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TypeScript Guide

TanStack Query is written in TypeScript and provides excellent type safety out of the box.

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Type Inference

TanStack Query infers types automatically from your query functions: 
const { data } = useQuery({
  queryKey: ['repo'],
  queryFn: async () => {
    const response = await fetch('https://api.github.com/repos/TanStack/query')
    return response.json() as Repository
  },
})

// data is automatically typed as Repository | undefined
Use type assertions in your query function to inform TypeScript about the returned data shape.

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Explicit Type Parameters

For more control, specify types explicitly: 
interface Repository {
  name: string
  description: string
  stargazers_count: number
  forks_count: number
}

const { data, error } = useQuery<
  Repository,  // TQueryFnData - Type returned by queryFn
  Error,       // TError - Type of errors
  Repository,  // TData - Type of data (after select)
  ['repo']     // TQueryKey - Type of queryKey
>({
  queryKey: ['repo'],
  queryFn: async () => {
    const response = await fetch('https://api.github.com/repos/TanStack/query')
    return response.json()
  },
})

// data is Repository | undefined
// error is Error | null
In most cases, you only need to specify the first type parameter (TQueryFnData). The others can be inferred.

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Typing Query Keys

Strong typing for query keys helps prevent errors: 
type QueryKey = 
  | ['todos']
  | ['todo', number]
  | ['user', string, { includeProjects: boolean }]

const { data } = useQuery<Todo, Error, Todo, ['todo', number]>({
  queryKey: ['todo', todoId],
  queryFn: ({ queryKey }) => {
    const [_key, id] = queryKey // id is typed as number
    return fetchTodo(id)
  },
})

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Using queryOptions Helper

The queryOptions helper provides better type inference: 
import { queryOptions, useQuery } from '@tanstack/react-query'

interface Post {
  id: number
  title: string
  body: string
}

function postOptions(postId: number) {
  return queryOptions({
    queryKey: ['post', postId],
    queryFn: async (): Promise<Post> => {
      const response = await fetch(`/api/posts/${postId}`)
      return response.json()
    },
    staleTime: 1000 * 60 * 5,
  })
}

// Fully typed, reusable query options
function Post({ postId }: { postId: number }) {
  const { data } = useQuery(postOptions(postId))
  // data is typed as Post | undefined
  
  return <div>{data?.title}</div>
}
Use queryOptions to create reusable, well-typed query configurations that can be shared across components.

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Typing Mutations

Mutations support full type safety for variables, data, and errors: 
import { useMutation } from '@tanstack/react-query'

interface Todo {
  id: number
  title: string
}

interface CreateTodoVariables {
  title: string
}

function TodoForm() {
  const mutation = useMutation<
    Todo,                    // TData - Mutation response type
    Error,                   // TError - Error type
    CreateTodoVariables,     // TVariables - Variables type
    { previousTodos?: Todo[] } // TContext - Context type
  >({
    mutationFn: async (variables) => {
      // variables is typed as CreateTodoVariables
      const response = await fetch('/api/todos', {
        method: 'POST',
        body: JSON.stringify(variables),
      })
      return response.json()
    },
    onSuccess: (data, variables, context) => {
      // data is typed as Todo
      // variables is typed as CreateTodoVariables
      // context is typed as { previousTodos?: Todo[] }
      console.log('Created:', data.title)
    },
  })

  return (
    <button
      onClick={() => {
        mutation.mutate({ title: 'New Todo' })
      }}
    >
      Add Todo
    </button>
  )
}

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Typing the QueryClient

The QueryClient is fully typed and provides type-safe methods: 
import { QueryClient, useQueryClient } from '@tanstack/react-query'

const queryClient = new QueryClient()

function Component() {
  const client = useQueryClient()

  // Type-safe query data access
  const todos = client.getQueryData<Todo[]>(['todos'])
  // todos is Todo[] | undefined

  // Type-safe query data updates
  client.setQueryData<Todo[]>(['todos'], (old) => {
    // old is Todo[] | undefined
    return old ? [...old, newTodo] : [newTodo]
  })
}

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Typing Query Results with Initial Data

When providing initial data, the result type changes: 
const { data } = useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
  initialData: [] as Todo[],
})

// data is Todo[] (not Todo[] | undefined)
// Because initial data is always present
With initialData, the data property is never undefined, making it easier to work with.

