---
title: Special Files
---
# Special Files
There are a few special files that React Router looks for in your project. Not all of these files are required
## react-router.config.ts
**This file is optional**
The config file is used to configure certain aspects of your app, such as whether you are using server-side rendering, where certain directories are located, and more.
```tsx filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";
export default {
// Config options...
} satisfies Config;
```
See the details on [react-router config API][react-router-config] for more information.
## root.tsx
**This file is required**
The "root" route (`app/root.tsx`) is the only _required_ route in your React Router application because it is the parent to all routes in your `routes/` directory and is in charge of rendering the root `` document.
Because the root route manages your document, it is the proper place to render a handful of "document-level" components React Router provides. These components are to be used once inside your root route and they include everything React Router figured out or built in order for your page to render properly.
```tsx filename=app/root.tsx
import type { LinksFunction } from "react-router";
import {
Links,
Meta,
Outlet,
Scripts,
ScrollRestoration,
} from "react-router";
import "./global-styles.css";
export default function App() {
return (
{/* All `meta` exports on all routes will render here */}
{/* All `link` exports on all routes will render here */}
{/* Child routes render here */}
{/* Manages scroll position for client-side transitions */}
{/* If you use a nonce-based content security policy for scripts, you must provide the `nonce` prop. Otherwise, omit the nonce prop as shown here. */}
{/* Script tags go here */}
{/* If you use a nonce-based content security policy for scripts, you must provide the `nonce` prop. Otherwise, omit the nonce prop as shown here. */}
);
}
```
### Layout export
The root route supports all [route module exports][route-module].
The root route also supports an additional optional `Layout` export. The `Layout` component serves 2 purposes:
1. Avoid duplicating your document's "app shell" across your root component, `HydrateFallback`, and `ErrorBoundary`
2. Prevent React from re-mounting your app shell elements when switching between the root component/`HydrateFallback`/`ErrorBoundary` which can cause a FOUC if React removes and re-adds `` tags from your `` component.
```tsx filename=app/root.tsx lines=[10-31]
export function Layout({ children }) {
return (
{/* children will be the root Component, ErrorBoundary, or HydrateFallback */}
{children}
);
}
export default function App() {
return ;
}
export function ErrorBoundary() {}
```
**A note on `useLoaderData`in the `Layout` Component**
`useLoaderData` is not permitted to be used in `ErrorBoundary` components because it is intended for the happy-path route rendering, and its typings have a built-in assumption that the `loader` ran successfully and returned something. That assumption doesn't hold in an `ErrorBoundary` because it could have been the `loader` that threw and triggered the boundary! In order to access loader data in `ErrorBoundary`'s, you can use `useRouteLoaderData` which accounts for the loader data potentially being `undefined`.
Because your `Layout` component is used in both success and error flows, this same restriction holds. If you need to fork logic in your `Layout` depending on if it was a successful request or not, you can use `useRouteLoaderData("root")` and `useRouteError()`.
Because your `` component is used for rendering the `ErrorBoundary`, you should be _very defensive_ to ensure that you can render your `ErrorBoundary` without encountering any render errors. If your `Layout` throws another error trying to render the boundary, then it can't be used and your UI will fall back to the very minimal built-in default `ErrorBoundary`.
```tsx filename=app/root.tsx lines=[6-7,19-29,32-34]
export function Layout({
children,
}: {
children: React.ReactNode;
}) {
const data = useRouteLoaderData("root");
const error = useRouteError();
return (
{data ? (
) : null}
{children}
);
}
```
## routes.ts
**This file is required**
The `routes.ts` file is used to configure which url patterns are matched to which route modules.
```tsx filename=app/routes.ts
import {
type RouteConfig,
route,
} from "@react-router/dev/routes";
export default [
route("some/path", "./some/file.tsx"),
// pattern ^ ^ module file
] satisfies RouteConfig;
```
See the [routing guide][routing] for more information.
## entry.client.tsx
**This file is optional**
By default, React Router will handle hydrating your app on the client for you. You can reveal the default entry client file with the following:
```shellscript nonumber
react-router reveal
```
This file is the entry point for the browser and is responsible for hydrating the markup generated by the server in your [server entry module][server-entry], however you can also initialize any other client-side code here.
```tsx filename=app/entry.client.tsx
import { startTransition, StrictMode } from "react";
import { hydrateRoot } from "react-dom/client";
import { HydratedRouter } from "react-router/dom";
startTransition(() => {
hydrateRoot(
document,
);
});
```
This is the first piece of code that runs in the browser. You can initialize client side libraries, add client only providers, etc.
## entry.server.tsx
**This file is optional**
By default, React Router will handle generating the HTTP Response for you. You can reveal the default entry server file with the following:
```shellscript nonumber
react-router reveal
```
The `default` export of this module is a function that lets you create the response, including HTTP status, headers, and HTML, giving you full control over the way the markup is generated and sent to the client.
