veritatisdoloremque/docs-exp/firestore_.firestoredataconverter.md

Monorepo for the Firebase JavaScript SDK

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[Home](./index.md) &gt; [@firebase/firestore](./firestore.md) &gt; [/](./firestore_.md) &gt; [FirestoreDataConverter](./firestore_.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.

<b>Signature:</b>

```typescript
export declare interface FirestoreDataConverter<T> 
```

## 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,
    options: firebase.firestore.SnapshotOptions
  ): 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, options)](./firestore_.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: <code>snapshot.data(options)</code>. |
|  [toFirestore(modelObject)](./firestore_.firestoredataconverter.tofirestore.md) | Called by the Firestore SDK to convert a custom model object of type <code>T</code> into a plain JavaScript object (suitable for writing directly to the Firestore database). To use <code>set()</code> with <code>merge</code> and <code>mergeFields</code>, <code>toFirestore()</code> must be defined with <code>Partial&lt;T&gt;</code>. |
|  [toFirestore(modelObject, options)](./firestore_.firestoredataconverter.tofirestore_1.md) | Called by the Firestore SDK to convert a custom model object of type <code>T</code> into a plain JavaScript object (suitable for writing directly to the Firestore database). Used with ,  and  with <code>merge:true</code> or <code>mergeFields</code>. |