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Hydration Strategies

Hydration is the process of attaching React event handlers to server-rendered HTML, making it interactive. React Router provides several strategies to optimize this process.

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Basic Hydration

By default, React Router hydrates immediately with server data: 
// Client entry
import { HydratedRouter } from "react-router/dom";

ReactDOM.hydrateRoot(
  document.getElementById("root"),
  <HydratedRouter />
);
This uses the hydration data embedded in the HTML: 
<!-- Server-rendered -->
<script>
  window.__staticRouterHydrationData = {
    loaderData: { root: {...}, about: {...} },
    errors: null
  };
</script>

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Client Loaders

Run additional logic on the client during hydration: 
export async function clientLoader({ serverLoader }) {
  // Get server data
  const serverData = await serverLoader();
  
  // Augment with client-only data
  const clientData = localStorage.getItem("preferences");
  
  return {
    ...serverData,
    preferences: JSON.parse(clientData),
  };
}

export async function loader() {
  return { user: await getUser() };
}

export function Component() {
  const data = useLoaderData();
  // data includes both server and client data
  return <div>Welcome {data.user.name}</div>;
}

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Hydrate Flag

Control whether client loaders run on hydration: 
export async function clientLoader({ serverLoader }) {
  return await serverLoader();
}

// Don't run on hydration - use server data only
clientLoader.hydrate = false;

export async function clientLoader() {
  return await getClientOnlyData();
}

// Always run on hydration
clientLoader.hydrate = true;

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Default Behavior

  • hydrate = true: When there’s no server loader
  • hydrate = false: When calling serverLoader()
  • hydrate = true: When not calling serverLoader()

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HydrateFallback

Show a loading state during client loader execution: 
export function HydrateFallback() {
  return <div>Loading preferences...</div>;
}

export async function clientLoader() {
  // This runs on hydration
  const data = await fetch("/api/user").then(r => r.json());
  return data;
}

clientLoader.hydrate = true;

export function Component() {
  const data = useLoaderData();
  return <div>Welcome {data.name}</div>;
}

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HydrateFallback Behavior

  1. Only runs on initial hydration: Not on client-side navigations
  2. Requires clientLoader: With hydrate = true
  3. Bubbles up: Renders parent’s HydrateFallback if none provided
  4. No Outlet: Cannot render children (they may not have data yet)

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Partial Hydration

Hydrate different parts of your app at different times: 
// Root route - hydrates immediately
export function Component() {
  return (
    <div>
      <header>Instant header</header>
      <Outlet />
    </div>
  );
}

// Dashboard route - deferred hydration
export function HydrateFallback() {
  return <div>Loading dashboard...</div>;
}

export async function clientLoader() {
  // Heavy client-side initialization
  await loadAnalytics();
  await loadCharts();
  return { ready: true };
}

clientLoader.hydrate = true;

export function Component() {
  return <div>Dashboard ready!</div>;
}

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Streaming Hydration

Combine with deferred data for progressive hydration: 
import { defer } from "react-router";

export async function loader() {
  return defer({
    critical: await getCriticalData(),
    deferred: getDeferredData(), // Promise
  });
}

export function Component() {
  const data = useLoaderData();
  
  return (
    <div>
      <h1>{data.critical.title}</h1>
      
      <Suspense fallback={<Spinner />}>
        <Await resolve={data.deferred}>
          {(deferred) => <Content data={deferred} />}
        </Await>
      </Suspense>
    </div>
  );
}
The page hydrates with critical data, then streams in deferred data.

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Optimistic Hydration

Assume server data is available and hydrate eagerly: 
// Server
export async function loader() {
  return { count: await getCount() };
}

// Client
export async function clientLoader({ serverLoader }) {
  // Optimistically return server data
  const data = await serverLoader();
  
  // Then update from API in the background
  fetch("/api/count")
    .then(r => r.json())
    .then(fresh => {
      // Update state with fresh data
    });
  
  return data;
}

clientLoader.hydrate = false; // Use server data immediately

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Selective Hydration

Only hydrate interactive components: 
import { hydrateRoot } from "react-dom/client";
import { HydratedRouter } from "react-router/dom";

// Hydrate interactive parts only
const root = document.getElementById("root");
const interactive = root.querySelector("[data-interactive]");

if (interactive) {
  hydrateRoot(interactive, <HydratedRouter />);
} else {
  // Just static content, no hydration needed
}

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Island Architecture

Hydrate isolated interactive regions: 
// Static layout, no hydration
export function Layout() {
  return (
    <div>
      <nav>Static navigation</nav>
      <Outlet />
    </div>
  );
}

// Interactive island
export function Component() {
  const [count, setCount] = useState(0);
  
  return (
    <div data-island>
      <button onClick={() => setCount(c => c + 1)}>
        Count: {count}
      </button>
    </div>
  );
}

​
Custom Hydration

Full control over the hydration process: 
import { matchRoutes } from "react-router";

// Find routes that need hydration
const lazyMatches = matchRoutes(
  routes,
  window.location
)?.filter(m => m.route.lazy);

// Preload lazy routes
if (lazyMatches?.length) {
  await Promise.all(
    lazyMatches.map(async (m) => {
      const module = await m.route.lazy();
      Object.assign(m.route, { ...module, lazy: undefined });
    })
  );
}

// Create router with hydration data
const router = createBrowserRouter(routes, {
  hydrationData: window.__staticRouterHydrationData,
});

// Hydrate
ReactDOM.hydrateRoot(
  document.getElementById("root"),
  <RouterProvider router={router} />
);

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Hydration Errors

Debug mismatches between server and client: 
if (import.meta.env.DEV) {
  const root = document.getElementById("root");
  const originalError = console.error;
  
  console.error = (...args) => {
    if (args[0]?.includes?.("Hydration")) {
      console.warn("Hydration mismatch:", args);
      // Log server HTML
      console.log("Server HTML:", root.innerHTML);
    }
    originalError(...args);
  };
}

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Performance Monitoring

Track hydration performance: 
export async function clientLoader({ serverLoader }) {
  const start = performance.now();
  const data = await serverLoader();
  const duration = performance.now() - start;
  
  // Send to analytics
  analytics.timing("hydration", duration);
  
  return data;
}

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Best Practices

  1. Use server data by default: Set hydrate = false when possible
  2. Show loading states: Use HydrateFallback for better UX
  3. Minimize client loaders: Keep hydration fast
  4. Test without JavaScript: Ensure server-rendered content works
  5. Monitor hydration time: Track performance metrics

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Common Patterns

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User Preferences

export async function clientLoader({ serverLoader }) {
  const [serverData, theme] = await Promise.all([
    serverLoader(),
    getLocalTheme(),
  ]);
  
  return { ...serverData, theme };
}

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Authentication State

export async function clientLoader({ serverLoader }) {
  const serverData = await serverLoader();
  const token = localStorage.getItem("token");
  
  return {
    ...serverData,
    isAuthenticated: !!token,
  };
}

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Feature Flags

export async function clientLoader({ serverLoader }) {
  const [serverData, flags] = await Promise.all([
    serverLoader(),
    getFeatureFlags(),
  ]);
  
  return { ...serverData, features: flags };
}

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