Server Functions

What are Server Functions?

Server functions allow you to specify logic that can be invoked anywhere (even the client), but run only on the server. In fact, they are not so different from an API route, but with a few key differences:

They are not publicly accessible via HTTP
They can be called from anywhere in your application, including loaders, hooks, components, etc.

However, they are similar to regular API routes in that:

They have access to the request context, allowing you to read headers, set cookies, and more
They can access sensitive information, such as environment variables, without exposing them to the client
They can be used to perform any kind of server-side logic, such as fetching data from a database, sending emails, or interacting with other services
They can return any value, including primitives, JSON-serializable objects, and even raw Response objects
They can throw errors, including redirects and notFounds, which can be handled automatically by the router

How are server functions different from "React Server Functions"?

TanStack Server Functions are not tied to a specific front-end framework, and can be used with any front-end framework or none at all.

TanStack Server Functions are backed by standard HTTP requests and can be called as often as you like without suffering from serial-execution bottlenecks.

How do they work?

Server functions can be defined anywhere in your application, but must be defined at the top level of a file. They can be called from anywhere in your application, including loaders, hooks, etc. Traditionally, this pattern is known as a Remote Procedure Call (RPC), but due to the isomorphic nature of these functions, we refer to them as server functions.

On the server bundle, server functions logic is left alone. Nothing needs to be done since they are already in the correct place.
On the client, server functions are modified to remove server-only logic out of the client bundle and replaced with a function that, when called, makes a fetch request to the server instructing it to execute the server function in the server bundle and then send the response back to the client.

Server Function Middleware

Server functions can use middleware to share logic, context, common operations, prerequisites, and much more. To learn more about server function middleware, be sure to read about them in the Middleware guide.

Defining Server Functions

We'd like to thank the tRPC team for both the inspiration of TanStack Start's server function design and guidance while implementing it. We love (and recommend) using tRPC for API routes so much that we insisted on server functions getting the same 1st class treatment and developer experience. Thank you!

Server functions are defined using the createServerFn function, exported from the @tanstack/start package. This function must be called with an HTTP verb, and an async function that will be executed on the server. Here's an example:

tsx

// getServerTime.ts
import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn().handler(async () => {
  // Wait for 1 second
  await new Promise((resolve) => setTimeout(resolve, 1000))
  // Return the current time
  return new Date().toISOString()
})

// getServerTime.ts
import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn().handler(async () => {
  // Wait for 1 second
  await new Promise((resolve) => setTimeout(resolve, 1000))
  // Return the current time
  return new Date().toISOString()
})

Where can I call server functions?

From server-side code
From client-side code
From other server functions
Anywhere, really!

Accepting Parameters

Server functions accept a single parameter, which can be a variety of types:

Primitives
- string
- number
- boolean
- null
- Array
- Object
FormData
ReadableStream (of any of the above)
Promise (of any of the above)

Here's an example of a server function that accepts a simple string parameter:

tsx

import { createServerFn } from '@tanstack/start'

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

greet({
  data: 'John',
})

import { createServerFn } from '@tanstack/start'

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

greet({
  data: 'John',
})

Validation / Runtime Type Safety

Server functions can be configured to validate their input data at runtime. This is useful for ensuring that the input is of the correct type before executing the server function, can provide more friendly error messages, and even power type-safety if configured correctly.

To enable validation, call the validator method on the server function. You can pass a variety of things:

A function that returns the valid value and type
A thrown error, to indicate that the input is invalid
Schema objects that define the shape of the input data (the exact shape of the schema object depends on the validation library you choose to use)

Basic Validation

Here's a simple example of a server function that validates the input parameter:

tsx

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown): Person => {
    if (typeof person !== 'object' || person === null) {
      throw new Error('Person must be an object')
    }

    if ('name' in person && typeof person.name !== 'string') {
      throw new Error('Person.name must be a string')
    }

    return person as Person
  })
  .handler(async ({ data }) => {
    return `Hello, ${data.name}!`
  })

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown): Person => {
    if (typeof person !== 'object' || person === null) {
      throw new Error('Person must be an object')
    }

    if ('name' in person && typeof person.name !== 'string') {
      throw new Error('Person.name must be a string')
    }

    return person as Person
  })
  .handler(async ({ data }) => {
    return `Hello, ${data.name}!`
  })

