index1.js

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

exports.createUserAccount = functions.auth.user().onCreate(event => {
    const email = event.data.email;
    const uid = event.data.uid;
    const photoURL = event.data.photoURL || "";
    var username = event.data.displayName;

    /**
     * if the username is null we use the fist part of his email
     * as the the default username 
     */
    if (username == null) {
        username = email.split("@")[0];
    }
    return admin.database().ref("/users").child(uid).set({
        email,
        username,
        photoURL
    });
});