Tanstack Router Best Practices

# ctx-root-context: Define Context at Root Route

## Priority: LOW

## Explanation

Use `createRootRouteWithContext` to define typed context that flows through your entire route tree. This enables dependency injection for things like query clients, auth state, and services.

## Bad Example

```tsx
// No context - importing globals directly
// routes/__root.tsx
import { createRootRoute } from '@tanstack/react-router'
import { queryClient } from '@/lib/query-client'  // Global import

export const Route = createRootRoute({
  component: RootComponent,
})

// routes/posts.tsx
import { queryClient } from '@/lib/query-client'  // Import again

export const Route = createFileRoute('/posts')({
  loader: async () => {
    // Using global - harder to test, couples to implementation
    return queryClient.ensureQueryData(postQueries.list())
  },
})
```

## Good Example

```tsx
// routes/__root.tsx
import { createRootRouteWithContext, Outlet } from '@tanstack/react-router'
import { QueryClient } from '@tanstack/react-query'

// Define the context interface
interface RouterContext {
  queryClient: QueryClient
  auth: {
    user: User | null
    isAuthenticated: boolean
  }
}

export const Route = createRootRouteWithContext<RouterContext>()({
  component: RootComponent,
})

function RootComponent() {
  return (
    <>
      <Header />
      <main>
        <Outlet />
      </main>
      <Footer />
    </>
  )
}

// router.tsx - Provide context when creating router
import { createRouter } from '@tanstack/react-router'
import { QueryClient } from '@tanstack/react-query'
import { setupRouterSsrQueryIntegration } from '@tanstack/react-router-ssr-query'
import { routeTree } from './routeTree.gen'

export function getRouter(auth: RouterContext['auth'] = { user: null, isAuthenticated: false }) {
  const queryClient = new QueryClient()

  const router = createRouter({
    routeTree,
    context: {
      queryClient,
      auth,
    },
    defaultPreload: 'intent',
    defaultPreloadStaleTime: 0,
    scrollRestoration: true,
  })

  setupRouterSsrQueryIntegration({ router, queryClient })

  return router
}

// routes/posts.tsx - Use context in loaders
export const Route = createFileRoute('/posts')({
  loader: async ({ context: { queryClient } }) => {
    // Context is typed and injected
    return queryClient.ensureQueryData(postQueries.list())
  },
})
```

## Good Example: Auth-Protected Routes

```tsx
// routes/__root.tsx
interface RouterContext {
  queryClient: QueryClient
  auth: AuthState
}

export const Route = createRootRouteWithContext<RouterContext>()({
  component: RootComponent,
})

// routes/_authenticated.tsx - Layout route for protected pages
export const Route = createFileRoute('/_authenticated')({
  beforeLoad: async ({ context, location }) => {
    if (!context.auth.isAuthenticated) {
      throw redirect({
        to: '/login',
        search: { redirect: location.href },
      })
    }
  },
  component: AuthenticatedLayout,
})

// routes/_authenticated/dashboard.tsx
export const Route = createFileRoute('/_authenticated/dashboard')({
  loader: async ({ context: { queryClient, auth } }) => {
    // We know user is authenticated from parent beforeLoad
    return queryClient.ensureQueryData(
      dashboardQueries.forUser(auth.user!.id)
    )
  },
})
```

## Extending Context with beforeLoad

```tsx
// routes/posts/$postId.tsx
export const Route = createFileRoute('/posts/$postId')({
  beforeLoad: async ({ context, params }) => {
    // Extend context with route-specific data
    const post = await fetchPost(params.postId)

    return {
      post,  // Available to this route and children
    }
  },
  loader: async ({ context }) => {
    // context now includes 'post' from beforeLoad
    const comments = await fetchComments(context.post.id)
    return { comments }
  },
})
```

## Context vs. Loader Data

| Context | Loader Data |
|---------|-------------|
| Available in beforeLoad, loader, and component | Only available in component |
|