Using a Validation Library

You can also use a validation library like Zod to validate the input parameter:

tsx

import { createServerFn } from '@tanstack/start'

import { z } from 'zod'

const Person = z.object({
  name: z.string(),
})

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown) => {
    return Person.parse(person)
  })
  .handler(async (ctx) => {
    return `Hello, ${ctx.data.name}!`
  })

greet({
  data: {
    name: 'John',
  },
})

import { createServerFn } from '@tanstack/start'

import { z } from 'zod'

const Person = z.object({
  name: z.string(),
})

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown) => {
    return Person.parse(person)
  })
  .handler(async (ctx) => {
    return `Hello, ${ctx.data.name}!`
  })

greet({
  data: {
    name: 'John',
  },
})

Type Safety

Since server-functions cross the network boundary, it's important to ensure that the data being passed to them is not only just the right type, but also validated at runtime. This is especially important wheen dealing with user input, as it can be unpredictable. To help ensure developers validate their I/O data, types are reliant on using the input validation, which is then used to infer the input and output types of the server function.

tsx

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown): Person => {
    if (typeof person !== 'object' || person === null) {
      throw new Error('Person must be an object')
    }

    if ('name' in person && typeof person.name !== 'string') {
      throw new Error('Person.name must be a string')
    }

    return person as Person
  })
  .handler(
    async ({
      data, // Person
    }) => {
      return `Hello, ${data.name}!`
    },
  )

function test() {
  greet({ data: { name: 'John' } }) // OK
  greet({ data: { name: 123 } }) // Error: Argument of type '{ name: number; }' is not assignable to parameter of type 'Person'.
}

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((person: unknown): Person => {
    if (typeof person !== 'object' || person === null) {
      throw new Error('Person must be an object')
    }

    if ('name' in person && typeof person.name !== 'string') {
      throw new Error('Person.name must be a string')
    }

    return person as Person
  })
  .handler(
    async ({
      data, // Person
    }) => {
      return `Hello, ${data.name}!`
    },
  )

function test() {
  greet({ data: { name: 'John' } }) // OK
  greet({ data: { name: 123 } }) // Error: Argument of type '{ name: number; }' is not assignable to parameter of type 'Person'.
}

Inference

Server functions infer their input and output types based on the validator handler and the return type of the handler function. In fact, the validator you pass can even have it's own separate input/output types, which can be useful if your validator performs some kind of transformation on the input data.

To illustrate this, let's take a look at an example using the zod validation library:

tsx

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

const transactionSchema = z.object({
  amount: z.string().transform((val) => parseInt(val, 10)),
})

const createTransaction = createServerFn()
  .validator(transactionSchema)
  .handler(({ data }) => {
    return data.amount // Returns a number
  })

createTransaction({
  data: {
    amount: '123', // Accepts a string
  },
})

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

const transactionSchema = z.object({
  amount: z.string().transform((val) => parseInt(val, 10)),
})

const createTransaction = createServerFn()
  .validator(transactionSchema)
  .handler(({ data }) => {
    return data.amount // Returns a number
  })

createTransaction({
  data: {
    amount: '123', // Accepts a string
  },
})

Non-Validated Inference

While we highly recommend using a validation library to validate your network I/O data, you may for whatever reason not want to validate your data, but still have the type-safety. To do this, you can still provide type information to the server function using an identity function as the validator handler that casts the input and or output to the correct type:

tsx

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((d: Person) => d)
  .handler(async (ctx) => {
    return `Hello, ${ctx.data.name}!`
  })

greet({
  data: {
    name: 'John',
  },
})

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
}

export const greet = createServerFn({ method: 'GET' })
  .validator((d: Person) => d)
  .handler(async (ctx) => {
    return `Hello, ${ctx.data.name}!`
  })

greet({
  data: {
    name: 'John',
  },
})

