tanstack-query-intent-core-selectors-and-derived-state

Core Patterns

Use select to subscribe a component to the data shape it actually needs while keeping the full response in the cache.

Select a stable slice

import { queryOptions, useSuspenseQuery } from '@tanstack/react-query'

const productOptions = (id: string) =>
  queryOptions({
    queryKey: ['product', id],
    queryFn: () => fetchProduct(id),
  })

function ProductTitle({ id }: { id: string }) { const title = useSuspenseQuery({ ...productOptions(id), select: (product) => product.title, })

return {title.data} }

### Stabilize expensive selectors

```tsx
const selectAverageRating = (products: Array<Product>) =>
  expensiveAverage(products)

function ProductSummary({ filters }: { filters: ProductFilters }) {
  return useSuspenseQuery({
    ...productsOptions(filters),
    select: selectAverageRating,
  })
}

Use React.useCallback when the selector closes over component props. Move it outside the component when it has no dependencies.

Derive client state instead of syncing it

const selectedUser = usersQuery.data?.find((user) => user.id === selectedUserId)
const visibleSelectedUserId = selectedUser ? selectedUserId : null

When server data changes, derived values update naturally during render.

Common Mistakes

HIGH Syncing derived state through an effect

Wrong:

React.useEffect(() => {
  if (!users?.some((user) => user.id === selectedUserId)) {
    setSelectedUserId(null)
  }
}, [users, selectedUserId])

Correct:

const selectedUser = users?.find((user) => user.id === selectedUserId)
const visibleSelectedUserId = selectedUser ? selectedUserId : null

Prefer deriving client state from server state during render when no side effect is required.

Source: https://tkdodo.eu/blog/deriving-client-state-from-server-state

MEDIUM Inline expensive select reruns on unrelated renders

Wrong:

useSuspenseQuery({
  ...productsOptions(filters),
  select: (data) => expensiveSuperTransformation(data),
})

Correct:

const selectProducts = (data: Array<Product>) =>
  expensiveSuperTransformation(data)

useSuspenseQuery({
  ...productsOptions(filters),
  select: selectProducts,
})

TanStack Query reruns select when data changes or the selector function identity changes.

Source: https://tkdodo.eu/blog/react-query-selectors-supercharged

MEDIUM Using select to throw domain errors

Wrong:

useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
  select: (todos) => {
    if (!todos.length) throw new Error('No todos')
    return todos
  },
})

Correct:

useQuery({
  queryKey: ['todos'],
  queryFn: async () => {
    const todos = await fetchTodos()
    if (!todos.length) throw new Error('No todos')
    return todos
  },
})

select transforms successful data. Query errors belong in the query function.

Source: TanStack/query:docs/framework/react/guides/render-optimizations.md