Merge array to Firestore arrayUnion

'use strict';

// Import packages
import admin from 'firebase-admin';
const FieldValue = admin.firestore.FieldValue; 

// Add credential service account details
import serviceAccount from ".path-to-file.json";

// Initialize firebase admin sdk config
admin.initializeApp({
	credential: admin.credential.cert(serviceAccount),
});

const codes = ['8034', '1146', '2423', '8744'];
const docRef = admin.firestore().collection('testing').doc('TgTjKUFs3tJCxrdnRcBj');
docRef.update("subscriptions", FieldValue.arrayUnion(...codes));
// docRef.update("subscriptions", FieldValue.arrayUnion(null, codes));

// output in firestore
subscriptions
(array) 
0: "6"
(string) 
1: "7"
(string) 
10: "2423"
(string) 
11: "8744"
(string) 
2: "1"
(string) 
3: "2"
(string) 
4: "8"
(string) 
5: "46"
(string) 
6: "3"
(string) 
7: "4"
(string) 
8: "8034"
(string) 
9: "1146"