index.js

/**
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
'use strict';

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

// Keeps track of the length of the 'likes' child list in a separate property.
exports.countlikechange = functions.database.ref('/posts/{postid}/likes/{likeid}').onWrite(
    async (change) => {
      const collectionRef = change.after.ref.parent;
      const countRef = collectionRef.parent.child('likes_count');

      let increment;
      if (change.after.exists() && !change.before.exists()) {
        increment = 1;
      } else if (!change.after.exists() && change.before.exists()) {
        increment = -1;
      } else {
        return null;
      }

      // Return the promise from countRef.transaction() so our function
      // waits for this async event to complete before it exits.
      await countRef.transaction((current) => {
        return (current || 0) + increment;
      });
      console.log('Counter updated.');
      return null;
    });

// If the number of likes gets deleted, recount the number of likes
exports.recountlikes = functions.database.ref('/posts/{postid}/likes_count').onDelete(async (snap) => {
  const counterRef = snap.ref;
  const collectionRef = counterRef.parent.child('likes');

  // Return the promise from counterRef.set() so our function
  // waits for this async event to complete before it exits.
  const messagesData = await collectionRef.once('value');
  return await counterRef.set(messagesData.numChildren());
});