JSON Parameters

Server functions can accept JSON-serializable objects as parameters. This is useful for passing complex data structures to the server:

tsx

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
  age: number
}

export const greet = createServerFn({ method: 'GET' })
  .validator((data) => data)
  .handler(async ({ data }) => {
    return `Hello, ${data.name}! You are ${data.age} years old.`
  })

greet({
  data: {
    name: 'John',
    age: 34,
  },
})

import { createServerFn } from '@tanstack/start'

type Person = {
  name: string
  age: number
}

export const greet = createServerFn({ method: 'GET' })
  .validator((data) => data)
  .handler(async ({ data }) => {
    return `Hello, ${data.name}! You are ${data.age} years old.`
  })

greet({
  data: {
    name: 'John',
    age: 34,
  },
})

FormData Parameters

Server functions can accept FormData objects as parameters

tsx

import { createServerFn } from '@tanstack/start'

export const greetUser = createServerFn()
  .validator((data) => {
    if (!(data instanceof FormData)) {
      throw new Error('Invalid form data')
    }
    const name = data.get('name')
    const age = data.get('age')

    if (!name || !age) {
      throw new Error('Name and age are required')
    }

    return {
      name: name.toString(),
      age: parseInt(age.toString(), 10),
    }
  })
  .handler(async ({ data: { name, age } }) => {
    return `Hello, ${name}! You are ${age} years old.`
  })

// Usage
function Test() {
  return (
    <form
      onSubmit={async (event) => {
        event.preventDefault()
        const formData = new FormData(event.target)
        const response = await greetUser({ data: formData })
        console.log(response)
      }}
    >
      <input name="name" />
      <input name="age" />
      <button type="submit">Submit</button>
    </form>
  )
}

import { createServerFn } from '@tanstack/start'

export const greetUser = createServerFn()
  .validator((data) => {
    if (!(data instanceof FormData)) {
      throw new Error('Invalid form data')
    }
    const name = data.get('name')
    const age = data.get('age')

    if (!name || !age) {
      throw new Error('Name and age are required')
    }

    return {
      name: name.toString(),
      age: parseInt(age.toString(), 10),
    }
  })
  .handler(async ({ data: { name, age } }) => {
    return `Hello, ${name}! You are ${age} years old.`
  })

// Usage
function Test() {
  return (
    <form
      onSubmit={async (event) => {
        event.preventDefault()
        const formData = new FormData(event.target)
        const response = await greetUser({ data: formData })
        console.log(response)
      }}
    >
      <input name="name" />
      <input name="age" />
      <button type="submit">Submit</button>
    </form>
  )
}

Server Function Context

In addition to the single parameter that server functions accept, you can also access server request context from within any server function using many utilities from vinxi/http. Under the hood, Vinxi uses unjs's h3 package to perform cross-platform HTTP requests.

There are many context functions available to you for things like:

Accessing the request context
Accessing/setting headers
Accessing/setting sessions/cookies
Setting response status codes and status messages
Dealing with multi-part form data
Reading/Setting custom server context properties

For a full list of available context functions, see all of the available h3 Methods or inspect the Vinxi Exports Source Code.

For starters, here are a few examples:

Accessing the Request Context

Let's use Vinxi's getWebRequest function to access the request itself from within a server function:

tsx

import { createServerFn } from '@tanstack/start'
import { getWebRequest } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    const request = getWebRequest()

    console.log(request.method) // GET

    console.log(request.headers.get('User-Agent')) // Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3
  },
)

import { createServerFn } from '@tanstack/start'
import { getWebRequest } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    const request = getWebRequest()

    console.log(request.method) // GET

    console.log(request.headers.get('User-Agent')) // Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3
  },
)

Accessing Headers

Use Vinxi's getHeaders function to access all headers from within a server function:

tsx

import { createServerFn } from '@tanstack/start'
import { getHeaders } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    console.log(getHeaders())
    // {
    //   "accept": "*/*",
    //   "accept-encoding": "gzip, deflate, br",
    //   "accept-language": "en-US,en;q=0.9",
    //   "connection": "keep-alive",
    //   "host": "localhost:3000",
    //   ...
    // }
  },
)

import { createServerFn } from '@tanstack/start'
import { getHeaders } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    console.log(getHeaders())
    // {
    //   "accept": "*/*",
    //   "accept-encoding": "gzip, deflate, br",
    //   "accept-language": "en-US,en;q=0.9",
    //   "connection": "keep-alive",
    //   "host": "localhost:3000",
    //   ...
    // }
  },
)

