[Home](./index.md) > [@firebase/firestore](./firestore.md) > [/](./firestore_.md) > [onSnapshot](./firestore_.onsnapshot_3.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. Signature: ```typescript export declare function onSnapshot(reference: DocumentReference, options: SnapshotListenOptions, onNext: (snapshot: DocumentSnapshot) => void, onError?: (error: FirestoreError) => void, onCompletion?: () => void): Unsubscribe; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | reference | [DocumentReference](./firestore_.documentreference.md)<T> | A reference to the document to listen to. | | options | [SnapshotListenOptions](./firestore_.snapshotlistenoptions.md) | Options controlling the listen behavior. | | onNext | (snapshot: [DocumentSnapshot](./firestore_.documentsnapshot.md)<T>) => void | A callback to be called every time a new DocumentSnapshot is available. | | onError | (error: [FirestoreError](./firestore_.firestoreerror.md)) => void | A callback to be called if the listen fails or is cancelled. No further callbacks will occur. | | onCompletion | () => void | Can be provided, but will not be called since streams are never ending. | Returns: [Unsubscribe](./firestore_.unsubscribe.md) An unsubscribe function that can be called to cancel the snapshot listener.