Firebase pagination

firebase-pagination.js

const admin = require("./firebase-admin");
admin.initializeApp({
  credential: **yourCredential**
  databaseURL: **yourDatabaseURL**
});
const db = admin.database();

const dbPromise = (db_ref, options = {}) => {
  const {
    can_null,
    null_errormsg = "Can not find the specified data"
  } = options;
  return new Promise((resolve, reject) => {
    db_ref.once("value", snapshot => {
      const val = snapshot.val();
      if (val || can_null) {
        resolve(val)
      } else {
        reject(null_errormsg)
      }
    }, reject)
  })
}

const usersRef = db.ref("users");

function formatMap(map) {
	const res = [];
	for (let k in map) {
		const item = map[k];
		const res_item = {
			nickname: item.nickname,
			create_time: item.create_time
		}
		res.push(res_item);
	}
	res.sort((a, b) => b.create_time - a.create_time)
	return res;
}

async function run(page, num) {

	const _page = (parseInt(page) || 1) - 1;
	const _num = parseInt(num) || 10;
	const _from = _page * _num;
	const _to = _from + _num;
	let list;

	if (_from) {
		const pre_map = await dbPromise(usersRef.orderByChild("create_time").limitToLast(_from));

		const pre_map_noop = function() {};
		pre_map_noop.prototype = pre_map;

		const pre_map_keys = Object.keys(pre_map);
		let min_create_time = Infinity;
		const min_item_key_set = new Set();
		for (var i = 0; i < pre_map_keys.length; i += 1) {
			var pre_map_key = pre_map_keys[i]
			var pre_map_item_create_time = pre_map[pre_map_key].create_time;
			if (pre_map_item_create_time < min_create_time) {
				min_create_time = pre_map_item_create_time;
				min_item_key_set.clear()
				min_item_key_set.add(pre_map_key)
			} else if (pre_map_item_create_time === min_create_time) {
				min_item_key_set.add(pre_map_key)
			}
		}

		const min_item_key_set_size = min_item_key_set.size;

		const map = await dbPromise(usersRef.orderByChild("create_time")
			.endAt(min_create_time, "create_time")
			.limitToLast(_num + min_item_key_set_size)
		);

		const map_noop = function() {};
		map_noop.prototype = map; // 让nodejs引擎优化成fast-object
		const map_keys = Object.keys(map); // Object.keys比for in快

		const res_list = [];
		for (var i = 0; i < map_keys.length; i += 1) {
			var map_key = map_keys[i];
			if (!min_item_key_set.has(map_key)) {
				res_list[res_list.length] = map[map_key];
			}
		}

		list = res_list;
	} else {
		const map = await dbPromise(usersRef.orderByChild("create_time").limitToLast(_to));
		const noop = function() {};
		noop.prototype = map;
		const map_keys = Object.keys(map);

		const res_list = [];
		for (var i = 0, last_i = map_keys.length - 1; i <= last_i; i += 1) {
			res_list[last_i - i] = map[map_keys[i]];
		}
		list = res_list;
	}

	list.sort((a, b) => b.create_time - a.create_time);
	return {
		page: _page + 1,
		num: _num,
		list
	}
};

run(2, 4).then(res => {
	console.log(JSON.stringify(res.list.map(item => {
		return {
			nickname: item.nickname,
			create_time: item.create_time
		}
	}), null, 2));

	setTimeout(() => {
		console.log("EXIT SERVER")
		process.exit(0);
	}, 100)
}).catch(e => {
	console.log("ERRRR", e)
	process.exit(0);
})