Unhandled rejection from Cloud Function, but it runs sometimes

Question

I have some code that works sometimes, but sometimes it does not. When it does not run, I get this error:

This is my code in Javascript:

exports.handleRequest = functions.database.ref('/Request/{usersUID}/{autoID}/{request}').onWrite(event => {
  let request = event.data.val();
  let additionalRequest = event.data.key;
  let usersUID = event.params.usersUID;
  const generatedAutoID = event.params.autoID;
  event.data.adminRef.remove();
  if (event.data.previous.exists()) {
        return;
  }
  if (!event.data.exists()) {
        return;
  }
  const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  admin.initializeApp(functions.config().firebase);
  var db = admin.database();

  var MasterAllCards = ["2_of_clubs", "2_of_spades", "2_of_diamonds", "2_of_hearts", "3_of_clubs", "3_of_spades", "3_of_diamonds", "3_of_hearts", "4_of_clubs", "4_of_spades", "4_of_diamonds", "4_of_hearts", "5_of_clubs", "5_of_spades", "5_of_diamonds", "5_of_hearts", "6_of_clubs", "6_of_spades", "6_of_diamonds", "6_of_hearts", "7_of_clubs", "7_of_spades", "7_of_diamonds", "7_of_hearts", "8_of_clubs", "8_of_spades", "8_of_diamonds", "8_of_hearts", "9_of_clubs", "9_of_spades", "9_of_diamonds", "9_of_hearts", "10_of_clubs", "10_of_spades", "10_of_diamonds", "10_of_hearts", "jack_of_clubs", "jack_of_spades", "jack_of_diamonds", "jack_of_hearts", "queen_of_clubs", "queen_of_spades", "queen_of_diamonds", "queen_of_hearts", "king_of_clubs", "king_of_spades", "king_of_diamonds", "king_of_hearts", "ace_of_clubs", "ace_of_spades", "ace_of_diamonds", "ace_of_hearts"]
  var MasterAllValues = [2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11, 12, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 14]
  function getRandomInt(min, max) {
        return Math.floor(Math.random() * (max - min + 1)) + min;
  }
  let pathToUsersTickets = db.ref('/users/' + usersUID + '/serverSideValues/Tickets');
  var usersTickets = 0;
  pathToUsersTickets.transaction(function(current) {
        return (current || 0) - 1;
  }).then(function(ticket) {
        usersTickets = Number(ticket.snapshot.val())
  });
  if (usersTickets >= 0) {
        let pathSettingUpGame = db.ref('/Checkrequests/' + usersUID + '/' + request); 
        let pathServer = db.ref('/Checkrequests/' + usersUID + '/' + request + '/Server');
        var tags = [];
        var images = [];
        var allCards = MasterAllCards
        var allCardsTags = MasterAllValues
        var i = 0;
        while (i < 5) {
              let randomc = getRandomInt(0, 51 - i);
              images.push(allCards[randomc])
              tags.push(allCardsTags[randomc])
              allCards.splice(randomc, 1);
              allCardsTags.splice(randomc, 1);
              i++
        }
        console.log(images);
        console.log(tags[0])
        console.log(allCards);
        console.log(allCardsTags) 
        pathSettingUpGame.update({
              "mastercard": images[0],
              "highlowgametier" : 1
        })    
        pathServer.update({
              "child1image" : images[1],
              "child2image" : images[2],
              "child3image" : images[3],
              "child4image" : images[4],
              "child1tag" : tags[1],
              "child2tag" : tags[2],
              "child3tag" : tags[3],
              "child4tag" : tags[4],
        })
   }
})

But sometimes it DOES run without any errors. You can check the code on here, where images[0] always is a string and never empty. How can this problem sometimes occur? No error:

And sometimes I get this, notice the undefined in the third value:

Answer 1

The intermittent failures may be the result of your not returning a Promise for the numerous asychronous Firebase operations your code performs. The Cloud Functions Guide explains:

It's important to manage the lifecycle of a function to ensure that it resolves properly... Also, you can make sure that the Cloud Functions instance running your function does not shut down before your function successfully reaches its terminating condition or state.

• Resolve functions that perform asynchronous processing by returning a JavaScript promise

Your code contains calls to remove(), transaction() and update(). Each of these completes asynchronously and returns a Promise. You need to chain or combine (see Promise.all()) the returned Promises, as needed, to ensure your function returns a Promise for any ongoing, asynchronous Firebase operation.

For example, to handle the two calls to update() at the end of the posted code:

const prom1 = pathSettingUpGame.update({
    ...
});

const prom2 = pathServer.update({
    ...
});

return Promise.all([prom1, prom2]);

There is also a Firebase video on returning Promises.