---
title: Pre-Rendering
---

# Pre-Rendering

[MODES: framework]

<br/>
<br/>

Pre-Rendering allows you to speed up page loads for static content by rendering pages at build time instead of at runtime.

## Configuration

Pre-rendering is enabled via the `prerender` config in `react-router.config.ts`.

The simplest configuration is a boolean `true` which will pre-render all off the applications static paths based on `routes.ts`:

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

export default {
  prerender: true,
} satisfies Config;
```

The boolean `true` will not include any dynamic paths (i.e., `/blog/:slug`) because the parameter values are unknown.

To configure specific paths including dynamic values, you can specify an array of paths:

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

let slugs = getPostSlugs();

export default {
  prerender: [
    "/",
    "/blog",
    ...slugs.map((s) => `/blog/${s}`),
  ],
} satisfies Config;
```

If you need to perform more complex and/or asynchronous logic to determine the paths, you can also provide a function that returns an array of paths. This function provides you with a `getStaticPaths` method you can use to avoid manually adding all of the static paths in your application:

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

export default {
  async prerender({ getStaticPaths }) {
    let slugs = await getPostSlugsFromCMS();
    return [
      ...getStaticPaths(), // "/" and "/blog"
      ...slugs.map((s) => `/blog/${s}`),
    ];
  },
} satisfies Config;
```

### Concurrency

By default, pages are pre-rendered one path at a time. You can enable concurrency to pre-render multiple paths in parallel which can speed up build times in many cases. You should experiment with the value that provides the best performance for your app.

To specify concurrency, move your `prerender` config down into a `prerender.paths` field and you can specify the concurrency in `prerender.concurrency`:

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

let slugs = getPostSlugs();

export default {
  prerender: {
    paths: [
      "/",
      "/blog",
      ...slugs.map((s) => `/blog/${s}`),
    ],
    concurrency: 4,
  },
} satisfies Config;
```

## Pre-Rendering with/without a Runtime Server

Pre-Rendering can be used in two ways based on the `ssr` config value:

- Alongside a runtime SSR server with `ssr:true` (the default value)
- Deployed to a static file server with `ssr:false`

### Pre-rendering with `ssr:true`

When pre-rendering with `ssr:true`, you're indicating you will still have a runtime server but you are choosing to pre-render certain paths for quicker Response times.

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

export default {
  // Can be omitted - defaults to true
  ssr: true,
  prerender: ["/", "/blog", "/blog/popular-post"],
} satisfies Config;
```

#### Data Loading and Pre-rendering

There is no extra application API for pre-rendering. Routes being pre-rendered use the same route `loader` functions as server rendering:

```tsx
export async function loader({ request, params }) {
  let post = await getPost(params.slug);
  return post;
}

export function Post({ loaderData }) {
  return <div>{loaderData.title}</div>;
}
```

Instead of a request coming to your route on a deployed server, the build creates a `new Request()` and runs it through your app just like a server would.

When server rendering, requests to paths that have not been pre-rendered will be server rendered as usual.

#### Static File Output

The rendered result will be written out to your `build/client` directory. You'll notice two files for each path:

- `[url].html` HTML file for initial document requests
- `[url].data` file for client side navigation browser requests

The output of your build will indicate what files were pre-rendered:

```sh
> react-router build
vite v5.2.11 building for production...
...
vite v5.2.11 building SSR bundle for production...
...
Prerender: Generated build/client/index.html
Prerender: Generated build/client/blog.data
Prerender: Generated build/client/blog/index.html
Prerender: Generated build/client/blog/my-first-post.data
Prerender: Generated build/client/blog/my-first-post/index.html
...
```

During development, pre-rendering doesn't save the rendered results to the public directory, this only happens for `react-router build`.

### Pre-rendering with `ssr:false`

The above examples assume you are deploying a runtime server but are pre-rendering some static pages to avoid hitting the server, resulting in faster loads.

To disable runtime SSR and configure pre-rendering to be served from a static file server, you can set the `ssr:false` config flag:

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

export default {
  ssr: false, // disable runtime server rendering
  prerender: true, // pre-render all static routes
} satisfies Config;
```

If you specify `ssr:false` without a `prerender` config, React Router refers to that as [SPA Mode](./spa). In SPA Mode, we render a single HTML file that is capable of hydrating for _any_ of your application paths. It can do this because it only renders the `root` route into the HTML file and then determines which child routes to load based on the browser URL during hydration. This means you can use a `loader` on the root route, but not on any other routes because we don't know which routes to load until hydration in the browser.

If you want to pre-render paths with `ssr:false`, those matched routes _can_ have loaders because we'll pre-render all of the matched routes for those paths, not just the root. You cannot include `actions` or `headers` functions in any routes when `ssr:false` is set because there will be no runtime server to run them on.

#### Pre-rendering with a SPA Fallback

If you want `ssr:false` but don't want to pre-render _all_ of your routes - that's fine too! You may have some paths where you need the performance/SEO benefits of pre-rendering, but other pages where a SPA would be fine.

You can do this using the combination of config options as well - just limit your `prerender` config to the paths that you want to pre-render and React Router will also output a "SPA Fallback" HTML file that can be served to hydrate any other paths (using the same approach as [SPA Mode](./spa)).

This will be written to one of the following paths:

- `build/client/index.html` - If the `/` path is not pre-rendered
- `build/client/__spa-fallback.html` - If the `/` path is pre-rendered

```ts filename=react-router.config.ts
import type { Config } from "@react-router/dev/config";

export default {
  ssr: false,

  // SPA fallback will be written to build/client/index.html
  prerender: ["/about-us"],

  // SPA fallback will be written to build/client/__spa-fallback.html
  prerender: ["/", "/about-us"],
} satisfies Config;
```

You can configure your deployment server to serve this file for any path that otherwise would 404. Some hosts do this by default, but others don't. As an example, a host may support a `_redirects` file to do this:

```
# If you did not pre-render the `/` route
/*    /index.html   200

# If you pre-rendered the `/` route
/*    /__spa-fallback.html   200
```

If you're getting 404s at valid routes for your app, it's likely you need to configure your host.

Here's another example of how you can do this with the [`sirv-cli`](https://www.npmjs.com/package/sirv-cli#user-content-single-page-applications) tool:

```sh
# If you did not pre-render the `/` route
sirv-cli build/client --single index.html

# If you pre-rendered the `/` route
sirv-cli build/client --single __spa-fallback.html
```

#### Invalid Exports

When pre-rendering with `ssr:false`, React Router will error at build time if you have invalid exports to help prevent some mistakes that can be easily overlooked.

- `headers`/`action` functions are prohibited in all routes because there will be no runtime server on which to run them
- When using `ssr:false` without a `prerender` config (SPA Mode), a `loader` is permitted on the root route only
- When using `ssr:false` with a `prerender` config, a `loader` is permitted on any route matched by a `prerender` path
  - If you are using a `loader` on a pre-rendered route that has child routes, you will need to make sure the parent `loaderData` can be determined at run-time properly by either:
    - Pre-rendering all child routes so that the parent `loader` can be called at build-time for each child route path and rendered into a `.data` file, or
    - Use a `clientLoader` on the parent that can be called at run-time for non-pre-rendered child paths