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Type-Safe Select

Transform query data with full type safety: 
interface Todo {
  id: number
  title: string
  completed: boolean
}

const { data } = useQuery({
  queryKey: ['todos'],
  queryFn: async (): Promise<Todo[]> => {
    const response = await fetch('/api/todos')
    return response.json()
  },
  select: (todos) => {
    // todos is typed as Todo[]
    return todos.filter((todo) => !todo.completed)
  },
})

// data is typed as Todo[] | undefined

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Discriminated Unions

Use TypeScript’s discriminated unions with query status: 
const result = useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
})

if (result.status === 'pending') {
  // TypeScript knows data is undefined here
  return <Spinner />
}

if (result.status === 'error') {
  // TypeScript knows error is defined here
  return <div>Error: {result.error.message}</div>
}

// TypeScript knows data is defined here
return <div>{result.data.length} todos</div>

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Generic Query Hook

Create reusable typed query hooks: 
import { useQuery, UseQueryOptions, UseQueryResult } from '@tanstack/react-query'

function useTypedQuery<TData, TError = Error>(
  key: unknown[],
  fetcher: () => Promise<TData>,
  options?: Omit<UseQueryOptions<TData, TError>, 'queryKey' | 'queryFn'>
): UseQueryResult<TData, TError> {
  return useQuery<TData, TError>({
    queryKey: key,
    queryFn: fetcher,
    ...options,
  })
}

// Usage
interface User {
  id: string
  name: string
}

function useUser(userId: string) {
  return useTypedQuery<User>(
    ['user', userId],
    () => fetchUser(userId),
    { staleTime: 1000 * 60 * 5 }
  )
}

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Infinite Queries

Infinite queries have special type requirements: 
import { useInfiniteQuery } from '@tanstack/react-query'

interface Page {
  data: Todo[]
  nextCursor: number | null
}

const { data, fetchNextPage, hasNextPage } = useInfiniteQuery<
  Page,  // TQueryFnData - Single page type
  Error,
  Page,  // TData - After select
  ['todos'],
  number // TPageParam - Page param type
>({
  queryKey: ['todos'],
  queryFn: async ({ pageParam = 0 }) => {
    // pageParam is typed as number
    const response = await fetch(`/api/todos?cursor=${pageParam}`)
    return response.json()
  },
  getNextPageParam: (lastPage) => lastPage.nextCursor,
  initialPageParam: 0,
})

// data.pages is Page[]
Always provide initialPageParam when using useInfiniteQuery to ensure proper typing.

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Best Practices

1

Define interfaces for your data

Create TypeScript interfaces for all API responses:
interface User {
  id: string
  name: string
  email: string
}
2

Use queryOptions for reusability

Create typed, reusable query configurations:
const userOptions = (id: string) => queryOptions({
  queryKey: ['user', id],
  queryFn: () => fetchUser(id),
})
3

Let TypeScript infer when possible

Avoid over-specifying types. Let TypeScript infer from your query functions.
4

Use discriminated unions

Take advantage of status checks for better type narrowing.

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Common Type Errors

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”Type ‘undefined’ is not assignable to type…”

This happens when you forget that data can be undefined: 
// ❌ Wrong
const { data } = useQuery({ queryKey: ['user'], queryFn: fetchUser })
return <div>{data.name}</div> // Error: data might be undefined

// ✅ Correct
const { data } = useQuery({ queryKey: ['user'], queryFn: fetchUser })
if (!data) return null
return <div>{data.name}</div>

// ✅ Also correct with optional chaining
const { data } = useQuery({ queryKey: ['user'], queryFn: fetchUser })
return <div>{data?.name}</div>
Use the status field or provide initialData to avoid undefined checks.

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Next Steps

DevTools

Set up DevTools to inspect your typed queries

Essential Concepts

Review core concepts with TypeScript examples

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