[Home](./index.md) > [@firebase/firestore](./firestore.md) > [lite](./firestore_lite.md) > [FirestoreDataConverter](./firestore_lite.firestoredataconverter.md)
## FirestoreDataConverter interface
Converter used by `withConverter()` to transform user objects of type `T` into Firestore data.
Using the converter allows you to specify generic type arguments when storing and retrieving objects from Firestore.
Signature:
```typescript
export declare interface FirestoreDataConverter
```
## Example
```typescript
class Post {
constructor(readonly title: string, readonly author: string) {}
toString(): string {
return this.title + ', by ' + this.author;
}
}
const postConverter = {
toFirestore(post: Post): firebase.firestore.DocumentData {
return {title: post.title, author: post.author};
},
fromFirestore(snapshot: firebase.firestore.QueryDocumentSnapshot): Post {
const data = snapshot.data(options)!;
return new Post(data.title, data.author);
}
};
const postSnap = await firebase.firestore()
.collection('posts')
.withConverter(postConverter)
.doc().get();
const post = postSnap.data();
if (post !== undefined) {
post.title; // string
post.toString(); // Should be defined
post.someNonExistentProperty; // TS error
}
```
## Methods
| Method | Description |
| --- | --- |
| [fromFirestore(snapshot)](./firestore_lite.firestoredataconverter.fromfirestore.md) | Called by the Firestore SDK to convert Firestore data into an object of type T. You can access your data by calling: snapshot.data(). |
| [toFirestore(modelObject)](./firestore_lite.firestoredataconverter.tofirestore.md) | 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). Used with , and . |
| [toFirestore(modelObject, options)](./firestore_lite.firestoredataconverter.tofirestore_1.md) | 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). Used with , and with merge:true or mergeFields. |