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Loaders

Loaders provide data to route components before they render. They run on the server during SSR and on the client during client-side navigation.

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Basic Loader

Loaders are async functions that return data for your route:
filename=app/routes/product.tsx
import type { Route } from "./+types/product";

export async function loader({ params }: Route.LoaderArgs) {
  const product = await db.product.findUnique({
    where: { id: params.id },
  });
  
  if (!product) {
    throw new Response("Not Found", { status: 404 });
  }
  
  return { product };
}

export default function Product({ loaderData }: Route.ComponentProps) {
  return (
    <div>
      <h1>{loaderData.product.name}</h1>
      <p>{loaderData.product.description}</p>
    </div>
  );
}

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Using useLoaderData

Access loader data in your component with the useLoaderData hook: 
import { useLoaderData } from "react-router";

export async function loader() {
  return await fakeDb.invoices.findAll();
}

export default function Invoices() {
  const invoices = useLoaderData<typeof loader>();
  return (
    <ul>
      {invoices.map((invoice) => (
        <li key={invoice.id}>{invoice.name}</li>
      ))}
    </ul>
  );
}

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Loader Arguments

Loaders receive an object with these properties: 
export async function loader({
  request,  // Fetch Request object
  params,   // URL parameters from the route path
  context,  // Router context (set via middleware)
}: Route.LoaderArgs) {
  // Access URL search params
  const url = new URL(request.url);
  const query = url.searchParams.get("q");
  
  // Access route params
  const userId = params.userId;
  
  // Access context values
  const user = context.get(userContext);
  
  return { query, userId, user };
}

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Returning Responses

Loaders can return various types of data:

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Plain Objects

export async function loader() {
  return { message: "Hello" };
}

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Response Objects

export async function loader() {
  return new Response(JSON.stringify({ data: "value" }), {
    headers: {
      "Content-Type": "application/json",
      "Cache-Control": "max-age=300",
    },
  });
}

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Redirects

import { redirect } from "react-router";

export async function loader({ context }: Route.LoaderArgs) {
  const user = context.get(userContext);
  
  if (!user) {
    return redirect("/login");
  }
  
  return { user };
}

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Error Responses

export async function loader({ params }: Route.LoaderArgs) {
  const post = await db.post.findUnique({
    where: { slug: params.slug },
  });
  
  if (!post) {
    throw new Response("Post not found", { status: 404 });
  }
  
  return { post };
}

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Parallel Data Loading

Loaders for all matching routes run in parallel: 
// app/routes/dashboard.tsx
export async function loader() {
  return { layout: "data" };
}

// app/routes/dashboard.stats.tsx
export async function loader() {
  return { stats: await getStats() };
}

// Both loaders run simultaneously when navigating to /dashboard/stats

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TypeScript

Use the generated Route types for full type safety: 
import type { Route } from "./+types/users";

interface User {
  id: string;
  name: string;
  email: string;
}

export async function loader({ params }: Route.LoaderArgs) {
  const users: User[] = await db.user.findMany();
  return { users };
}

export default function Users({ loaderData }: Route.ComponentProps) {
  // loaderData.users is fully typed as User[]
  return (
    <ul>
      {loaderData.users.map((user) => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
}

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Accessing Parent Loader Data

Use useRouteLoaderData to access data from parent routes: 
import { useRouteLoaderData } from "react-router";

function SomeComponent() {
  // Access loader data from the root route
  const { user } = useRouteLoaderData("root");
  return <p>Hello, {user.name}!</p>;
}
Route IDs are automatically created from the file path:
Route FilenameRoute ID
app/root.tsx"root"
app/routes/teams.tsx"routes/teams"
app/routes/teams.$id.tsx"routes/teams.$id"

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Error Handling

Handle loader errors with ErrorBoundary: 
export async function loader({ params }: Route.LoaderArgs) {
  const user = await db.user.findUnique({
    where: { id: params.id },
  });
  
  if (!user) {
    throw new Response("User not found", { status: 404 });
  }
  
  return { user };
}

export function ErrorBoundary() {
  const error = useRouteError();
  
  if (isRouteErrorResponse(error)) {
    return (
      <div>
        <h1>{error.status} {error.statusText}</h1>
        <p>{error.data}</p>
      </div>
    );
  }
  
  return <div>Something went wrong</div>;
}

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Client Loaders

Use clientLoader to load data only on the client: 
export async function loader() {
  // Runs on server during SSR
  return { serverData: "from server" };
}

export async function clientLoader({ serverLoader }: Route.ClientLoaderArgs) {
  // Runs on client-side navigation
  const serverData = await serverLoader();
  const clientData = localStorage.getItem("cachedData");
  
  return { ...serverData, clientData };
}

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Best Practices

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Keep Loaders Fast

// Good: Parallel queries
export async function loader() {
  const [user, posts, comments] = await Promise.all([
    db.user.findUnique({ where: { id } }),
    db.post.findMany({ where: { userId: id } }),
    db.comment.findMany({ where: { userId: id } }),
  ]);
  return { user, posts, comments };
}

// Bad: Sequential queries
export async function loader() {
  const user = await db.user.findUnique({ where: { id } });
  const posts = await db.post.findMany({ where: { userId: id } });
  const comments = await db.comment.findMany({ where: { userId: id } });
  return { user, posts, comments };
}

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Return Only What You Need

// Good: Return minimal data
export async function loader() {
  const user = await db.user.findUnique({
    where: { id },
    select: { id: true, name: true, email: true },
  });
  return { user };
}

// Bad: Return entire objects
export async function loader() {
  const user = await db.user.findUnique({ where: { id } });
  return { user }; // Includes sensitive fields, relations, etc.
}

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