Server Functions

What are Server Functions?

Server functions let you define server-only logic that can be called from anywhere in your application - loaders, components, hooks, or other server functions. They run on the server but can be invoked from client code seamlessly.

import { createServerFn } from '@tanstack/react-start'

export const getServerTime = createServerFn().handler(async () => {
  // This runs only on the server
  return new Date().toISOString()
})

// Call from anywhere - components, loaders, hooks, etc.
const time = await getServerTime()

import { createServerFn } from '@tanstack/react-start'

export const getServerTime = createServerFn().handler(async () => {
  // This runs only on the server
  return new Date().toISOString()
})

// Call from anywhere - components, loaders, hooks, etc.
const time = await getServerTime()

Server functions provide server capabilities (database access, environment variables, file system) while maintaining type safety across the network boundary.

Basic Usage

Server functions are created with createServerFn() and can specify HTTP method:

import { createServerFn } from '@tanstack/react-start'

// GET request (default)
export const getData = createServerFn().handler(async () => {
  return { message: 'Hello from server!' }
})

// POST request
export const saveData = createServerFn({ method: 'POST' }).handler(async () => {
  // Server-only logic
  return { success: true }
})

import { createServerFn } from '@tanstack/react-start'

// GET request (default)
export const getData = createServerFn().handler(async () => {
  return { message: 'Hello from server!' }
})

// POST request
export const saveData = createServerFn({ method: 'POST' }).handler(async () => {
  // Server-only logic
  return { success: true }
})

Where to Call Server Functions

Call server functions from:

Route loaders - Perfect for data fetching
Components - Use with useServerFn() hook
Other server functions - Compose server logic
Event handlers - Handle form submissions, clicks, etc.

// In a route loader
export const Route = createFileRoute('/posts')({
  loader: () => getPosts(),
})

// In a component
function PostList() {
  const getPosts = useServerFn(getServerPosts)

  const { data } = useQuery({
    queryKey: ['posts'],
    queryFn: () => getPosts(),
  })
}

// In a route loader
export const Route = createFileRoute('/posts')({
  loader: () => getPosts(),
})

// In a component
function PostList() {
  const getPosts = useServerFn(getServerPosts)

  const { data } = useQuery({
    queryKey: ['posts'],
    queryFn: () => getPosts(),
  })
}

Parameters & Validation

Server functions accept a single data parameter. Since they cross the network boundary, validation ensures type safety and runtime correctness.

Basic Parameters

import { createServerFn } from '@tanstack/react-start'

export const greetUser = createServerFn({ method: 'GET' })
  .inputValidator((data: { name: string }) => data)
  .handler(async ({ data }) => {
    return `Hello, ${data.name}!`
  })

await greetUser({ data: { name: 'John' } })

import { createServerFn } from '@tanstack/react-start'

export const greetUser = createServerFn({ method: 'GET' })
  .inputValidator((data: { name: string }) => data)
  .handler(async ({ data }) => {
    return `Hello, ${data.name}!`
  })

await greetUser({ data: { name: 'John' } })

Validation with Zod

For robust validation, use schema libraries like Zod:

import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'

const UserSchema = z.object({
  name: z.string().min(1),
  age: z.number().min(0),
})

export const createUser = createServerFn({ method: 'POST' })
  .inputValidator(UserSchema)
  .handler(async ({ data }) => {
    // data is fully typed and validated
    return `Created user: ${data.name}, age ${data.age}`
  })

import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'

const UserSchema = z.object({
  name: z.string().min(1),
  age: z.number().min(0),
})

export const createUser = createServerFn({ method: 'POST' })
  .inputValidator(UserSchema)
  .handler(async ({ data }) => {
    // data is fully typed and validated
    return `Created user: ${data.name}, age ${data.age}`
  })

Form Data

Handle form submissions with FormData:

export const submitForm = createServerFn({ method: 'POST' })
  .inputValidator((data) => {
    if (!(data instanceof FormData)) {
      throw new Error('Expected FormData')
    }

    return {
      name: data.get('name')?.toString() || '',
      email: data.get('email')?.toString() || '',
    }
  })
  .handler(async ({ data }) => {
    // Process form data
    return { success: true }
  })

export const submitForm = createServerFn({ method: 'POST' })
  .inputValidator((data) => {
    if (!(data instanceof FormData)) {
      throw new Error('Expected FormData')
    }

    return {
      name: data.get('name')?.toString() || '',
      email: data.get('email')?.toString() || '',
    }
  })
  .handler(async ({ data }) => {
    // Process form data
    return { success: true }
  })

Error Handling & Redirects

Server functions can throw errors, redirects, and not-found responses that are handled automatically when called from route lifecycles or components using useServerFn().

Basic Errors

import { createServerFn } from '@tanstack/react-start'

export const riskyFunction = createServerFn().handler(async () => {
  if (Math.random() > 0.5) {
    throw new Error('Something went wrong!')
  }
  return { success: true }
})

// Errors are serialized to the client
try {
  await riskyFunction()
} catch (error) {
  console.log(error.message) // "Something went wrong!"
}

import { createServerFn } from '@tanstack/react-start'

export const riskyFunction = createServerFn().handler(async () => {
  if (Math.random() > 0.5) {
    throw new Error('Something went wrong!')
  }
  return { success: true }
})

// Errors are serialized to the client
try {
  await riskyFunction()
} catch (error) {
  console.log(error.message) // "Something went wrong!"
}

Redirects

Use redirects for authentication, navigation, etc:

import { createServerFn } from '@tanstack/react-start'
import { redirect } from '@tanstack/react-router'

export const requireAuth = createServerFn().handler(async () => {
  const user = await getCurrentUser()

  if (!user) {
    throw redirect({ to: '/login' })
  }

  return user
})

import { createServerFn } from '@tanstack/react-start'
import { redirect } from '@tanstack/react-router'

export const requireAuth = createServerFn().handler(async () => {
  const user = await getCurrentUser()

  if (!user) {
    throw redirect({ to: '/login' })
  }

  return user
})

Not Found

Throw not-found errors for missing resources:

import { createServerFn } from '@tanstack/react-start'
import { notFound } from '@tanstack/react-router'

export const getPost = createServerFn()
  .inputValidator((data: { id: string }) => data)
  .handler(async ({ data }) => {
    const post = await db.findPost(data.id)

    if (!post) {
      throw notFound()
    }

    return post
  })

import { createServerFn } from '@tanstack/react-start'
import { notFound } from '@tanstack/react-router'

export const getPost = createServerFn()
  .inputValidator((data: { id: string }) => data)
  .handler(async ({ data }) => {
    const post = await db.findPost(data.id)

    if (!post) {
      throw notFound()
    }

    return post
  })

Advanced Topics

For more advanced server function patterns and features, see these dedicated guides:

Server Context & Request Handling

Access request headers, cookies, and response customization:

getRequest() - Access the full request object
getRequestHeader() - Read specific headers
setResponseHeader() - Set custom response headers
setResponseStatus() - Custom status codes

Note: Server functions use a compilation process that extracts server code from client bundles while maintaining seamless calling patterns. On the client, calls become fetch requests to the server.