You can also access individual headers using the getHeader function:

tsx

import { createServerFn } from '@tanstack/start'
import { getHeader } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    console.log(getHeader('User-Agent')) // Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3
  },
)

import { createServerFn } from '@tanstack/start'
import { getHeader } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    console.log(getHeader('User-Agent')) // Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3
  },
)

Returning Values

Server functions can return a few different types of values:

Primitives
JSON-serializable objects
redirect errors (can also be thrown)
notFound errors (can also be thrown)
Raw Response objects

Returning Primitives and JSON

To return any primitive or JSON-serializable object, simply return the value from the server function:

tsx

import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    return new Date().toISOString()
  },
)

export const getServerData = createServerFn({ method: 'GET' }).handler(
  async () => {
    return {
      message: 'Hello, World!',
    }
  },
)

import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    return new Date().toISOString()
  },
)

export const getServerData = createServerFn({ method: 'GET' }).handler(
  async () => {
    return {
      message: 'Hello, World!',
    }
  },
)

By default, server functions assume that any non-Response object returned is either a primitive or JSON-serializable object.

Responding with Custom Headers

To respond with custom headers, you can use Vinxi's setHeader function:

tsx

import { createServerFn } from '@tanstack/start'
import { setHeader } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    setHeader('X-Custom-Header', 'value')
    return new Date().toISOString()
  },
)

import { createServerFn } from '@tanstack/start'
import { setHeader } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    setHeader('X-Custom-Header', 'value')
    return new Date().toISOString()
  },
)

Responding with Custom Status Codes

To respond with a custom status code, you can use Vinxi's setResponseStatus function:

tsx

import { createServerFn } from '@tanstack/start'
import { setResponseStatus } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    setResponseStatus(201)
    return new Date().toISOString()
  },
)

import { createServerFn } from '@tanstack/start'
import { setResponseStatus } from 'vinxi/http'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    setResponseStatus(201)
    return new Date().toISOString()
  },
)

Returning Raw Response objects

To return a raw Response object, simply return a Response object from the server function:

tsx

import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    // Read a file from s3
    return fetch('https://example.com/time.txt')
  },
)

import { createServerFn } from '@tanstack/start'

export const getServerTime = createServerFn({ method: 'GET' }).handler(
  async () => {
    // Read a file from s3
    return fetch('https://example.com/time.txt')
  },
)

Throwing Errors

Aside from special redirect and notFound errors, server functions can throw any custom error. These errors will be serialized and sent to the client as a JSON response along with a 500 status code.

tsx

import { createServerFn } from '@tanstack/start'

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  throw new Error('Something went wrong!')
})

// Usage
function Test() {
  try {
    await doStuff()
  } catch (error) {
    console.error(error)
    // {
    //   message: "Something went wrong!",
    //   stack: "Error: Something went wrong!\n    at doStuff (file:///path/to/file.ts:3:3)"
    // }
  }
}

import { createServerFn } from '@tanstack/start'

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  throw new Error('Something went wrong!')
})

// Usage
function Test() {
  try {
    await doStuff()
  } catch (error) {
    console.error(error)
    // {
    //   message: "Something went wrong!",
    //   stack: "Error: Something went wrong!\n    at doStuff (file:///path/to/file.ts:3:3)"
    // }
  }
}

Calling server functions from within route lifecycles

Server functions can be called normally from route loaders, beforeLoads, or any other router-controlled APIs. These APIs are equipped to handle errors, redirects, and notFounds thrown by server functions automatically.

tsx

import { getServerTime } from './getServerTime'

export const Route = createFileRoute('/time')({
  loader: async () => {
    const time = await getServerTime()

    return {
      time,
    }
  },
})

import { getServerTime } from './getServerTime'

export const Route = createFileRoute('/time')({
  loader: async () => {
    const time = await getServerTime()

    return {
      time,
    }
  },
})

