lifecycle/migrate-from-nextjs

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Step-by-step migration from Next.js App Router to TanStack Start: route definition conversion, API mapping, server function conversion from Server Actions, middleware conversion, data fetching pattern changes.

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Migrate from Next.js App Router to TanStack Start

This is a step-by-step migration checklist. Complete tasks in order.

CRITICAL: TanStack Start is isomorphic by default. ALL code runs in both environments unless you use createServerFn. This is the opposite of Next.js Server Components, where code is server-only by default.

CRITICAL: TanStack Start uses createServerFn, NOT "use server" directives. Do not carry over any "use server" or "use client" directives.

CRITICAL: Types are FULLY INFERRED in TanStack Router/Start. Never cast, never annotate inferred values.

Pre-Migration

Create a migration branch

git checkout -b migrate-to-tanstack-start

Install TanStack Start

npm i @tanstack/react-start @tanstack/react-router
npm i -D vite @vitejs/plugin-react

Remove Next.js

npm uninstall next @next/font @next/image

Concept Mapping

Next.js App Router	TanStack Start
`app/page.tsx`	`src/routes/index.tsx`
`app/layout.tsx`	`src/routes/__root.tsx`
`app/posts/[id]/page.tsx`	`src/routes/posts/$postId.tsx`
`app/api/users/route.ts`	`src/routes/api/users.ts` (server property)
`"use server"` + Server Actions	`createServerFn()`
`"use client"`	Not needed (everything is isomorphic)
Server Components (default)	All components are isomorphic; use `createServerFn` for server-only logic
`next/navigation` `useRouter`	`useRouter()` from `@tanstack/react-router`
`next/link` `Link`	`<Link>` from `@tanstack/react-router`
`next/head` or `metadata` export	`head` property on route
`middleware.ts` (edge)	`createMiddleware()` in `src/start.ts`
`next.config.js`	`vite.config.ts` with `tanstackStart()`
`generateStaticParams`	`prerender` config in `vite.config.ts`

Step 1: Vite Configuration

Replace next.config.js with:

// vite.config.ts
import { defineConfig } from 'vite'
import { tanstackStart } from '@tanstack/react-start/plugin/vite'
import viteReact from '@vitejs/plugin-react'

export default defineConfig({
  plugins: [
    tanstackStart(), // MUST come before react()
    viteReact(),
  ],
})

Update package.json:

{
  "type": "module",
  "scripts": {
    "dev": "vite dev",
    "build": "vite build",
    "start": "node .output/server/index.mjs"
  }
}

Step 2: Router Factory

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

export function getRouter() {
  const router = createRouter({
    routeTree,
    scrollRestoration: true,
  })
  return router
}

Step 3: Convert Layout → Root Route

Next.js:

// app/layout.tsx
export const metadata = { title: 'My App' }
export default function RootLayout({ children }) {
  return (
    <html>
      <body>{children}</body>
    </html>
  )
}

TanStack Start:

// src/routes/__root.tsx
import type { ReactNode } from 'react'
import {
  Outlet,
  createRootRoute,
  HeadContent,
  Scripts,
} from '@tanstack/react-router'

export const Route = createRootRoute({
  head: () => ({
    meta: [
      { charSet: 'utf-8' },
      { name: 'viewport', content: 'width=device-width, initial-scale=1' },
      { title: 'My App' },
    ],
  }),
  component: RootComponent,
})

function RootComponent() {
  return (
    <html>
      <head>
        <HeadContent />
      </head>
      <body>
        <Outlet />
        <Scripts />
      </body>
    </html>
  )
}

Step 4: Convert Pages → File Routes

Next.js:

// app/posts/[id]/page.tsx
export default function PostPage({ params }: { params: { id: string } }) {
  // ...
}

TanStack Start:

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

export const Route = createFileRoute('/posts/$postId')({
  component: PostPage,
})

function PostPage() {
  const { postId } = Route.useParams()
  // ...
}

Key differences:

Dynamic segments use $param not [param]
Params accessed via Route.useParams() not component props
Route path in filename uses . or / separators

Step 5: Convert Server Actions → Server Functions

Next.js:

// app/actions.ts
'use server'
export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  await db.posts.create({ title })
}

TanStack Start:

// src/utils/posts.functions.ts
import { createServerFn } from '@tanstack/react-start'

export const createPost = createServerFn({ method: 'POST' })
  .validator((data) => {
    if (!(data instanceof FormData)) throw new Error('Expected FormData')
    return { title: data.get('title')?.toString() || '' }
  })
  .handler(async ({ data }) => {
    await db.posts.create({ title: data.title })
    return { success: true }
  })

Step 6: Convert Data Fetching

Next.js Server Component:

// app/posts/page.tsx (Server Component — server-only by default)
export default async function PostsPage() {
  const posts = await db.posts.findMany()
  return <PostList posts={posts} />
}

TanStack Start:

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

const getPosts = createServerFn({ method: 'GET' }).handler(async () => {
  return db.posts.findMany()
})

export const Route = createFileRoute('/posts')({
  loader: () => getPosts(), // loader is isomorphic, getPosts runs on server
  component: PostsPage,
})

function PostsPage() {
  const posts = Route.useLoaderData()
  return <PostList posts={posts} />
}

Step 7: Convert API Routes → Server Routes

Next.js:

// app/api/users/route.ts
export async function GET() {
  const users = await db.users.findMany()
  return Response.json(users)
}

TanStack Start:

// src/routes/api/users.ts
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/api/users')({
  server: {
    handlers: {
      GET: async () => {
        const users = await db.users.findMany()
        return Response.json(users)
      },
    },
  },
})

Step 8: Convert Navigation

Next.js:

import Link from 'next/link'
;<Link href={`/posts/${post.id}`}>View Post</Link>

TanStack Start:

import { Link } from '@tanstack/react-router'
;<Link to="/posts/$postId" params={{ postId: post.id }}>
  View Post
</Link>

Never interpolate params into the to string. Use params prop.

Step 9: Convert Middleware

Next.js:

// middleware.ts
export function middleware(request: NextRequest) {
  const token = request.cookies.get('session')
  if (!token) return NextResponse.redirect(new URL('/login', request.url))
}
export const config = { matcher: ['/dashboard/:path*'] }

TanStack Start:

// src/start.ts — must be manually created
import { createStart, createMiddleware } from '@tanstack/react-start'
import { redirect } from '@tanstack/react-router'

const authMiddleware = createMiddleware().server(async ({ next, request }) => {
  const cookie = request.headers.get('cookie')
  if (!cookie?.includes('session=')) {
    throw redirect({ to: '/login' })
  }
  return next()
})

export const startInstance = createStart(() => ({
  requestMiddleware: [authMiddleware],
}))

Step 10: Convert Metadata/SEO

Next.js:

export const metadata = {
  title: 'Post Title',
  description: 'Post description',
}

TanStack Start:

export const Route = createFileRoute('/posts/$postId')({
  loader: async ({ params }) => fetchPost(params.postId),
  head: ({ loaderData }) => ({
    meta: [
      { title: loaderData.title },
      { name: 'description', content: loaderData.excerpt },
      { property: 'og:title', content: loaderData.title },
    ],
  }),
})

Post-Migration Checklist

Remove all "use server" and "use client" directives
Remove next.config.js / next.config.ts
Remove app/ directory (replaced by src/routes/)
Remove middleware.ts (replaced by src/start.ts)
Verify no next/* imports remain
Run npm run dev and check all routes
Verify server-only code is inside createServerFn (not bare in components/loaders)
Check that <Scripts /> is in the root route <body>

Common Mistakes

1. CRITICAL: Keeping Server Component mental model

// WRONG — treating component as server-only (Next.js habit)
function PostsPage() {
  const posts = await db.posts.findMany() // fails on client
  return <div>{posts.map(...)}</div>
}

// CORRECT — use server function + loader
const getPosts = createServerFn({ method: 'GET' }).handler(async () => {
  return db.posts.findMany()
})

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

2. CRITICAL: Using "use server" directive

// WRONG — "use server" is Next.js/React pattern
'use server'
export async function myAction() { ... }

// CORRECT — use createServerFn
export const myAction = createServerFn({ method: 'POST' })
  .handler(async () => { ... })

3. HIGH: Interpolating params into Link href

// WRONG — Next.js pattern
<Link to={`/posts/${post.id}`}>View</Link>

// CORRECT — TanStack Router pattern
<Link to="/posts/$postId" params={{ postId: post.id }}>View</Link>

Cross-References

react-start — full React Start setup
start-core/server-functions — server function patterns
start-core/execution-model — isomorphic execution