Pre-render routes with react-snap

Not server-side rendering but still want to speed up the performance of your React site? Try pre-rendering!

Houssein Djirdeh

react-snap is a third-party library that pre-renders pages on your site into static HTML files. This can improve First Paint times in your application.

Here's a comparison of the same application with and without pre-rendering loaded on a simulated 3G connection and mobile device:

Why is this useful?

The main performance problem with large single-page applications is that the user needs to wait for the JavaScript bundle(s) that make up the site to finish downloading before they can see any real content. The larger the bundles, the longer the user will have to wait.

To solve this, many developers take the approach of rendering the application on the server instead of only booting it up on the browser. With each page/route transition, the complete HTML is generated on the server and sent to the browser, which reduces First Paint times but comes at the cost of a slower Time to First Byte.

Pre-rendering is a separate technique that is less complex than server rendering, but also provides a way to improve First Paint times in your application. A headless browser, or a browser without a user interface, is used to generate static HTML files of every route during build time. These files can then be shipped along with the JavaScript bundles that are needed for the application.

react-snap

react-snap uses Puppeteer to create pre-rendered HTML files of different routes in your application. To begin, install it as a development dependency:

npm install --save-dev react-snap

Then add a postbuild script in your package.json:

"scripts": {
  //...
  "postbuild": "react-snap"
}

This would automatically run the react-snap command every time a new build of the applications made (npm build).

The last thing you will need to do is change how the application is booted. Change the src/index.js file to the following:

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));
const rootElement = document.getElementById("root");

if (rootElement.hasChildNodes()) {
  ReactDOM.hydrate(<App />, rootElement);
} else {
  ReactDOM.render(<App />, rootElement);
}

Instead of only using ReactDOM.render to render the root React element directly into the DOM, this checks to see if any child nodes are already present to determine whether HTML contents were pre-rendered (or rendered on the server). If that's the case, ReactDOM.hydrate is used instead to attach event listeners to the already created HTML instead of creating it anew.

Building the application will now generate static HTML files as payloads for each route that is crawled. You can take a look at what the HTML payload looks like by clicking the URL of the HTML request and then clicking the Previews tab within Chrome DevTools.