# QueryObserver

Defined in: [packages/query-core/src/queryObserver.ts:56](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L56)

A `QueryObserver` watches a single query in the `QueryCache` and computes a
`QueryObserverResult` from its state, recomputing and notifying subscribers
whenever the underlying query (or the observer's options) changes. It is
the primitive that framework adapters (e.g. `useQuery`) build their hooks
on top of, but it can also be used directly to observe and switch between
queries outside of any framework.

## Example

```ts
const observer = new QueryObserver(queryClient, {
  queryKey: ['posts'],
  queryFn: fetchPosts,
})

const unsubscribe = observer.subscribe((result) => {
  console.log(result.data)
})
```

## Extends

- `Subscribable`\<`QueryObserverListener`\<`TData`, `TError`\>\>

## Extended by

- [`InfiniteQueryObserver`](InfiniteQueryObserver.md)

## Type Parameters

### TQueryFnData

`TQueryFnData` = `unknown`

### TError

`TError` = [`DefaultError`](../type-aliases/DefaultError.md)

### TData

`TData` = `TQueryFnData`

### TQueryData

`TQueryData` = `TQueryFnData`

### TQueryKey

`TQueryKey` *extends* [`QueryKey`](../type-aliases/QueryKey.md) = [`QueryKey`](../type-aliases/QueryKey.md)

## Constructors

### Constructor

```ts
new QueryObserver<TQueryFnData, TError, TData, TQueryData, TQueryKey>(client: QueryClient, options: QueryObserverOptions<TQueryFnData, TError, TData, TQueryData, TQueryKey>): QueryObserver<TQueryFnData, TError, TData, TQueryData, TQueryKey>;
```

Defined in: [packages/query-core/src/queryObserver.ts:86](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L86)

#### Parameters

##### client

[`QueryClient`](QueryClient.md)

##### options

[`QueryObserverOptions`](../interfaces/QueryObserverOptions.md)\<`TQueryFnData`, `TError`, `TData`, `TQueryData`, `TQueryKey`\>

#### Returns

`QueryObserver`\<`TQueryFnData`, `TError`, `TData`, `TQueryData`, `TQueryKey`\>

#### Overrides

```ts
Subscribable<QueryObserverListener<TData, TError>>.constructor
```

## Properties

### options

```ts
options: QueryObserverOptions<TQueryFnData, TError, TData, TQueryData, TQueryKey>;
```

Defined in: [packages/query-core/src/queryObserver.ts:88](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L88)

## Methods

### destroy()

```ts
destroy(): void;
```

Defined in: [packages/query-core/src/queryObserver.ts:162](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L162)

Stops observing the current query: clears all listeners, cancels the
stale and refetch-interval timers, and removes this observer from the
query it was observing.

#### Returns

`void`

***

### fetchOptimistic()

```ts
fetchOptimistic(options: QueryObserverOptions<TQueryFnData, TError, TData, TQueryData, TQueryKey>): Promise<QueryObserverResult<TData, TError>>;
```

Defined in: [packages/query-core/src/queryObserver.ts:408](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L408)

Fetches a query defined by the given options without affecting this
observer's own tracked query or result, and returns a promise that
resolves with the `QueryObserverResult` for that fetch. This is useful
for prefetching data that another observer (e.g. a query about to be
navigated to) will need, ahead of time.

#### Parameters

##### options

[`QueryObserverOptions`](../interfaces/QueryObserverOptions.md)\<`TQueryFnData`, `TError`, `TData`, `TQueryData`, `TQueryKey`\>

The observer options of the query to fetch.

#### Returns

`Promise`\<[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>\>

A promise that resolves with the result for the fetched query.

#### Example

```ts
const result = await observer.fetchOptimistic({
  queryKey: ['posts', 2],
  queryFn: () => fetchPost(2),
})
console.log(result.data)
```

***

### getCurrentQuery()

```ts
getCurrentQuery(): Query<TQueryFnData, TError, TQueryData, TQueryKey>;
```

Defined in: [packages/query-core/src/queryObserver.ts:367](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L367)

Returns the `Query` instance this observer is currently observing.

#### Returns

[`Query`](Query.md)\<`TQueryFnData`, `TError`, `TQueryData`, `TQueryKey`\>

The observed query.

***

### getCurrentResult()

```ts
getCurrentResult(): QueryObserverResult<TData, TError>;
```

Defined in: [packages/query-core/src/queryObserver.ts:326](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L326)

Returns the most recently computed `QueryObserverResult` for the
observed query. This is a point-in-time read; to be notified of updates
as they happen, subscribe to the observer instead (its inherited
`subscribe` method).

#### Returns

[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>

The observer's latest result.

