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:

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]),
})

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.

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
  },
})

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:

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()
  },
})

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:

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()
}

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

• queryKey: QueryKey: Query Keys
• pageParam?: unknown
  • only for Infinite Queries
  • the page parameter used to fetch the current page
• signal?: AbortSignal
  • AbortSignal instance provided by TanStack Query
  • Can be used for Query Cancellation
• meta: Record<string, unknown> | undefined
  • an optional field you can fill with additional information about your query