking this alternative into consideration, I currently prefer the abstract API object method, as it make it unnecessary to take on the overhead of running additional server routes when the app is running server-side. Though I reserve the right to change my mind.

Abstract Rendering and Other Environment-Dependent Operations

Continuing in the vein of the previous section, I also found it necessary to abstract how the application handles a few other concerns: rendering, redirects, page not found, and errors. All of these require specialized handling that is environment-dependent. Rendering must use ReactDOM.render() client-side and renderToString() server-side. Redirects must be implemented as HTTP redirects server-side and as a route transition client-side. A page not found should result in a special page rendered from both environments, but with a 404 HTTP status returned on the server-side. Errors need to be logged differently depending on the environment. As before, my goal is to minimize environment-based conditionals in my application code.

My approach to handling these concerns was to pass callbacks as options to my App class that it can then invoke when it encounters these particular scenarios:

class App {
  constructor(options) {
    this.options = options;
    options.api = options.api || appApi; = createStore(options.api);
    // ...

  handleError(error) {
    let { onError } = this.options;

    if (onError) {

  redirect(path) {

  render(Component, props = {}) {
      <Provider store={}>
          <Component {...props}/>

  render404() {
    let { on404 } = this.options;

    if (on404) {


  // ...

As you can see, App has methods for handling these concerns, and each method delegates to the appropriate callback that was provided to the app on instantiation. This way, the server can pass in appropriate functions when instantiating the app:

let app = new App({
  renderer: (element) => {
    res.render('base', {
      assets: res.assets,
      csrfToken: req.csrfToken(),
      data: JSON.stringify(,
      html: renderToString(element),
      isDev: IS_DEV
  onRedirect: res.redirect.bind(res),
  onError: next,
  on404: () => res.status(404),

In the server's case, it uses React's renderToString to render the app as an HTML string, which it then passes to Express's rendering logic. For redirects, it uses the Express response object's redirect() method. For errors, it passes the Express route's next() callback, which will register an error if invoked with an argument. And for page not found, it simply sets a 404 status for the HTTP response.

On the client-side, appropriate handlers can also be passed in when running the initialization logic:

app = new App({
  renderer: appRenderer,
  onRedirect: page,
  onError: errorHandler

function appRenderer(element) {
  ReactDOM.render(element, rootEl);

function errorHandler(error) {
  console.error && console.error(error);

In this version, the appRenderer function will call ReactDOM.render(). onRedirect will trigger a route transition (I'm using Page.js for routing). For error handling, I'm currently just logging to console.error(), though in the future I hope to send errors to the server via Ajax for real production logging. In the client's case, I've omitted an on404 callback, as the default app behavior of rendering the 404 page is sufficient.

Use Separate Builds for Client and Server

I'm a huge fan of modern JavaScript's syntax and latest standard library additions (ES2015 and beyond). So naturally I want to use some of these niceties in writing my application's code. This of course makes transpilation necessary, as not all syntax and library features are available in every environment.

In addition to the need for transpilation, a modern JS app also needs its modules built and managed in a way that's appropriate for its target environment (or environments in my case). For my app code, I prefer to use ES6 modules. And besides your application code, you need a way to build and bundle all the third-party libraries you depend on. Finally, there are non-JavaScript assets you likely also need to manage, such as CSS files written with Sass.

My top choice for handling these concerns is Webpack. It's a feature-rich and extensible tool that handles all these needs efficiently. When it comes to building my app so it can run server-side and client-side, I've found the best approach is to output two builds: one optimized for the browser and one optimized for the server.

To facilitate this, I have a Webpack configuration for the client and a Webpack configuration for the server. Both share a few values from a shared configuration and then diverge to configure an optimized build for each environment. For example, I can take advantage of Webpack's CommonsChunkPlugin for the client build to pull third-party code and commonly-used utilities into their own JS files. And for the server, I can output the app as a CommonJS module for easy use in Node and configure Webpack to optimize for the Node environment:

module.exports = {
  entry: {
    app: './app/index.jsx'

  output: Object.assign({}, shared.output, {
    path: path.join(shared.output.path, 'server'),
    library: 'app',
    libraryTarget: 'commonjs2'

  resolve: shared.resolve,
  target: 'node',

  // ...

