preact-iso

Isomorphic async tools for Preact.

• Lazy-load components using lazy() and <ErrorBoundary>, which also enables progressive hydration.
• Generate static HTML for your app using prerender(), waiting for lazy() components and data dependencies.
• Implement async-aware client and server-side routing using <Router>, including seamless async transitions.

Routing

preact-iso offers a simple router for Preact with conventional and hooks-based APIs. The <Router> component is async-aware: when transitioning from one route to another, if the incoming route suspends (throws a Promise), the outgoing route is preserved until the new one becomes ready.

import { lazy, LocationProvider, ErrorBoundary, Router, Route } from 'preact-iso';

// Synchronous
import Home from './routes/home.js';

// Asynchronous (throws a promise)
const Profiles = lazy(() => import('./routes/profiles.js'));
const Profile = lazy(() => import('./routes/profile.js'));
const NotFound = lazy(() => import('./routes/_404.js'));

const App = () => (
	<LocationProvider>
		<ErrorBoundary>
			<Router>
				<Home path="/" />
				{/* Alternative dedicated route component for better TS support */}
				<Route path="/profiles" component={Profiles} />
				<Route path="/profiles/:id" component={Profile} />
				{/* `default` prop indicates a fallback route. Useful for 404 pages */}
				<NotFound default />
			</Router>
		</ErrorBoundary>
	</LocationProvider>
);

Progressive Hydration: When the app is hydrated on the client, the route (Home or Profile in this case) suspends. This causes hydration for that part of the page to be deferred until the route's import() is resolved, at which point that part of the page automatically finishes hydrating.

Seamless Routing: Switch switching between routes on the client, the Router is aware of asynchronous dependencies in routes. Instead of clearing the current route and showing a loading spinner while waiting for the next route (or its data), the router preserves the current route in-place until the incoming route has finished loading, then they are swapped.

Nested Routing: Nested routes are supported by using multiple Router components. Partially matched routes end with a wildcard /* and the remaining value will be past to continue matching with if there are any further routes.

Prerendering

prerender() renders a Virtual DOM tree to an HTML string using preact-render-to-string. The Promise returned from prerender() resolves to an Object with html and links[] properties. The html property contains your pre-rendered static HTML markup, and links is an Array of any non-external URL strings found in links on the generated page.

Primarily meant for use with prerendering via @preact/preset-vite or other prerendering systems that share the API. If you're server-side rendering your app via any other method, you can use preact-render-to-string (specifically renderToStringAsync()) directly.

import { LocationProvider, ErrorBoundary, Router, lazy, prerender as ssr } from 'preact-iso';

// Asynchronous (throws a promise)
const Foo = lazy(() => import('./foo.js'));

const App = () => (
	<LocationProvider>
		<ErrorBoundary>
			<Router>
				<Foo path="/" />
			</Router>
		</ErrorBoundary>
	</LocationProvider>
);

hydrate(<App />);

export async function prerender(data) {
	return await ssr(<App />);
}

API Docs

LocationProvider

A context provider that provides the current location to its children. This is required for the router to function.

Props:

• scope?: string | RegExp - Sets a scope for the paths that the router will handle (intercept). If a path does not match the scope, either by starting with the provided string or matching the RegExp, the router will ignore it and default browser navigation will apply.

Typically, you would wrap your entire app in this provider:

import { LocationProvider } from 'preact-iso';

const App = () => (
    <LocationProvider scope="/app">
        {/* Your app here */}
    </LocationProvider>
);

Router

Props:

• onRouteChange?: (url: string) => void - Callback to be called when a route changes.
• onLoadStart?: (url: string) => void - Callback to be called when a route starts loading (i.e., if it suspends). This will not be called before navigations to sync routes or subsequent navigations to async routes.
• onLoadEnd?: (url: string) => void - Callback to be called after a route finishes loading (i.e., if it suspends). This will not be called after navigations to sync routes or subsequent navigations to async routes.

import { LocationProvider, Router } from 'preact-iso';

const App = () => (
	<LocationProvider>
		<Router
			onRouteChange={(url) => console.log('Route changed to', url)}
			onLoadStart={(url) => console.log('Starting to load', url)}
			onLoadEnd={(url) => console.log('Finished loading', url)}
		>
			<Home path="/" />
			<Profile path="/profile" />
		</Router>
	</LocationProvider>
);

Route

There are two ways to define routes using preact-iso:

1. Append router params to the route components directly: <Home path="/" />
2. Use the Route component instead: <Route path="/" component={Home} />

Appending arbitrary props to components not unreasonable in JavaScript, as JS is a dynamic language that's perfectly happy to support dynamic & arbitrary interfaces. However, TypeScript, which many of us use even when writing JS (via TS's language server), is not exactly a fan of this sort of interface design.

TS does not (yet) allow for overriding a child's props from the parent component so we cannot, for instance, define <Home> as taking no props unless it's a child of a <Router>, in which case it can have a path prop. This leaves us with a bit of a dilemma: either we define all of our routes as taking path props so we don't see TS errors when writing <Home path="/" /> or we create wrapper components to handle the route definitions.

