Not server-side rendering but still want to speed up the performance of your React site? Try pre-rendering!
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.
Flash of unstyled content
Although static HTML is now rendered almost immediately, it still remains unstyled by default which may cause the issue of showing a "flash of unstyled content" (FOUC). This can be especially noticeable if you are using a CSS-in-JS library to generate selectors since the JavaScript bundle will have to finish executing before any styles can be applied.
To help prevent this, the critical CSS, or the minimum amount of CSS that is
needed for the initial page to render, can be inlined directly to the <head>
of the HTML document. react-snap
uses another third-party library under the
hood, minimalcss
, to extract any
critical CSS for different routes. You can enable this by specifying the
following in your package.json
file:
"reactSnap": {
"inlineCss": true
}
Taking a look at the response preview in Chrome DevTools will now show the styled page with critical CSS inlined.
Conclusion
If you are not server-side rendering routes in your application, use
react-snap
to pre-render static HTML to your users.
- Install it as a development dependency and begin with just the default settings.
- Use the experimental
inlineCss
option to inline critical CSS if it works for your site. - If you are using code splitting on a component level within any routes, be
careful not to pre-render a loading state to your users. The
react-snap
README covers this in more detail.