Firestore push multiple keys to map without overwriting

Question

Current data in firestore

fruits: { apple: 20, orange: 10 }

Now how to add more fruits to the fruits field without overwriting apple and orange

Fruits to be added

fruits: { pineapple: 20, cherry: 10, banana: 10 }

Tried doing this but apple and orange gets removed

batch.update(fs.doc(`config/254868`), {
  fruits: fruits,
});