While <Home path="/" /> is completely equivalent to <Route path="/" component={Home} />, TS users may find the latter preferable.

import { LocationProvider, Router, Route } from 'preact-iso';

const App = () => (
	<LocationProvider>
		<Router>
			{/* Both of these are equivalent */}
			<Home path="/" />
			<Route path="/" component={Home} />

			<Profile path="/profile" />
			<NotFound default />
		</Router>
	</LocationProvider>
);

Props for any route component:

• path: string - The path to match (read on)
• default?: boolean - If set, this route is a fallback/default route to be used when nothing else matches

Specific to the Route component:

• component: AnyComponent - The component to render when the route matches

Path Segment Matching

Paths are matched using a simple string matching algorithm. The following features may be used:

• :param - Matches any URL segment, binding the value to the label (can later extract this value from useRoute())
  • /profile/:id will match /profile/123 and /profile/abc
  • /profile/:id? will match /profile and /profile/123
  • /profile/:id* will match /profile, /profile/123, and /profile/123/abc
  • /profile/:id+ will match /profile/123, /profile/123/abc
• * - Matches one or more URL segments
  • /profile/* will match /profile/123, /profile/123/abc, etc.

These can then be composed to create more complex routes:

• /profile/:id/* will match /profile/123/abc, /profile/123/abc/def, etc.

The difference between /:id* and /:id/* is that in the former, the id param will include the entire path after it, while in the latter, the id is just the single path segment.

• /profile/:id*, with /profile/123/abc
  • id is 123/abc
• /profile/:id/*, with /profile/123/abc
  • id is 123

useLocation

A hook to work with the LocationProvider to access location context.

Returns an object with the following properties:

• url: string - The current path & search params
• path: string - The current path
• query: Record<string, string> - The current query string parameters (/profile?name=John -> { name: 'John' })
• route: (url: string, replace?: boolean) => void - A function to programmatically navigate to a new route. The replace param can optionally be used to overwrite history, navigating them away without keeping the current location in the history stack.

useRoute

A hook to access current route information. Unlike useLocation, this hook only works within <Router> components.

Returns an object with the following properties:

• path: string - The current path
• query: Record<string, string> - The current query string parameters (/profile?name=John -> { name: 'John' })
• params: Record<string, string> - The current route parameters (/profile/:id -> { id: '123' })

lazy

Make a lazily-loaded version of a Component.

lazy() takes an async function that resolves to a Component, and returns a wrapper version of that Component. The wrapper component can be rendered right away, even though the component is only loaded the first time it is rendered.

import { lazy, LocationProvider, Router } from 'preact-iso';

// Synchronous, not code-splitted:
import Home from './routes/home.js';

// Asynchronous, code-splitted:
const Profile = lazy(() => import('./routes/profile.js'));

const App = () => (
	<LocationProvider>
		<Router>
			<Home path="/" />
			<Profile path="/profile" />
		</Router>
	</LocationProvider>
);

The result of lazy() also exposes a preload() method that can be used to load the component before it's needed for rendering. Entirely optional, but can be useful on focus, mouse over, etc. to start loading the component a bit earlier than it otherwise would be.

const Profile = lazy(() => import('./routes/profile.js'));

function Home() {
    return (
        <a href="/profile" onMouseOver={() => Profile.preload()}>
            Profile Page -- Hover over me to preload the module!
        </a>
    );
}

ErrorBoundary

A simple component to catch errors in the component tree below it.

Props:

• onError?: (error: Error) => void - A callback to be called when an error is caught

import { LocationProvider, ErrorBoundary, Router } from 'preact-iso';

const App = () => (
	<LocationProvider>
		<ErrorBoundary onError={(e) => console.log(e)}>
			<Router>
				<Home path="/" />
				<Profile path="/profile" />
			</Router>
		</ErrorBoundary>
	</LocationProvider>
);

hydrate

A thin wrapper around Preact's hydrate export, it switches between hydrating and rendering the provided element, depending on whether the current page has been prerendered. Additionally, it checks to ensure it's running in a browser context before attempting any rendering, making it a no-op during SSR.

Pairs with the prerender() function.

Params:

• jsx: ComponentChild - The JSX element or component to render
• parent?: Element | Document | ShadowRoot | DocumentFragment - The parent element to render into. Defaults to document.body if not provided.

import { hydrate } from 'preact-iso';

const App = () => (
	<div class="app">
		<h1>Hello World</h1>
	</div>
);

hydrate(<App />);

However, it is just a simple utility method. By no means is it essential to use, you can always use Preact's hydrate export directly.

prerender

Renders a Virtual DOM tree to an HTML string using preact-render-to-string. The Promise returned from prerender() resolves to an Object with html and links[] properties. The html property contains your pre-rendered static HTML markup, and links is an Array of any non-external URL strings found in links on the generated page.

Pairs primarily with @preact/preset-vite's prerendering.

Params:

• jsx: ComponentChild - The JSX element or component to render

import { LocationProvider, ErrorBoundary, Router, lazy, prerender } from 'preact-iso';

// Asynchronous (throws a promise)
const Foo = lazy(() => import('./foo.js'));
const Bar = lazy(() => import('./bar.js'));

const App = () => (
	<LocationProvider>
		<ErrorBoundary>
			<Router>
				<Foo path="/" />
				<Bar path="/bar" />
			</Router>
		</ErrorBoundary>
	</LocationProvider>
);

const { html, links } = await prerender(<App />);

License

MIT