firestore_trigger_transaction_example.js

let batch = db.batch

let docToWrite = {
    stringField: "StringValue",
    dateField: {
        startDate: new Date('2018–09–07')
    }
}

let setOptions = { merge: true }

batch.set(db.doc('users/user1'), docToWrite, setOptions}

if (change.before.exists && change.before.data().date.endDate.toDate()) {
    let docToWrite2 = { field1: "dummyValue" }
    batch.update(db.doc('resources/resource1'), docToWrite2)
    return batch.commit()
} else {
    return batch.commit()
}