What are Firestore batch writes?

Firestore batch writes commit multiple write operations as a single atomic operation — all succeed or all fail. Unlike transactions, batches don't read data — they only write. Supported operations: set, update, delete. Example: const batch = writeBatch(db); batch.set(doc(db, "cities", "NYC"), { name: "New York", state: "NY" }); batch.update(doc(db, "cities", "LA"), { population: increment(1000) }); batch.delete(doc(db, "cities", "OldCity")); await batch.commit(). Batch limits: maximum 500 operations per batch (each set/update/delete counts as one). Benefits over individual writes: (1) Atomicity — all changes apply together or none; (2) Performance — one network round trip instead of N; (3) Consistency — no partial state visible to other readers during batch commit. For deleting large collections, combine pagination with batch deletes: query 500 docs, batch delete, repeat. Batches don't retry on failure and don't read documents (unlike transactions).