Router Core Data Loading

Data Loading

Setup

Basic loader returning data, consumed via useLoaderData:

// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  component: PostsComponent,
})

function PostsComponent() {
  const posts = Route.useLoaderData()
  return (
    <ul>
      {posts.map((post) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  )
}

In code-split components, use getRouteApi instead of importing Route:

import { getRouteApi } from '@tanstack/react-router'

const routeApi = getRouteApi('/posts')

function PostsComponent() {
  const posts = routeApi.useLoaderData()
  return <ul>{/* ... */}</ul>
}

Route Loading Lifecycle

The router executes this sequence on every URL/history update:

1. Route Matching (top-down)
   • route.params.parse
   • route.validateSearch
2. Route Pre-Loading (serial)
   • route.beforeLoad
   • route.onError → route.errorComponent
3. Route Loading (parallel)
   • route.component.preload?
   • route.loader
     • route.pendingComponent (optional)
     • route.component
   • route.onError → route.errorComponent

Key: beforeLoad runs before loader. beforeLoad for a parent runs before its children's beforeLoad. Throwing in beforeLoad prevents all children from loading.

Core Patterns

loaderDeps for Search-Param-Driven Cache Keys

Loaders don't receive search params directly. Use loaderDeps to declare which search params affect the cache key:

// src/routes/posts.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  validateSearch: (search) => ({
    offset: Number(search.offset) || 0,
    limit: Number(search.limit) || 10,
  }),
  loaderDeps: ({ search: { offset, limit } }) => ({ offset, limit }),
  loader: ({ deps: { offset, limit } }) => fetchPosts({ offset, limit }),
})

When deps change, the route reloads regardless of staleTime.

SWR Caching Configuration

TanStack Router has built-in Stale-While-Revalidate caching keyed on the route's parsed pathname + loaderDeps.

Defaults:

• staleTime: 0 — data is always considered stale, reloads in background on re-match
• preloadStaleTime: 30 seconds — preloaded data won't be refetched for 30s
• gcTime: 30 minutes — unused cache entries garbage collected after 30min

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  staleTime: 10_000, // 10s: data considered fresh for 10 seconds
  gcTime: 5 * 60 * 1000, // 5min: garbage collect after 5 minutes
})

Disable SWR caching entirely:

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  staleTime: Infinity,
})

Globally:

const router = createRouter({
  routeTree,
  defaultStaleTime: Infinity,
})

Pending States (pendingComponent / pendingMs / pendingMinMs)

By default, a pending component shows after 1 second (pendingMs: 1000) and stays for at least 500ms (pendingMinMs: 500) to avoid flash.

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  pendingMs: 500,
  pendingMinMs: 300,
  pendingComponent: () => <div>Loading posts...</div>,
  component: PostsComponent,
})

Router Context with createRootRouteWithContext (Factory Pattern)

createRootRouteWithContext is a factory that returns a function. You must call it twice — the first call passes the generic type, the second passes route options:

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

interface MyRouterContext {
  auth: { userId: string }
  fetchPosts: () => Promise<Post[]>
}

// NOTE: double call — createRootRouteWithContext<Type>()({...})
export const Route = createRootRouteWithContext<MyRouterContext>()({
  component: () => <Outlet />,
})

Supply the context when creating the router:

// src/router.tsx
import { createRouter } from '@tanstack/react-router'
import { routeTree } from './routeTree.gen'

const router = createRouter({
  routeTree,
  context: {
    auth: { userId: '123' },
    fetchPosts,
  },
})

Consume in loaders and beforeLoad:

// src/routes/posts.tsx
export const Route = createFileRoute('/posts')({
  loader: ({ context: { fetchPosts } }) => fetchPosts(),
})

To pass React hook values into the router context, call the hook above RouterProvider and inject via the context prop:

import { RouterProvider } from '@tanstack/react-router'

function InnerApp() {
  const auth = useAuth()
  return <RouterProvider router={router} context={{ auth }} />
}

function App() {
  return (
    <AuthProvider>
      <InnerApp />
    </AuthProvider>
  )
}

Route-level context via beforeLoad:

export const Route = createFileRoute('/posts')({
  beforeLoad: () => ({
    fetchPosts: () => fetch('/api/posts').then((r) => r.json()),
  }),
  loader: ({ context: { fetchPosts } }) => fetchPosts(),
})

Deferred Data Loading

Return unawaited promises from the loader for non-critical data. Use the Await component to render them:

import { createFileRoute, Await } from '@tanstack/react-router'

export const Route = createFileRoute('/posts/$postId')({
  loader: async ({ params: { postId } }) => {
    // Slow data — do NOT await
    const slowDataPromise = fetchComments(postId)
    // Fast data — await
    const post = await fetchPost(postId)

    return { post, deferredComments: slowDataPromise }
  },
  component: PostComponent,
})

function PostComponent() {
  const { post, deferredComments } = Route.useLoaderData()

  return (
    <div>
      <h1>{post.title}</h1>
      <Await
        promise={deferredComments}
        fallback={<div>Loading comments...</div>}
      >
        {(comments) => (
          <ul>
            {comments.map((c) => (
              <li key={c.id}>{c.body}</li>
            ))}
          </ul>
        )}
      </Await>
    </div>
  )
}

Invalidation After Mutations

router.invalidate() forces all active route loaders to re-run and marks all cached data as stale:

import { useRouter } from '@tanstack/react-router'

function AddPostButton() {
  const router = useRouter()

  const handleAdd = async () => {
    await fetch('/api/posts', { method: 'POST', body: '...' })
    router.invalidate()
  }

  return <button Post</button>
}

For synchronous invalidation (wait until loaders finish):

await router.invalidate({ sync: true })