Calling server functions from hooks and components

Server functions can throw redirects or notFounds and while not required, it is recommended to catch these errors and handle them appropriately. To make this easier, the @tanstack/start package exports a useServerFn hook that can be used to bind server functions to components and hooks:

tsx

import { useServerFn } from '@tanstack/start'
import { useQuery } from '@tanstack/react-query'
import { getServerTime } from './getServerTime'

export function Time() {
  const getTime = useServerFn(getServerTime)

  const timeQuery = useQuery({
    queryKey: 'time',
    queryFn: () => getTime(),
  })
}

import { useServerFn } from '@tanstack/start'
import { useQuery } from '@tanstack/react-query'
import { getServerTime } from './getServerTime'

export function Time() {
  const getTime = useServerFn(getServerTime)

  const timeQuery = useQuery({
    queryKey: 'time',
    queryFn: () => getTime(),
  })
}

Calling server functions anywhere else

Server functions are just async functions, so they can ultimately be called from anywhere in your application. However, be aware that any redirects or notFounds thrown by server functions will not be handled automatically unless called from a route lifecycle or a component that uses the useServerFn hook.

Redirects

Server functions can throw a redirect error to redirect the user to a different URL. This is useful for handling authentication, authorization, or other scenarios where you need to redirect the user to a different page.

During SSR, redirects are handled by sending a 302 response to the client with the new location
On the client, redirects are handled by the router automatically from within a route lifecycle or a component that uses the useServerFn hook. If you call a server function from anywhere else, redirects will not be handled automatically.

To throw a redirect, you can use the redirect function exported from the @tanstack/react-router package:

tsx

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page
  throw redirect({
    to: '/',
  })
})

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page
  throw redirect({
    to: '/',
  })
})

Redirects can utilize all of the same options as router.navigate, useNavigate() and <Link> components. So feel free to also pass:

Path Params
Search Params
Hash

Redirects can also set the status code of the response by passing a status option:

tsx

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page with a 301 status code
  throw redirect({
    to: '/',
    status: 301,
  })
})

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page with a 301 status code
  throw redirect({
    to: '/',
    status: 301,
  })
})

⚠️ Do not use Vinxi's sendRedirect function to send soft redirects from within server functions. This will send the redirect using the Location header and will force a full page hard navigation on the client.

Redirect Headers

You can also set custom headers on a redirect by passing a headers option:

tsx

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page with a custom header
  throw redirect({
    to: '/',
    headers: {
      'X-Custom-Header': 'value',
    },
  })
})

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

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Redirect the user to the home page with a custom header
  throw redirect({
    to: '/',
    headers: {
      'X-Custom-Header': 'value',
    },
  })
})

Not Found

While calling a server function from a loader or beforeLoad route lifecycle, a special notFound error can be thrown to indicate to the router that the requested resource was not found. This is more useful than a simple 404 status code, as it allows you to render a custom 404 page, or handle the error in a custom way. If notFound is thrown from a server function used outside of a route lifecycle, it will not be handled automatically.

To throw a notFound, you can use the notFound function exported from the @tanstack/start package:

tsx

import { createServerFn, notFound } from '@tanstack/start'

const getStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Randomly return a not found error
  if (Math.random() < 0.5) {
    throw notFound()
  }

  // Or return some stuff
  return {
    stuff: 'stuff',
  }
})

export const Route = createFileRoute('/stuff')({
  loader: async () => {
    const stuff = await getStuff()

    return {
      stuff,
    }
  },
})

import { createServerFn, notFound } from '@tanstack/start'

const getStuff = createServerFn({ method: 'GET' }).handler(async () => {
  // Randomly return a not found error
  if (Math.random() < 0.5) {
    throw notFound()
  }

  // Or return some stuff
  return {
    stuff: 'stuff',
  }
})

export const Route = createFileRoute('/stuff')({
  loader: async () => {
    const stuff = await getStuff()

    return {
      stuff,
    }
  },
})

Not found errors are a core feature of TanStack Router,

Handling Errors

If a server function throws a (non-redirect/non-notFound) error, it will be serialized and sent to the client as a JSON response along with a 500 status code. This is useful for debugging, but you may want to handle these errors in a more user-friendly way. You can do this by catching the error and handling it in your route lifecycle, component, or hook as you normally would.

tsx

import { createServerFn } from '@tanstack/start'

