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preact-iso

preact-iso is a collection of isomorphic async tools for Preact.

"Isomorphic" describes code that can run (ideally seamlessly) across both the browser and server. preact-iso is made for supporting these environments, allowing users to build apps without having to create separate browser and server routers or worry about differences in data or component loading. The same app code can be used in the browser and on a server during prerendering, no adjustments necessary.

Note: Whilst this is a routing library that comes from the Preact team, many other routers are available in the wider Preact/React ecosystem that you may prefer to use instead, including wouter and react-router. It's a great first option but you can bring your favorite router to Preact if you prefer.

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'));

function App() {
  return (
    <LocationProvider>
      <ErrorBoundary>
        <Router>
          <Home path="/" />
          {/* Alternative dedicated route component for better TS support */}
          <Route path="/profiles" component={Profiles} />
          <Route path="/profile/: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: When 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, the router preserves the current route in-place until the incoming route has finished loading, then they are swapped.

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'));

function App() {
  return (
    <LocationProvider>
      <ErrorBoundary>
        <Router>
          <Foo path="/" />
        </Router>
      </ErrorBoundary>
    </LocationProvider>
  );
}

hydrate(<App />);

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

Nested Routing

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

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

const NotFound = lazy(() => import('./routes/_404.js'));

function App() {
  return (
    <LocationProvider>
      <ErrorBoundary>
        <Router>
          <Route path="/movies/*" component={Movies} />
          <NotFound default />
        </Router>
      </ErrorBoundary>
    </LocationProvider>
  );
}

const TrendingMovies = lazy(() => import('./routes/movies/trending.js'));
const SearchMovies = lazy(() => import('./routes/movies/search.js'));
const MovieDetails = lazy(() => import('./routes/movies/details.js'));

function Movies() {
  return (
    <ErrorBoundary>
      <Router>
        <Route path="/trending" component={TrendingMovies} />
        <Route path="/search" component={SearchMovies} />
        <Route path="/:id" component={MovieDetails} />
      </Router>
    </ErrorBoundary>
  );
}

This will match the following routes:

  • /movies/trending
  • /movies/search
  • /movies/Inception

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';

function App() {
  return (
    <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';

function App() {
  return (
    <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="/" />
        <Profiles path="/profiles" />
        <Profile path="/profile/:id" />
      </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';

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

        <Profiles path="/profiles" />
        <Profile path="/profile/:id" />
        <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 Profiles = lazy(() => import('./routes/profiles.js').then(m => m.Profiles)); // Expects a named export called `Profiles`
const Profile = lazy(() => import('./routes/profile.js')); // Expects a default export

function App() {
  return (
    <LocationProvider>
      <Router>
        <Home path="/" />
        <Profiles path="/profiles" />
        <Profile path="/profile/:id" />
      </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/rschristian" 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';

function App() {
  return (
    <LocationProvider>
      <ErrorBoundary onError={(e) => console.log(e)}>
        <Router>
          <Home path="/" />
          <Profiles path="/profiles" />
          <Profile path="/profile/:id" />
        </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';

function App() {
  return (
    <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'));

function App() {
  return (
    <LocationProvider>
      <ErrorBoundary>
        <Router>
          <Foo path="/" />
          <Bar path="/bar" />
        </Router>
      </ErrorBoundary>
    </LocationProvider>
  );
}

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

Built by a bunch of lovely people like @rpetrich.