Page through large collections of data in a Firebase Cloud Function

firebase-forEachPage.js

const _ = require('lodash');

function forEachPage(ref, pageSize, callback, startAt) {
  var query = ref.orderByKey();
  if (startAt) query = query.startAt(startAt);
  return query.limitToFirst(pageSize + 1) // Ask for one extra
  .once('value')
  .then((snapshot) => {
    if (snapshot.numChildren() <= pageSize) {
      // There was no extra value returned, this is the last page
      return Promise.resolve(callback(snapshot.val()));
    } else {
      // We remove the extra value we asked for
      const lastKey = Object.keys(snapshot.val() || {}).reduce((a, b) => a>b ? a : b, undefined);
      const valuesWithoutExtra = _.omitBy(snapshot.val(), (val, key) => key === lastKey);
      return Promise.all([
        forEachPage(ref, pageSize, callback, lastKey), // Fetch the next page,
        Promise.resolve(callback(valuesWithoutExtra))
      ]);
    }
  });
}

module.exports = {
  forEachPage: forEachPage
};