Add Timestamp When Document Created Firestore

script.js

/*
Using Cloud Functions, here's what I've come up with for anyone who wants to 
add a timestamp field in a Firestore document. You need to have the Firebase Admin SDK installed in
addition to the general setup for Cloud Functions as detailed in the Firebase documentation.
*/

const functions = require('firebase-functions');

// The Firebase Admin SDK to access the Firebase Realtime Database.
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

// Use a wildcard to identify the document if you are unsure of its ID
exports.addServerTimestamp = functions.firestore
.document('collection/{itemID}')
.onCreate(event => {

const itemID = String(event.params.itemID)
const doc = admin.firestore().collection('collection').doc(itemID);

doc.set({
timeStamp: admin.firestore.FieldValue.serverTimestamp()}, 
{merge:true}
);

});