Error Handling

import {
  createFileRoute,
  ErrorComponent,
  useRouter,
} from '@tanstack/react-router'

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  errorComponent: ({ error, reset }) => {
    const router = useRouter()

    if (error instanceof CustomError) {
      return <div>{error.message}</div>
    }

    return (
      <div>
        <ErrorComponent error={error} />
        <button
          => {
            // For loader errors, invalidate to re-run loader + reset boundary
            router.invalidate()
          }}
        >
          Retry
        </button>
      </div>
    )
  },
})

Loader Parameters

The loader function receives:

• params — parsed path params
• deps — object from loaderDeps
• context — merged parent + beforeLoad context
• abortController — cancelled when route unloads or becomes stale
• cause — 'enter', 'stay', or 'preload'
• preload — true during preloading
• location — current location object
• parentMatchPromise — promise of parent route match
• route — the route object itself

export const Route = createFileRoute('/posts/$postId')({
  loader: ({ params: { postId }, abortController }) =>
    fetchPost(postId, { signal: abortController.signal }),
})

Common Mistakes

CRITICAL: Assuming loaders only run on the server

TanStack Router is client-first. Loaders run on the client by default. They also run on the server when using TanStack Start for SSR, but the default mental model is client-side execution.

// WRONG — this will crash in the browser
export const Route = createFileRoute('/posts')({
  loader: async () => {
    const fs = await import('fs') // Node.js only!
    return JSON.parse(fs.readFileSync('...')) // fails in browser
  },
})

// CORRECT — loaders run in the browser, use fetch or API calls
export const Route = createFileRoute('/posts')({
  loader: async () => {
    const res = await fetch('/api/posts')
    return res.json()
  },
})

Do NOT put database queries, filesystem access, or server-only code in loaders unless you are using TanStack Start server functions.

MEDIUM: Not understanding staleTime default is 0

Default staleTime is 0. This means data reloads in the background on every route re-match. This is intentional — it ensures fresh data. But if your data is expensive or static, set staleTime:

export const Route = createFileRoute('/posts')({
  loader: () => fetchPosts(),
  staleTime: 60_000, // Consider fresh for 1 minute
})

HIGH: Using reset() instead of router.invalidate() in error components

reset() only resets the error boundary UI. It does NOT re-run the loader. For loader errors, use router.invalidate() which re-runs loaders and resets the boundary:

// WRONG — resets boundary but loader still has stale error
function PostErrorComponent({ error, reset }) {
  return <button
}

// CORRECT — re-runs loader and resets the error boundary
function PostErrorComponent({ error }) {
  const router = useRouter()
  return <button => router.invalidate()}>Retry</button>
}

HIGH: Missing double parentheses on createRootRouteWithContext

createRootRouteWithContext<Type>() is a factory — it returns a function. Must call twice:

// WRONG — missing second call, passes options to the factory
const rootRoute = createRootRouteWithContext<{ auth: AuthState }>({
  component: RootComponent,
})

// CORRECT — factory()({options})
const rootRoute = createRootRouteWithContext<{ auth: AuthState }>()({
  component: RootComponent,
})

HIGH: Using React hooks in beforeLoad or loader

beforeLoad and loader are NOT React components. You cannot call hooks inside them. Use router context to inject values from hooks:

// WRONG — hooks cannot be called outside React components
export const Route = createFileRoute('/posts')({
  loader: () => {
    const auth = useAuth() // This will crash!
    return fetchPosts(auth.userId)
  },
})

// CORRECT — inject hook values via router context
// In your App component:
function InnerApp() {
  const auth = useAuth()
  return <RouterProvider router={router} context={{ auth }} />
}

// In your route:
export const Route = createFileRoute('/posts')({
  loader: ({ context: { auth } }) => fetchPosts(auth.userId),
})

HIGH: Property order affects TypeScript inference

Router infers types from earlier properties into later ones. Declaring beforeLoad after loader means context from beforeLoad is unknown in the loader:

// WRONG — context.user is unknown because beforeLoad declared after loader
export const Route = createFileRoute('/admin')({
  loader: ({ context }) => fetchData(context.user),
  beforeLoad: () => ({ user: getUser() }),
})

// CORRECT — validateSearch → loaderDeps → beforeLoad → loader
export const Route = createFileRoute('/admin')({
  beforeLoad: () => ({ user: getUser() }),
  loader: ({ context }) => fetchData(context.user),
})

HIGH: Returning entire search object from loaderDeps

// WRONG — loader re-runs on ANY search param change
loaderDeps: ({ search }) => search

// CORRECT — only re-run when page changes
loaderDeps: ({ search }) => ({ page: search.page })

Returning the whole search object means unrelated param changes (e.g., sortDirection, viewMode) trigger unnecessary reloads because deep equality fails on the entire object.

Tensions

• Client-first loaders vs SSR expectations: Loaders run on the client by default. When using SSR (TanStack Start), they run on both client and server. Browser-only APIs work by default but break under SSR. Server-only APIs (fs, db) break by default but work under Start server functions. See router-core-ssr/SKILL.md.
• Built-in SWR cache vs external cache coordination: Router has built-in caching. When using TanStack Query, set defaultPreloadStaleTime: 0 to avoid double-caching. See compositions-router-query/SKILL.md.

Cross-References

• See also: router-core-search-params/SKILL.md — loaderDeps consumes validated search params as cache keys
• See also: compositions-router-query/SKILL.md — for external cache coordination with TanStack Query