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Middleware

Middleware functions in React Router allow you to run code before your route loaders and actions execute, enabling authentication, authorization, logging, and more.

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Enable Middleware

Middleware is a future flag that must be enabled in react-router.config.ts: 
export default {
  future: {
    v8_middleware: true,
  },
} satisfies Config;

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Route Middleware

Define middleware in route modules: 
import type { Route } from "./+types/route-name";
import { redirect } from "react-router";

export async function middleware({ request, context }: Route.MiddlewareArgs) {
  // Check authentication
  const user = await context.auth.getUser(request);
  
  if (!user) {
    throw redirect("/login");
  }

  // Middleware can return data to pass to loader/action
  return { user };
}

export async function loader({ request, context }: Route.LoaderArgs) {
  // Access middleware data from context
  const user = context.user;
  
  return {
    user,
    posts: await db.post.findMany({ userId: user.id }),
  };
}

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Middleware Execution Order

Middleware executes in route hierarchy order, from parent to child: 
// routes.ts
import { route } from "@react-router/dev/routes";

export default [
  route("admin", "./admin.tsx", [
    route("users", "./admin.users.tsx"),
  ]),
];

// app/admin.tsx
export async function middleware({ request, context }) {
  console.log("1. Admin middleware");
  // Check if user is admin
  if (!context.user?.isAdmin) {
    throw redirect("/");
  }
  return { adminCheck: true };
}

// app/admin.users.tsx
export async function middleware({ request, context }) {
  console.log("2. Users middleware");
  // Parent middleware ran first
  // Access parent middleware data via context
  return { usersCheck: true };
}

export async function loader({ context }) {
  // Both middleware results available
  console.log(context.adminCheck); // true
  console.log(context.usersCheck); // true
}

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Middleware Return Values

Middleware can return data that gets merged into the context: 
export async function middleware({ request, context }) {
  const user = await getUserFromSession(request);
  const permissions = await getPermissions(user.id);
  
  // Return an object to add to context
  return {
    user,
    permissions,
  };
}

export async function loader({ context }) {
  // Access middleware data
  const { user, permissions } = context;
  
  if (!permissions.canViewPosts) {
    throw new Response("Forbidden", { status: 403 });
  }
  
  return { posts: await getPosts() };
}

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Client Middleware

Client-side middleware runs before client loaders and actions: 
import type { Route } from "./+types/route-name";

export async function clientMiddleware({ request, context }: Route.ClientMiddlewareArgs) {
  // Run on the client before clientLoader/clientAction
  const token = localStorage.getItem("token");
  
  if (!token) {
    throw redirect("/login");
  }
  
  return { token };
}

export async function clientLoader({ request, context }: Route.ClientLoaderArgs) {
  // Access client middleware data
  const { token } = context;
  
  const response = await fetch("/api/data", {
    headers: { Authorization: `Bearer ${token}` },
  });
  
  return response.json();
}

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Middleware Arguments

Middleware functions receive these arguments: 
export async function middleware({
  request,  // Web Request object
  params,   // Route parameters
  context,  // Server context + parent middleware data
}: Route.MiddlewareArgs) {
  // ...
}

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request

The Web Fetch API Request object: 
export async function middleware({ request }) {
  const url = new URL(request.url);
  const cookies = request.headers.get("Cookie");
  const method = request.method;
  
  if (method === "POST") {
    const formData = await request.formData();
  }
}

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params

Route parameters from the URL: 
// Route: posts/:postId/edit
export async function middleware({ params }) {
  const { postId } = params; // Typed as string
  const post = await getPost(postId);
  
  if (!post) {
    throw new Response("Not Found", { status: 404 });
  }
  
  return { post };
}

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context

Server context plus parent middleware data: 
export async function middleware({ context }) {
  // Access data from load context
  const db = context.db;
  
  // Access data from parent middleware
  const user = context.user;
}

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Throwing Responses

Middleware can throw responses to short-circuit execution: 
export async function middleware({ context }) {
  if (!context.user) {
    // Redirect to login
    throw redirect("/login");
  }
  
  if (!context.user.emailVerified) {
    // Return 403 Forbidden
    throw new Response("Email not verified", { status: 403 });
  }
  
  if (context.user.isBanned) {
    // Return JSON error
    throw new Response(
      JSON.stringify({ error: "Account banned" }),
      {
        status: 403,
        headers: { "Content-Type": "application/json" },
      }
    );
  }
}

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Common Use Cases

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Authentication

export async function middleware({ request, context }) {
  const session = await getSession(request.headers.get("Cookie"));
  const user = session ? await db.user.findUnique({ where: { id: session.userId } }) : null;
  
  if (!user) {
    throw redirect("/login");
  }
  
  return { user };
}

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Authorization

export async function middleware({ context, params }) {
  const { user } = context;
  const post = await db.post.findUnique({ where: { id: params.postId } });
  
  if (post.authorId !== user.id && !user.isAdmin) {
    throw new Response("Unauthorized", { status: 403 });
  }
  
  return { post };
}

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Logging

export async function middleware({ request }) {
  const start = Date.now();
  console.log(`[${request.method}] ${request.url}`);
  
  return {
    requestStart: start,
  };
}

export async function loader({ context }) {
  const data = await getData();
  const duration = Date.now() - context.requestStart;
  console.log(`Loader completed in ${duration}ms`);
  return data;
}

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Feature Flags

export async function middleware({ context }) {
  const features = await getFeatureFlags(context.user.id);
  
  if (!features.newDashboard) {
    throw redirect("/old-dashboard");
  }
  
  return { features };
}

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Rate Limiting

export async function middleware({ request, context }) {
  const ip = request.headers.get("X-Forwarded-For") || "unknown";
  const rateLimit = await context.redis.get(`ratelimit:${ip}`);
  
  if (rateLimit && parseInt(rateLimit) > 100) {
    throw new Response("Too Many Requests", { status: 429 });
  }
  
  await context.redis.incr(`ratelimit:${ip}`);
  await context.redis.expire(`ratelimit:${ip}`, 60); // 1 minute window
}

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Middleware vs. Loaders

Use middleware for:
  • Authentication and authorization
  • Request validation
  • Logging and analytics
  • Setting up shared context
  • Rate limiting
Use loaders for:
  • Fetching data to render
  • Database queries
  • API calls
  • Business logic specific to the route

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TypeScript Types

With type generation enabled: 
import type { Route } from "./+types/route-name";

// Server middleware
export const middleware: Route.MiddlewareFunction = async ({ request, params, context }) => {
  // Fully typed
  return { data: "..." };
};

// Client middleware
export const clientMiddleware: Route.ClientMiddlewareFunction = async ({ request, params, context }) => {
  // Fully typed
  return { data: "..." };
};

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Server-Only Middleware

Server middleware exports are automatically removed from client bundles: 
// app/routes/admin.tsx

// This runs ONLY on the server
export async function middleware({ context }) {
  // Safe to use server-only code
  const secret = process.env.SECRET_KEY;
  return { secret };
}

// This runs on server AND client
export async function clientMiddleware({ context }) {
  // Cannot access server-only code
  // Can access browser APIs
  const token = localStorage.getItem("token");
  return { token };
}

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Caveats

  1. Middleware runs for every request - Keep it fast
  2. Middleware blocks rendering - Avoid heavy computation
  3. Context is merged - Avoid key conflicts between parent/child middleware
  4. Throwing ends execution - No subsequent middleware/loaders run
  5. Client middleware is different - Runs separately from server middleware

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See Also

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