const query = firestore
    .collection(`posts`)
    .limit(200)

forEachDocs(query, docs => {
  docs.map(doc => {
    console.log(`${doc.id} do something`)
  })
})

async function forEachDocs(query, handler) {
  let snap = await query.get()
  while (snap.size > 0) {
    handler(snap.docs)
    snap = await query
        .startAfter(snap.docs.pop())
        .get()
  }
}