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  undefined.foo()
})

export const Route = createFileRoute('/stuff')({
  loader: async () => {
    try {
      await doStuff()
    } catch (error) {
      // Handle the error:
      // error === {
      //   message: "Cannot read property 'foo' of undefined",
      //   stack: "TypeError: Cannot read property 'foo' of undefined\n    at doStuff (file:///path/to/file.ts:3:3)"
    }
  },
})

import { createServerFn } from '@tanstack/start'

export const doStuff = createServerFn({ method: 'GET' }).handler(async () => {
  undefined.foo()
})

export const Route = createFileRoute('/stuff')({
  loader: async () => {
    try {
      await doStuff()
    } catch (error) {
      // Handle the error:
      // error === {
      //   message: "Cannot read property 'foo' of undefined",
      //   stack: "TypeError: Cannot read property 'foo' of undefined\n    at doStuff (file:///path/to/file.ts:3:3)"
    }
  },
})

No-JS Server Functions

Without JavaScript enabled, there's only one way to execute server functions: by submitting a form.

This is done by adding a form element to the page with the HTML attribute action.

Notice that we mentioned the HTML attribute action. This attribute only accepts a string in HTML, just like all other attributes.

While React 19 added support for passing a function to action, it's a React-specific feature and not part of the HTML standard.

The action attribute tells the browser where to send the form data when the form is submitted. In this case, we want to send the form data to the server function.

To do this, we can utilize the url property of the server function:

const yourFn = createServerFn()
  .validator((formData) => {
    const name = formData.get('name')

    if (!name) {
      throw new Error('Name is required')
    }

    return name
  })
  .handler(async ({ data: name }) => {
    console.log(name) // 'John'
  })

console.info(yourFn.url)

const yourFn = createServerFn()
  .validator((formData) => {
    const name = formData.get('name')

    if (!name) {
      throw new Error('Name is required')
    }

    return name
  })
  .handler(async ({ data: name }) => {
    console.log(name) // 'John'
  })

console.info(yourFn.url)

And pass this to the action attribute of the form:

tsx

function Component() {
  return (
    <form action={yourFn.url} method="POST">
      <input name="name" defaultValue="John" />
      <button type="submit">Click me!</button>
    </form>
  )
}

function Component() {
  return (
    <form action={yourFn.url} method="POST">
      <input name="name" defaultValue="John" />
      <button type="submit">Click me!</button>
    </form>
  )
}

When the form is submitted, the server function will be executed.

No-JS Server Function Arguments

To pass arguments to a server function when submitting a form, you can use the input element with the name attribute to attach the argument to the FormData passed to your server function:

tsx

const yourFn = createServerFn()
  .validator((formData) => {
    if (!(formData instanceof FormData)) {
      throw new Error('Invalid form data')
    }

    const age = formData.get('age')

    if (!age) {
      throw new Error('age is required')
    }

    return age.toString()
  })
  .handler(async ({ data: formData }) => {
    // `age` will be '123'
    const age = formData.get('age')
    // ...
  })

function Component() {
  return (
    //  We need to tell the server that our data type is `multipart/form-data` by setting the `encType` attribute on the form.
    <form action={yourFn.url} method="POST" encType="multipart/form-data">
      <input name="age" defaultValue="34" />
      <button type="submit">Click me!</button>
    </form>
  )
}

const yourFn = createServerFn()
  .validator((formData) => {
    if (!(formData instanceof FormData)) {
      throw new Error('Invalid form data')
    }

    const age = formData.get('age')

    if (!age) {
      throw new Error('age is required')
    }

    return age.toString()
  })
  .handler(async ({ data: formData }) => {
    // `age` will be '123'
    const age = formData.get('age')
    // ...
  })

function Component() {
  return (
    //  We need to tell the server that our data type is `multipart/form-data` by setting the `encType` attribute on the form.
    <form action={yourFn.url} method="POST" encType="multipart/form-data">
      <input name="age" defaultValue="34" />
      <button type="submit">Click me!</button>
    </form>
  )
}

When the form is submitted, the server function will be executed with the form's data as an argument.

No-JS Server Function Return Value

Regardless of whether JavaScript is enabled, the server function will return a response to the HTTP request made from the client.

