quodolores/docs-exp/firestore_.onsnapshot_2.md

Monorepo for the Firebase JavaScript SDK

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[Home](./index.md) &gt; [@firebase/firestore](./firestore.md) &gt; [/](./firestore_.md) &gt; [onSnapshot](./firestore_.onsnapshot_2.md)

## onSnapshot() function

Attaches a listener for `DocumentSnapshot` events. You may either pass individual `onNext` and `onError` callbacks or pass a single observer object with `next` and `error` callbacks.

NOTE: Although an `onCompletion` callback can be provided, it will never be called because the snapshot stream is never-ending.

<b>Signature:</b>

```typescript
export declare function onSnapshot<T>(reference: DocumentReference<T>, onNext: (snapshot: DocumentSnapshot<T>) => void, onError?: (error: FirestoreError) => void, onCompletion?: () => void): Unsubscribe;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  reference | [DocumentReference](./firestore_.documentreference.md)<!-- -->&lt;T&gt; | A reference to the document to listen to. |
|  onNext | (snapshot: [DocumentSnapshot](./firestore_.documentsnapshot.md)<!-- -->&lt;T&gt;) =&gt; void | A callback to be called every time a new <code>DocumentSnapshot</code> is available. |
|  onError | (error: [FirestoreError](./firestore_.firestoreerror.md)<!-- -->) =&gt; void | A callback to be called if the listen fails or is cancelled. No further callbacks will occur. |
|  onCompletion | () =&gt; void | Can be provided, but will not be called since streams are never ending. |

<b>Returns:</b>

[Unsubscribe](./firestore_.unsubscribe.md)

An unsubscribe function that can be called to cancel the snapshot listener.