React Hooks for loading Firebase Data

firebase-hook.ts

import React, { useReducer, useEffect, useRef } from 'react';
import firebase from 'firebase/app';
import equal from 'deep-equal';

function filterKeys(raw, allowed) {
  if (!raw) {
    return raw;
  }
  let s = new Set(allowed);
  return Object.keys(raw)
    .filter(key => s.has(key))
    .reduce((obj, key) => {
      obj[key] = raw[key];
      return obj;
    }, {});
}

export const useDbData = (paths) => {
  let unsubscribes = useRef({})
  let [data, dispatch] = useReducer((d, action) => {
    let {type, path, payload} = action
    switch (type) {
      case 'upsert':
        if (payload) {
          return Object.assign({}, d, {[path]: payload})
        } else {
          let newData = Object.assign({}, d)
          delete newData[path]
          return newData
        }
      default:
        throw new Error('bad type to reducer', type)
    }
  }, {})
  useEffect(() => {
    for (let path of Object.keys(paths)) {
      if (unsubscribes.current.hasOwnProperty(path)) {
        continue
      }
      let ref = firebase.database().ref(path)
      let lastVal = undefined
      let f = ref.on('value', snap => {
        let val = snap.val()
        val = paths[path] ? filterKeys(val, paths[path]) : val
        if (!equal(val, lastVal)) {
          dispatch({type: 'upsert', payload: val, path})
          lastVal = val
        }
      })
      unsubscribes.current[path] = () => ref.off('value', f)
    }
    let pathSet = new Set(Object.keys(paths))
    for (let path of Object.keys(unsubscribes.current)) {
      if (!pathSet.has(path)) {
        unsubscribes.current[path]()
        delete unsubscribes.current[path]
        dispatch({type: 'upsert', path})
      }
    }
  })
  useEffect(() => {
    return () => {
      for (let unsubscribe of Object.values(unsubscribes.current)) {
        unsubscribe()
      }
    }
  }, [])
  return data
}

export const useDbDatum = (path, allowed=null) => {
  let datum = useDbData(path ? {[path]: allowed} : {})
  if (datum[path]) {
    return datum[path]
  }
  return null
}