#### Example

```ts
const result = observer.getCurrentResult()
console.log(result.status, result.data)
```

***

### getOptimisticResult()

```ts
getOptimisticResult(options: DefaultedQueryObserverOptions<TQueryFnData, TError, TData, TQueryData, TQueryKey>): QueryObserverResult<TData, TError>;
```

Defined in: [packages/query-core/src/queryObserver.ts:277](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L277)

Computes the result the observer would produce for the given (already-defaulted) options
right now, building the underlying `Query` if it doesn't exist yet, without waiting for a
subscription callback. Called by framework adapters on every render (e.g. `useQuery`) so the
returned value is available synchronously, ahead of `setOptions` triggering an actual fetch.

#### Parameters

##### options

[`DefaultedQueryObserverOptions`](../type-aliases/DefaultedQueryObserverOptions.md)\<`TQueryFnData`, `TError`, `TData`, `TQueryData`, `TQueryKey`\>

The defaulted observer options to compute the result for.

#### Returns

[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>

The result for the given options.

***

### hasListeners()

```ts
hasListeners(): boolean;
```

Defined in: [packages/query-core/src/subscribable.ts:43](https://github.com/TanStack/query/blob/main/packages/query-core/src/subscribable.ts#L43)

Returns `true` while at least one listener is registered, `false` once they have all unsubscribed.

#### Returns

`boolean`

`true` if at least one listener is registered.

#### Inherited from

```ts
Subscribable.hasListeners
```

***

### refetch()

```ts
refetch(options: RefetchOptions): Promise<QueryObserverResult<TData, TError>>;
```

Defined in: [packages/query-core/src/queryObserver.ts:383](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L383)

Refetches the observed query and returns a promise that resolves with
the resulting `QueryObserverResult`.

#### Parameters

##### options

[`RefetchOptions`](../interfaces/RefetchOptions.md) = `{}`

Set `cancelRefetch` to `false` to keep a running fetch instead of cancelling
it, and `throwOnError` to `true` to reject when the refetch fails.

#### Returns

`Promise`\<[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>\>

A promise that resolves with the result after the refetch.

#### Example

```ts
const result = await observer.refetch({ cancelRefetch: false })
console.log(result.data)
```

***

### setOptions()

```ts
setOptions(options: QueryObserverOptions<TQueryFnData, TError, TData, TQueryData, TQueryKey>): void;
```

Defined in: [packages/query-core/src/queryObserver.ts:185](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L185)

Updates the observer's options. This will re-resolve the query being
observed (switching to a different query if the `queryKey` changed),
trigger a fetch if the new options require one and the observer has
subscribers, recompute the current result, and reschedule the stale and
refetch-interval timers as needed.

#### Parameters

##### options

[`QueryObserverOptions`](../interfaces/QueryObserverOptions.md)\<`TQueryFnData`, `TError`, `TData`, `TQueryData`, `TQueryKey`\>

The new observer options. They are defaulted with [QueryClient#defaultQueryOptions](QueryClient.md#defaultqueryoptions) before being applied.

#### Returns

`void`

#### Throws

If `enabled` is neither a boolean nor a function.

#### Example

```ts
observer.setOptions({ queryKey: ['posts', 1], queryFn: () => fetchPost(1) })
// later: switch to a different query, reusing the same observer
observer.setOptions({ queryKey: ['posts', 2], queryFn: () => fetchPost(2) })
```

***

### shouldFetchOnReconnect()

```ts
shouldFetchOnReconnect(): boolean;
```

Defined in: [packages/query-core/src/queryObserver.ts:135](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L135)

Returns whether the observed query is currently stale and configured
(via the `refetchOnReconnect` option) to refetch when the network
reconnects.

#### Returns

`boolean`

`true` if the observer should refetch the query on reconnect.

***

### shouldFetchOnWindowFocus()

```ts
shouldFetchOnWindowFocus(): boolean;
```

Defined in: [packages/query-core/src/queryObserver.ts:149](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L149)

Returns whether the observed query is currently stale and configured
(via the `refetchOnWindowFocus` option) to refetch when the window
regains focus.

#### Returns

`boolean`

`true` if the observer should refetch the query on window focus.

***

### subscribe()

```ts
subscribe(listener: QueryObserverListener): () => void;
```

Defined in: [packages/query-core/src/subscribable.ts:28](https://github.com/TanStack/query/blob/main/packages/query-core/src/subscribable.ts#L28)

Registers a listener to be called on every update this object notifies about. Returns a function
that removes the listener again — call it to stop listening. The base class never drops a listener
on its own, though some subclasses clear all of theirs in `destroy()`.

#### Parameters

##### listener

`QueryObserverListener`

Called on each update, with whatever the subclass passes to its subscribers.

