[Home](./index.md) > [@firebase/firestore](./firestore.md) > [/](./firestore_.md) > [onSnapshot](./firestore_.onsnapshot.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, observer: { next?: (snapshot: DocumentSnapshot) => void; error?: (error: FirestoreError) => void; complete?: () => void; }): Unsubscribe; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | reference | [DocumentReference](./firestore_.documentreference.md)<T> | A reference to the document to listen to. | | observer | { next?: (snapshot: [DocumentSnapshot](./firestore_.documentsnapshot.md)<T>) => void; error?: (error: [FirestoreError](./firestore_.firestoreerror.md)) => void; complete?: () => void; } | A single object containing next and error callbacks. | Returns: [Unsubscribe](./firestore_.unsubscribe.md) An unsubscribe function that can be called to cancel the snapshot listener.