You may think this means I have to run Webpack twice each time I build, but Webpack will accept an array of configuration objects and then run all of them during one execution. Here's what my top-level configuration file that I pass to Webpack looks like:

module.exports = [

Let's look a little closer at how using separate build configurations allows me to optimize build output for each environment.

Handling Transpilation

As I mentioned before, I need to transpile my application code so I can take advantage of the latest and greatest capabilities of the JavaScript language. But that doesn't mean I need to transpile in the same way for the server and for the client. I'm using Node.js version 6 to run my server-side code, which has excellent ES6 support. It needs very little transpiled, mostly just bleeding-edge features like Object spread. For the client, however, I want to support even some older browsers that don't support any ES6 features, and so I want to transpile the code entirely to ES5. Using separate build configurations, it's easy to set up optimal transpilation for both environments.

I'm using Babel for transpilation since it's become the industry standard tool. For the client, I'm using babel-preset-es2015 and babel-preset-react to transpile my app code for widest browser support. For the server, I'm using babel-preset-es2015-node and babel-preset-react to transpile only the features that Node version 6 doesn't already support.

I think it makes sense to only transpile as much as needed, and take advantage of native feature support when it's available. Babel's excellent preset system combined with my two-build setup makes it easy for me to produce the best bundle for client and server.

Handling Styles

I also want to call out handling styles during build. For my styles, I'm using Sass to write my CSS and Webpack to run it through the Sass compiler. This allows me to declare styling dependencies directly in my JavaScript modules, which I think helps with keeping dependencies explicit and related code together. For example, I have a PostPage/index.jsx module that defines a PostPage component and also declares a dependency on the PostPage/styles.scss Sass file that lives next to it.

import './styles.scss';
import React from 'react';
import SafeOutput from 'app/utils/components/SafeOutput';

export default function PostPage({post}) {
  return (
    <section className="post-view-page post">
      <h2 className="page-title">{post.title}</h2>

      <SafeOutput className="post__body card" content={post.body}/>

But this poses an issue when it comes to outputting a build of the app to run server-side. Styles aren't needed in that environment. Thankfully, there's a handy null-loader for Webpack you can use to ignore a file type during a build. I simply apply it to Sass files in my server build configuration and the issue is no more (Note that I'm using Webpack 2 beta's new configuration API).

module.exports = {
  // ...

  module: {
    rules: [
      // ...

        test: /\.scss$/,
        use: [

Now I don't need to worry about styles for the server-side and for the client-side I can configure advanced tools like Autoprefixer for my styles.

Enable Hot-Reloading of the App Server-Side during Development

The last tip I have pertains to reloading app code during development on the server-side. For my actual server code, I use Nodemon to automatically restart my server when I change code. But for built application code, I instead hot-reload it server-side so I don't even need a server restart to pick up changes (which is safe because I re-instantiate the app for each request). To do this, I use the require-reload module, which removes a module from Node's require cache so the next time it's required, it reruns from scratch. I capture loading of the app in a utility function which clears the cache for the app's built JS when in development:

const IS_DEV = process.env.NODE_ENV === 'development';

let App = require(APP_SRC);

module.exports = function loadApp() {
  if (IS_DEV) {
    let srcFiles = fs.readdirSync(APP_DIST_DIR);
    srcFiles.forEach((file) => reload(`${APP_DIST_DIR}/${file}`));
    App = require(APP_SRC);

  return App.default;

Note that there are multiple JS files even for the server side build because I'm using Webpack's code-splitting functionality to lazy-load code client-side and this unfortunately results in split code for the server too.


Those were some helpful tips I discovered while building my blog as a universal JS app. I hope they'll be of use to others embarking on similar adventures. If you have any feedback for me, feel free to reach out to me on Twitter at @WillHPower (yes, I plan to add commenting functionality to my blog in the near future).