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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:

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

Handling and Throwing Errors

For React Query to determine a query has errored, the query function must throw. Any error that is thrown in the query function will be persisted on the error state of the query.

tsx
const { error } = useQuery(['todos', todoId], async () => {
if (somethingGoesWrong) {
throw new Error('Oh no!')
}
return data
})

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:

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

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:

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

QueryFunctionContext

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

  • queryKey: QueryKey: Query Keys
  • pageParam: unknown | undefined
  • signal?: AbortSignal
  • meta?: Record<string, unknown>
    • an optional field you can fill with additional information about your query

Using a Query Object instead of parameters

Anywhere the [queryKey, queryFn, config] signature is supported throughout React Query's API, you can also use an object to express the same configuration:

tsx
import { useQuery } from '@tanstack/react-query'
useQuery({
queryKey: ['todo', 7],
queryFn: fetchTodo,
...config,
})