#### Returns

A function that removes the listener.

```ts
(): void;
```

##### Returns

`void`

#### Example

```ts
const unsubscribe = subscribable.subscribe(() => {
  // react to the update
})

unsubscribe()
```

#### Inherited from

```ts
Subscribable.subscribe
```

***

### trackProp()

```ts
trackProp(key: 
  | "error"
  | "data"
  | "isError"
  | "isPending"
  | "isLoading"
  | "isLoadingError"
  | "isRefetchError"
  | "isSuccess"
  | "isPlaceholderData"
  | "status"
  | "dataUpdatedAt"
  | "errorUpdatedAt"
  | "failureCount"
  | "failureReason"
  | "errorUpdateCount"
  | "isFetched"
  | "isFetchedAfterMount"
  | "isFetching"
  | "isInitialLoading"
  | "isPaused"
  | "isRefetching"
  | "isStale"
  | "isEnabled"
  | "refetch"
  | "fetchStatus"): void;
```

Defined in: [packages/query-core/src/queryObserver.ts:359](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L359)

Records that the given `QueryObserverResult` property was read, so a subsequent update only
notifies this observer if a tracked property actually changed. Normally called indirectly via
[QueryObserver#trackResult](#trackresult)'s proxy; exposed directly for adapters that track property
access themselves (e.g. through their own reactivity system) instead of via the proxy.

#### Parameters

##### key

The name of the property that was read.

`"error"` | `"data"` | `"isError"` | `"isPending"` | `"isLoading"` | `"isLoadingError"` | `"isRefetchError"` | `"isSuccess"` | `"isPlaceholderData"` | `"status"` | `"dataUpdatedAt"` | `"errorUpdatedAt"` | `"failureCount"` | `"failureReason"` | `"errorUpdateCount"` | `"isFetched"` | `"isFetchedAfterMount"` | `"isFetching"` | `"isInitialLoading"` | `"isPaused"` | `"isRefetching"` | `"isStale"` | `"isEnabled"` | `"refetch"` | `"fetchStatus"`

#### Returns

`void`

***

### trackResult()

```ts
trackResult(result: QueryObserverResult<TData, TError>, onPropTracked?: (key: 
  | "error"
  | "data"
  | "isError"
  | "isPending"
  | "isLoading"
  | "isLoadingError"
  | "isRefetchError"
  | "isSuccess"
  | "isPlaceholderData"
  | "status"
  | "dataUpdatedAt"
  | "errorUpdatedAt"
  | "failureCount"
  | "failureReason"
  | "errorUpdateCount"
  | "isFetched"
  | "isFetchedAfterMount"
  | "isFetching"
  | "isInitialLoading"
  | "isPaused"
  | "isRefetching"
  | "isStale"
  | "isEnabled"
  | "refetch"
| "fetchStatus") => void): QueryObserverResult<TData, TError>;
```

Defined in: [packages/query-core/src/queryObserver.ts:339](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L339)

Wraps a `QueryObserverResult` in a `Proxy` that records which properties are read, via
[QueryObserver#trackProp](#trackprop) (and an optional `onPropTracked` callback). Used by framework
adapters when `notifyOnChangeProps` is not set, to implement its default "only re-render on
properties you actually read" behavior.

#### Parameters

##### result

[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>

The result to wrap.

##### onPropTracked?

(`key`: 
  \| `"error"`
  \| `"data"`
  \| `"isError"`
  \| `"isPending"`
  \| `"isLoading"`
  \| `"isLoadingError"`
  \| `"isRefetchError"`
  \| `"isSuccess"`
  \| `"isPlaceholderData"`
  \| `"status"`
  \| `"dataUpdatedAt"`
  \| `"errorUpdatedAt"`
  \| `"failureCount"`
  \| `"failureReason"`
  \| `"errorUpdateCount"`
  \| `"isFetched"`
  \| `"isFetchedAfterMount"`
  \| `"isFetching"`
  \| `"isInitialLoading"`
  \| `"isPaused"`
  \| `"isRefetching"`
  \| `"isStale"`
  \| `"isEnabled"`
  \| `"refetch"`
  \| `"fetchStatus"`) => `void`

Called with the name of each property that is read.

#### Returns

[`QueryObserverResult`](../type-aliases/QueryObserverResult.md)\<`TData`, `TError`\>

A proxy of `result` that tracks property reads.

***

### updateResult()

```ts
updateResult(): void;
```

Defined in: [packages/query-core/src/queryObserver.ts:746](https://github.com/TanStack/query/blob/main/packages/query-core/src/queryObserver.ts#L746)

Recomputes and stores the current result from the current query/options, notifying listeners
if it changed. Framework adapters call this right after subscribing to make sure no query
update was missed in the gap between creating the observer and subscribing to it.

#### Returns

`void`