Server-Side Rendering

Server-Side Rendering (often abbreviated as "SSR") allows you to render your application to an HTML string that can be sent to the client to improve load time. Outside of that there are other scenarios, like testing, where SSR proves really useful.

Installation

The server-side renderer for Preact lives in its own repository and can be installed via your packager of choice:

npm install -S preact-render-to-string

After the command above finished, we can start using it right away.

Basic Usage

Basic functionality can be best explained via a simple snippet:

import render from 'preact-render-to-string';
import { h } from 'preact';

const name = 'Preact User!'
const App = <div class="foo">Hello {name}</div>;

console.log(render(App));
// <div class="foo">Hello Preact User!</div>

Asynchronous Rendering with Suspense & lazy

You may find that you need to render components that are dynamically loaded, like when using Suspense and lazy to facilitate code splitting (along with some other use cases). The async renderer will await the resolution of promises, allowing you to fully construct your HTML string:

// page/home.js
export default () => {
    return <h1>Home page</h1>;
};

// main.js
import { Suspense, lazy } from 'preact/compat';

// Creation of the lazy component
const HomePage = lazy(() => import('./pages/home'));

const Main = () => {
    return (
        <Suspense fallback={<p>Loading</p>}>
            <HomePage />
        </Suspense>
    );
};

The above is a very typical setup for a Preact application that uses code splitting, with no changes necessary to make use of server-side rendering.

To render this, we will deviate slightly from the basic usage example and use the renderToStringAsync export to render our application:

import { renderToStringAsync } from 'preact-render-to-string';
import { Main } from './main';

const main = async () => {
    // Rendering of lazy components
    const html = await renderToStringAsync(<Main />);

    console.log(html);
    // <h1>Home page</h1>
};

// Execution & error handling
main().catch((error) => {
    console.error(error);
});

Shallow Rendering

For some purposes it's often preferable to not render the whole tree, but only one level. For that we have a shallow renderer which will print child components by name, instead of their return value.

import { shallow } from 'preact-render-to-string';
import { h } from 'preact';

const Foo = () => <div>foo</div>;
const App = <div class="foo"><Foo /></div>;

console.log(shallow(App));
// <div class="foo"><Foo /></div>

Pretty Mode

If you need to get the rendered output in a more human friendly way, we've got you covered! By passing the pretty option, we'll preserve whitespace and indent the output as expected.

import render from 'preact-render-to-string/jsx';
import { h } from 'preact';

const Foo = () => <div>foo</div>;
const App = <div class="foo"><Foo /></div>;

console.log(render(App, {}, { pretty: true }));
// Logs:
// <div class="foo">
//   <div>foo</div>
// </div>

JSX Mode

The JSX rendering mode is especially useful if you're doing any kind of snapshot testing. It renders the output as if it was written in JSX.

import render from 'preact-render-to-string/jsx';
import { h } from 'preact';

const App = <div data-foo={true} />;

console.log(render(App));
// Logs: <div data-foo={true} />