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clientLoader

A client-side data loading function that runs in the browser, allowing you to fetch data from APIs, access browser storage, or augment server loader data.

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Signature

export function clientLoader(args: ClientLoaderFunctionArgs): Promise<Data> | Data
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args
ClientLoaderFunctionArgs
required
Arguments passed to the clientLoader function
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return
Data | Promise<Data>
Data to be used by the route component (accessible via useLoaderData())

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Basic Example

export async function clientLoader({ params }: Route.ClientLoaderArgs) {
  const product = await fetch(`/api/products/${params.id}`).then(r => r.json());
  return { product };
}

export default function Product() {
  const { product } = useLoaderData<typeof clientLoader>();
  return <h1>{product.name}</h1>;
}

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Calling Server Loader

export async function loader({ params }: Route.LoaderArgs) {
  const product = await db.product.findUnique({ where: { id: params.id } });
  return { product };
}

export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  // Get data from server
  const serverData = await serverLoader<typeof loader>();
  
  // Augment with client-only data
  const reviews = await fetchReviewsFromAPI(serverData.product.id);
  
  return {
    ...serverData,
    reviews,
  };
}

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Skipping Server Loader

// Only run on client - no server loader
export async function clientLoader({ params }: Route.ClientLoaderArgs) {
  const data = await fetch(`https://api.example.com/data/${params.id}`)
    .then(r => r.json());
  return { data };
}

export default function Component() {
  const { data } = useLoaderData<typeof clientLoader>();
  return <div>{data.title}</div>;
}

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Hydrate Option

Control whether clientLoader runs on initial page load: 
// Run on hydration (initial page load)
export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  const data = await serverLoader();
  // Add client-side data
  return { ...data, timestamp: Date.now() };
}
clientLoader.hydrate = true;

export function HydrateFallback() {
  return <div>Loading...</div>;
}
Without hydrate = true, clientLoader only runs on client-side navigation: 
export async function loader() {
  return { data: await fetchFromDB() };
}

// Only runs on client navigation, not initial load
export async function clientLoader({ params }: Route.ClientLoaderArgs) {
  return { data: await fetchFromAPI(params.id) };
}

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HydrateFallback

Show a loading state during hydration: 
export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  const data = await serverLoader();
  // Do some client-side processing
  await new Promise(r => setTimeout(r, 1000));
  return data;
}
clientLoader.hydrate = true;

export function HydrateFallback() {
  return (
    <div className="loading">
      <Spinner />
      <p>Loading product details...</p>
    </div>
  );
}

export default function Product() {
  const data = useLoaderData<typeof clientLoader>();
  return <div>{data.product.name}</div>;
}

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Client-Side Caching

const cache = new Map();

export async function clientLoader({ params }: Route.ClientLoaderArgs) {
  const cacheKey = params.id;
  
  // Check cache first
  if (cache.has(cacheKey)) {
    return cache.get(cacheKey);
  }
  
  // Fetch and cache
  const product = await fetch(`/api/products/${params.id}`).then(r => r.json());
  cache.set(cacheKey, { product });
  
  return { product };
}

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Accessing Browser APIs

export async function clientLoader({ params }: Route.ClientLoaderArgs) {
  // Access localStorage
  const preferences = JSON.parse(
    localStorage.getItem("preferences") || "{}"
  );
  
  // Access IndexedDB
  const cachedData = await idb.get(params.id);
  
  // Use browser-only APIs
  const online = navigator.onLine;
  
  return {
    preferences,
    cachedData,
    online,
  };
}

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Combining Server and Client Data

export async function loader({ params }: Route.LoaderArgs) {
  // Server-side: fetch from database
  const article = await db.article.findUnique({
    where: { slug: params.slug }
  });
  return { article };
}

export async function clientLoader({ 
  params, 
  serverLoader 
}: Route.ClientLoaderArgs) {
  // Get server data
  const { article } = await serverLoader<typeof loader>();
  
  // Add client-only data
  const [viewCount, userBookmarked] = await Promise.all([
    fetch(`/api/views/${article.id}`).then(r => r.json()),
    checkIfBookmarked(article.id),
  ]);
  
  return {
    article,
    viewCount,
    userBookmarked,
  };
}
clientLoader.hydrate = true;

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Progressive Enhancement

// Server loader for SSR and no-JS
export async function loader({ params }: Route.LoaderArgs) {
  return { products: await db.product.findMany() };
}

// Client loader for enhanced experience
export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  const { products } = await serverLoader<typeof loader>();
  
  // Add real-time prices from external API
  const prices = await fetch("/api/prices").then(r => r.json());
  
  const enrichedProducts = products.map(product => ({
    ...product,
    currentPrice: prices[product.id],
  }));
  
  return { products: enrichedProducts };
}
clientLoader.hydrate = true;

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Error Handling

export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  try {
    // Try to get fresh data from server
    return await serverLoader();
  } catch (error) {
    // Fall back to cached data
    const cached = await getCachedData();
    
    if (cached) {
      return { ...cached, fromCache: true };
    }
    
    // Re-throw if no cache available
    throw error;
  }
}

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Conditional Server Calls

export async function clientLoader({ 
  params, 
  serverLoader 
}: Route.ClientLoaderArgs) {
  // Check if we have fresh cache
  const cached = cache.get(params.id);
  const isFresh = cached && Date.now() - cached.timestamp < 60000;
  
  if (isFresh) {
    return cached.data;
  }
  
  // Cache miss or stale - call server
  const data = await serverLoader();
  cache.set(params.id, { data, timestamp: Date.now() });
  
  return data;
}

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Best Practices

clientLoader is perfect for features that require browser APIs:
export async function clientLoader() {
  return {
    userPreferences: JSON.parse(localStorage.getItem("prefs") || "{}"),
    deviceInfo: {
      online: navigator.onLine,
      battery: await navigator.getBattery(),
    },
  };
}
If you need both server and client data on initial load:
export async function loader() {
  return { serverData: await fetchFromDB() };
}

export async function clientLoader({ serverLoader }) {
  const server = await serverLoader();
  const client = await fetchFromAPI();
  return { ...server, ...client };
}
clientLoader.hydrate = true; // ✅ Run on initial load
Use clientLoader to enhance, not replace:
// ❌ Bad - duplicating server logic
export async function loader({ params }) {
  return { user: await db.user.find(params.id) };
}

export async function clientLoader({ params }) {
  return { user: await fetch(`/api/users/${params.id}`).then(r => r.json()) };
}

// ✅ Good - augmenting server data
export async function loader({ params }) {
  return { user: await db.user.find(params.id) };
}

export async function clientLoader({ serverLoader }) {
  const { user } = await serverLoader();
  const activity = await fetchUserActivity(user.id);
  return { user, activity };
}
Provide fallbacks when server is unreachable:
export async function clientLoader({ serverLoader }) {
  try {
    return await serverLoader();
  } catch (error) {
    // Return cached data or offline message
    const cached = await getFromIndexedDB();
    if (cached) {
      return { ...cached, offline: true };
    }
    throw new Response("Offline", { status: 503 });
  }
}

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See Also

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