When JavaScript is enabled, this response can be accessed as the return value of the server function in the client's JavaScript code.

const yourFn = createServerFn().handler(async () => {
  return 'Hello, world!'
})

// `.then` is not available when JavaScript is disabled
yourFn().then(console.log)

const yourFn = createServerFn().handler(async () => {
  return 'Hello, world!'
})

// `.then` is not available when JavaScript is disabled
yourFn().then(console.log)

However, when JavaScript is disabled, there is no way to access the return value of the server function in the client's JavaScript code.

Instead, the server function can provide a response to the client, telling the browser to navigate in a certain way.

When combined with a loader from TanStack Router, we're able to provide an experience similar to a single-page application, even when JavaScript is disabled; all by telling the browser to reload the current page with new data piped through the loader:

tsx

import * as fs from 'fs'
import { createFileRoute } from '@tanstack/react-router'
import { createServerFn } from '@tanstack/start'

const filePath = 'count.txt'

async function readCount() {
  return parseInt(
    await fs.promises.readFile(filePath, 'utf-8').catch(() => '0'),
  )
}

const getCount = createServerFn({
  method: 'GET',
}).handler(() => {
  return readCount()
})

const updateCount = createServerFn()
  .validator((formData) => {
    if (!(formData instanceof FormData)) {
      throw new Error('Invalid form data')
    }

    const addBy = formData.get('addBy')

    if (!addBy) {
      throw new Error('addBy is required')
    }

    return parseInt(addBy.toString())
  })
  .handler(async ({ data: addByAmount }) => {
    const count = await readCount()
    await fs.promises.writeFile(filePath, `${count + addByAmount}`)
    // Reload the page to trigger the loader again
    return new Response('ok', { status: 301, headers: { Location: '/' } })
  })

export const Route = createFileRoute('/')({
  component: Home,
  loader: async () => await getCount(),
})

function Home() {
  const state = Route.useLoaderData()

  return (
    <div>
      <form
        action={updateCount.url}
        method="POST"
        encType="multipart/form-data"
      >
        <input type="number" name="addBy" defaultValue="1" />
        <button type="submit">Add</button>
      </form>
      <pre>{state}</pre>
    </div>
  )
}

import * as fs from 'fs'
import { createFileRoute } from '@tanstack/react-router'
import { createServerFn } from '@tanstack/start'

const filePath = 'count.txt'

async function readCount() {
  return parseInt(
    await fs.promises.readFile(filePath, 'utf-8').catch(() => '0'),
  )
}

const getCount = createServerFn({
  method: 'GET',
}).handler(() => {
  return readCount()
})

const updateCount = createServerFn()
  .validator((formData) => {
    if (!(formData instanceof FormData)) {
      throw new Error('Invalid form data')
    }

    const addBy = formData.get('addBy')

    if (!addBy) {
      throw new Error('addBy is required')
    }

    return parseInt(addBy.toString())
  })
  .handler(async ({ data: addByAmount }) => {
    const count = await readCount()
    await fs.promises.writeFile(filePath, `${count + addByAmount}`)
    // Reload the page to trigger the loader again
    return new Response('ok', { status: 301, headers: { Location: '/' } })
  })

export const Route = createFileRoute('/')({
  component: Home,
  loader: async () => await getCount(),
})

function Home() {
  const state = Route.useLoaderData()

  return (
    <div>
      <form
        action={updateCount.url}
        method="POST"
        encType="multipart/form-data"
      >
        <input type="number" name="addBy" defaultValue="1" />
        <button type="submit">Add</button>
      </form>
      <pre>{state}</pre>
    </div>
  )
}

How are server functions compiled?

Under the hood, server functions are extracted out of the client bundle and into a separate server bundle. On the server, they are executed as-is, and the result is sent back to the client. On the client, server functions proxy the request to the server, which executes the function and sends the result back to the client, all via fetch.

The process looks like this:

When createServerFn is found in a file, the inner function is checked for a use server directive
If the use server directive is missing, it is added to the top of the function
On the client, the inner function is extracted out of the client bundle and into a separate server bundle
The client-side server function is replaced with a proxy function that sends a request to the server to execute the function that was extracted
On the server, the server function is no extracted and is executed as-is
After extraction occurs, each bundle applies a dead-code elimination process to remove any unused code from each bundle.