# React Query

* removes boilerplate of loading state, error state, and setting state for async data requests
* Auto retries up to default of 3 times with backoff

```bash
npm install @tanstack/react-query
```

## Basic

```tsx
import { QueryClient, QueryClientProvider, useQuery } from '@tanstack/react-query'

const queryClient = new QueryClient()

export default function App() {
  return (
    <QueryClientProvider client={queryClient}>
      <Example />
    </QueryClientProvider>
  )
}

function Example() {
  const { isLoading, error, data } = useQuery(['repoData', page], () => fetchProjects(page)
    )
  )

  if (isLoading) return 'Loading...'
  
  
  if (error) return 'An error has occurred: ' + error.message

  return (
    <div>
      <h1>{data.name}</h1>
      <p>{data.description}</p>
      <strong>👀 {data.subscribers_count}</strong>{' '}
      <strong>✨ {data.stargazers_count}</strong>{' '}
      <strong>🍴 {data.forks_count}</strong>
    </div>
  )
}
```

* Loading States
  * Could check if data exists instead of `isLoading`, error especially if you are setting enabled
  * `isLoading` true on initial fetch but `isFetching` is true on fetch or refetch, so even if data is cached

#### UseQuery

* unique key is used for caching and deduping, can be string or array of serializable data
  * `['todo', {type: 'done', userId}])` will change when userId changes
* Query Fun

#### Mutation

```tsx
function App() {
  const mutation = useMutation(newTodo => {
    return axios.post('/todos', newTodo)
  })

  return (
    <div>
      {mutation.isLoading ? (
        'Adding todo...'
      ) : (
        <>
          {mutation.isError ? (
            <div>An error occurred: {mutation.error.message}</div>
          ) : null}

          {mutation.isSuccess ? <div>Todo added!</div> : null}

          <button
            onClick={() => {
              mutation.mutate({ id: new Date(), title: 'Do Laundry' })
            }}
          >
            Create Todo
          </button>
        </>
      )}
    </div>
  )
}
```

## Advanced

* Can also use query object

  ```ts
  useQuery({
    queryKey: ['todo', 7],
    queryFn: fetchTodo,
    config: {
      // The query will not execute until the userId exists
      enabled: !!userId,
      retryDelay: 1000,
      retry: 6,
      //initialData is appended to/kept in cache
      initalData: {},
      placeholderData: {}
    },
  })
  ```
* `useQueries` for changing number of queries per render

#### Infinite Scroll Queries

```tsx
import { useInfiniteQuery } from 'react-query'

function Projects() {
  const fetchProjects = ({ pageParam = 0 }) =>
    fetch('/api/projects?cursor=' + pageParam)

  const {
    data,
    error,
    fetchNextPage,
    hasNextPage,
    isFetching,
    isFetchingNextPage,
    status,
  } = useInfiniteQuery('projects', fetchProjects, {
    getNextPageParam: (lastPage, pages) => lastPage.nextCursor,
  })

  return status === 'loading' ? (
    <p>Loading...</p>
  ) : status === 'error' ? (
    <p>Error: {error.message}</p>
  ) : (
    <>
      {data.pages.map((group, i) => (
        <React.Fragment key={i}>
          {group.projects.map(project => (
            <p key={project.id}>{project.name}</p>
          ))}
        </React.Fragment>
      ))}
      <div>
        <button
          onClick={() => fetchNextPage()}
          disabled={!hasNextPage || isFetchingNextPage}
        >
          {isFetchingNextPage
            ? 'Loading more...'
            : hasNextPage
            ? 'Load More'
            : 'Nothing more to load'}
        </button>
      </div>
      <div>{isFetching && !isFetchingNextPage ? 'Fetching...' : null}</div>
    </>
  )
}
```

#### Paginated Queries

```tsx
function Todos() {
  const [page, setPage] = React.useState(0)

  const fetchProjects = (page = 0) => fetch('/api/projects?page=' + page).then((res) => res.json())

  const {
    isLoading,
    isError,
    error,
    data,
    isFetching,
    isPreviousData,
  } = useQuery(['projects', page], () => fetchProjects(page), { keepPreviousData : true })

  return (
    <div>
      {isLoading ? (
        <div>Loading...</div>
      ) : isError ? (
        <div>Error: {error.message}</div>
      ) : (
        <div>
          {data.projects.map(project => (
            <p key={project.id}>{project.name}</p>
          ))}
        </div>
      )}
      <span>Current Page: {page + 1}</span>
      <button
        onClick={() => setPage(old => Math.max(old - 1, 0))}
        disabled={page === 0}
      >
        Previous Page
      </button>{' '}
      <button
        onClick={() => {
          if (!isPreviousData && data.hasMore) {
            setPage(old => old + 1)
          }
        }}
        // Disable the Next Page button until we know a next page is available
        disabled={isPreviousData || !data?.hasMore}
      >
        Next Page
      </button>
      {isFetching ? <span> Loading...</span> : null}{' '}
    </div>
  )
}
```

#### Prefetch

```tsx
const prefetchTodos = async () => {
  // The results of this query will be cached like a normal query
  await queryClient.prefetchQuery('todos', fetchTodos)
}
```

#### More

```tsx
// Invalidate every query in the cache
queryClient.invalidateQueries()
// Invalidate every query with a key that starts with `todos`
queryClient.invalidateQueries({ queryKey: ['todos'] })


// When this mutation succeeds, invalidate any queries with the `todos` or `reminders` query key
const mutation = useMutation(addTodo, {
  onSuccess: () => {
    queryClient.invalidateQueries('todos')
    queryClient.invalidateQueries('reminders')
  },
})
```

* Suspense
* Caching
* FIlters


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://openai.gitbook.io/code-cheatsheets/react/data_loading/react-query.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.