function cleanupFoos(startAfter) {
  let query = db.collection('myCollection').orderBy('createdAt').limit(200)
  if (startAfter) query.startAfter(startAfter)
  return query.get().then(querySnapshot => {
    startAfter = _.last(querySnapshot.docs)
    let batch = db.batch()
    let promises = querySnapshot.docs.map(doc => cleanupFoo(doc, batch))
    return Promise.all(promises).then(() => batch)
  }).then(batch => {
    return batch.commit()
  }).then(() => {
    return startAfter ? cleanupFoos(startAfter) : null
  })
}

function cleanupFoo(doc, batch) {
  // return a promise
}

// call it
cleanupFoos()

startAfter = _.last(querySnapshot.docs) // --> { just this one doc }