This module should render the markup for the current page using a `` element with the `context` and `url` for the current request. This markup will (optionally) be re-hydrated once JavaScript loads in the browser using the [client entry module][client-entry].
### `streamTimeout`
If you are [streaming] responses, you can export an optional `streamTimeout` value (in milliseconds) that will control the amount of time the server will wait for streamed promises to settle before rejecting outstanding promises them and closing the stream.
It's recommended to decouple this value from the timeout in which you abort the React renderer. You should always set the React rendering timeout to a higher value so it has time to stream down the underlying rejections from your `streamTimeout`.
```tsx lines=[1-2,13-15]
// Reject all pending promises from handler functions after 10 seconds
export const streamTimeout = 10000;
export default function handleRequest(...) {
return new Promise((resolve, reject) => {
// ...
const { pipe, abort } = renderToPipeableStream(
,
{ /* ... */ }
);
// Abort the streaming render pass after 11 seconds to allow the rejected
// boundaries to be flushed
setTimeout(abort, streamTimeout + 1000);
});
}
```
### `handleDataRequest`
You can export an optional `handleDataRequest` function that will allow you to modify the response of a data request. These are the requests that do not render HTML, but rather return the loader and action data to the browser once client-side hydration has occurred.
```tsx
export function handleDataRequest(
response: Response,
{
request,
params,
context,
}: LoaderFunctionArgs | ActionFunctionArgs
) {
response.headers.set("X-Custom-Header", "value");
return response;
}
```
### `handleError`
By default, React Router will log encountered server-side errors to the console. If you'd like more control over the logging, or would like to also report these errors to an external service, then you can export an optional `handleError` function which will give you control (and will disable the built-in error logging).
```tsx
export function handleError(
error: unknown,
{
request,
params,
context,
}: LoaderFunctionArgs | ActionFunctionArgs
) {
if (!request.signal.aborted) {
sendErrorToErrorReportingService(error);
console.error(formatErrorForJsonLogging(error));
}
}
```
_Note that you generally want to avoid logging when the request was aborted, since React Router's cancellation and race-condition handling can cause a lot of requests to be aborted._
### Streaming Rendering Errors
When you are streaming your HTML responses via [`renderToPipeableStream`][rendertopipeablestream] or [`renderToReadableStream`][rendertoreadablestream], your own `handleError` implementation will only handle errors encountered during the initial shell render. If you encounter a rendering error during subsequent streamed rendering you will need to handle these errors manually since the React Router server has already sent the Response by that point.
For `renderToPipeableStream`, you can handle these errors in the `onError` callback function. You will need to toggle a boolean in `onShellReady` so you know if the error was a shell rendering error (and can be ignored) or an async
For an example, please refer to the default [`entry.server.tsx`][node-streaming-entry-server] for Node.
**Thrown Responses**
Note that this does not handle thrown `Response` instances from your `loader`/`action` functions. The intention of this handler is to find bugs in your code which result in unexpected thrown errors. If you are detecting a scenario and throwing a 401/404/etc. `Response` in your `loader`/`action` then it's an expected flow that is handled by your code. If you also wish to log, or send those to an external service, that should be done at the time you throw the response.
## `.server` modules
While not strictly necessary, `.server` modules are a good way to explicitly mark entire modules as server-only.
The build will fail if any code in a `.server` file or `.server` directory accidentally ends up in the client module graph.
```txt
app
├── .server 👈 marks all files in this directory as server-only
│ ├── auth.ts
│ └── db.ts
├── cms.server.ts 👈 marks this file as server-only
├── root.tsx
└── routes.ts
```
`.server` modules must be within your app directory.
Refer to the Route Module section in the sidebar for more information.
## `.client` modules
While uncommon, you may have a file or dependency that uses module side effects in the browser. You can use `*.client.ts` on file names or nest files within `.client` directories to force them out of server bundles.
```ts filename=feature-check.client.ts
// this would break the server
export const supportsVibrationAPI =
"vibrate" in window.navigator;
```
Note that values exported from this module will all be `undefined` on the server, so the only places to use them are in [`useEffect`][use_effect] and user events like click handlers.
```ts
import { supportsVibrationAPI } from "./feature-check.client.ts";
console.log(supportsVibrationAPI);
// server: undefined
// client: true | false
```
[react-router-config]: https://api.reactrouter.com/v7/types/_react_router_dev.config.Config.html
[route-module]: ../start/framework/route-module
[routing]: ../start/framework/routing
[server-entry]: #entryservertsx
[client-entry]: #entryclienttsx
[rendertopipeablestream]: https://react.dev/reference/react-dom/server/renderToPipeableStream
[rendertoreadablestream]: https://react.dev/reference/react-dom/server/renderToReadableStream
[node-streaming-entry-server]: https://github.com/remix-run/react-router/blob/dev/packages/react-router-dev/config/defaults/entry.server.node.tsx
[streaming]: ../how-to/suspense
[use_effect]: https://react.dev/reference/react/useEffect