Is it possible for Firestore document Snapshot data to be undefined?

February 19, 2022

Using typescript I get

QueryDocumentSnapshot<DocumentData>.data(options?: SnapshotOptions | undefined): DocumentData

The question is.. Do I really have to check if data is possibly undefined

if (snapDoc.data()) //...
else //...

or it is safe to do snapDoc.data()! for the fact that the data can never be undefined?

If it can be undefined then when or at what case does the data will return undefined?

>Solution :

If you are querying a single document using getDoc(<DoucmentReference>) that returns a DocumentSnapshot then yes, data() will return undefined if that document does not exist.

const docSnap = await getDoc(docRef);

if (docSnap.exists()) {
  console.log("Document data:", docSnap.data());
} else {
  // doc.data() will be undefined in this case
  console.log("No such document!");
}

If you are querying multiple documents using getDocs(<query>), a QuerySnapshot is returned that has .docs property (array of QueryDocumentSnapshot) that contains all matched documents that exist for sure. So .data() will never be undefined in that case as mentioned in the documentation.

const querySnapshot = await getDocs(q);
querySnapshot.forEach((doc) => {
  // doc.data() is never undefined for query doc snapshots
  console.log(doc.id, " => ", doc.data());
});