Firebase Firestore: Append/Remove items from document array

Update elements in an array

If your document contains an array field, you can use arrayUnion() and arrayRemove() to add and remove elements. arrayUnion() adds elements to an array but only elements not already present. arrayRemove() removes all instances of each given element.

let washingtonRef = db.collection("cities").document("DC")

// Atomically add a new region to the "regions" array field.
washingtonRef.updateData([
    "regions": FieldValue.arrayUnion(["greater_virginia"])
])

// Atomically remove a region from the "regions" array field.
washingtonRef.updateData([
    "regions": FieldValue.arrayRemove(["east_coast"])
])

See documentation here

Actually, nowadays it is possible. With latest updates db.collection.updateData method actually appends new item to array instead of replacing it. Example usage can be found in Firebase documentation.

If you need to do it manually, you can use

FieldValue.arrayUnion([user.uid])

Nope. This isn't possible.

Arrays tend to be problematic in an environment like Cloud Firestore where many clients could theoretically append or remove elements from an array at the same time -- if instructions arrive in a slightly different order, you could end up with out-of-bounds errors, corrupted data, or just a really bad time. So you either need to use a dictionary (where you can specify individual keys) or replace the entire array.