---
id: query-functions
title: Query Functions
---

A query function can be literally any function that **returns a promise**. The promise that is returned should either **resolve the data** or **throw an error**.

All of the following are valid query function configurations:

[//]: # 'Example'

```tsx
useQuery({ queryKey: ['todos'], queryFn: fetchAllTodos })
useQuery({ queryKey: ['todos', todoId], queryFn: () => fetchTodoById(todoId) })
useQuery({
  queryKey: ['todos', todoId],
  queryFn: async () => {
    const data = await fetchTodoById(todoId)
    return data
  },
})
useQuery({
  queryKey: ['todos', todoId],
  queryFn: ({ queryKey }) => fetchTodoById(queryKey[1]),
})
```

[//]: # 'Example'

## Handling and Throwing Errors

For TanStack Query to determine a query has errored, the query function **must throw** or return a **rejected Promise**. Any error that is thrown in the query function will be persisted on the `error` state of the query.

[//]: # 'Example2'

```tsx
const { error } = useQuery({
  queryKey: ['todos', todoId],
  queryFn: async () => {
    if (somethingGoesWrong) {
      throw new Error('Oh no!')
    }
    if (somethingElseGoesWrong) {
      return Promise.reject(new Error('Oh no!'))
    }

    return data
  },
})
```

[//]: # 'Example2'

## Usage with `fetch` and other clients that do not throw by default

While most utilities like `axios` or `graphql-request` automatically throw errors for unsuccessful HTTP calls, some utilities like `fetch` do not throw errors by default. If that's the case, you'll need to throw them on your own. Here is a simple way to do that with the popular `fetch` API:

[//]: # 'Example3'

```tsx
useQuery({
  queryKey: ['todos', todoId],
  queryFn: async () => {
    const response = await fetch('/todos/' + todoId)
    if (!response.ok) {
      throw new Error('Network response was not ok')
    }
    return response.json()
  },
})
```

[//]: # 'Example3'

## Query Function Variables

Query keys are not just for uniquely identifying the data you are fetching, but are also conveniently passed into your query function as part of the QueryFunctionContext. While not always necessary, this makes it possible to extract your query functions if needed:

[//]: # 'Example4'

```tsx
function Todos({ status, page }) {
  const result = useQuery({
    queryKey: ['todos', { status, page }],
    queryFn: fetchTodoList,
  })
}

// Access the key, status and page variables in your query function!
function fetchTodoList({ queryKey }) {
  const [_key, { status, page }] = queryKey
  return new Promise()
}
```

[//]: # 'Example4'

### QueryFunctionContext

The `QueryFunctionContext` is the object passed to each query function. It consists of:

- `queryKey: QueryKey`: [Query Keys](../query-keys.md)
- `client: QueryClient`: [QueryClient](../../../../reference/QueryClient.md)
- `signal?: AbortSignal`
  - [AbortSignal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) instance provided by TanStack Query
  - Can be used for [Query Cancellation](../query-cancellation.md)
- `meta: Record<string, unknown> | undefined`
  - an optional field you can fill with additional information about your query

Additionally, [Infinite Queries](../infinite-queries.md) get the following options passed:

- `pageParam: TPageParam`
  - the page parameter used to fetch the current page
- `direction: 'forward' | 'backward'`
  - **deprecated**
  - the direction of the current page fetch
  - To get access to the direction of the current page fetch, please add a direction to `pageParam` from `getNextPageParam` and `getPreviousPageParam`.