[Home](./index.md) > [@firebase/firestore](./firestore.md) > [/](./firestore_.md) > [documentId](./firestore_.documentid.md) ## documentId() function Returns a special sentinel `FieldPath` to refer to the ID of a document. It can be used in queries to sort or filter by the document ID. Signature: ```typescript export declare function documentId(): FieldPath; ``` Returns: [FieldPath](./firestore_.fieldpath.md)