[Home](./index.md) > [@firebase/firestore](./firestore.md) > [/](./firestore_.md) > [FirestoreDataConverter](./firestore_.firestoredataconverter.md) > [toFirestore](./firestore_.firestoredataconverter.tofirestore.md) ## FirestoreDataConverter.toFirestore() method Called by the Firestore SDK to convert a custom model object of type `T` into a plain JavaScript object (suitable for writing directly to the Firestore database). To use `set()` with `merge` and `mergeFields`, `toFirestore()` must be defined with `Partial`. Signature: ```typescript toFirestore(modelObject: T): DocumentData; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | modelObject | T | | Returns: [DocumentData](./firestore_.documentdata.md)