Gist for medium article: https://medium.com/evenbit/create-user-data-for-new-firebase-auth-accounts-c730719b140a

createUserDoc.ts

import * as functions from "firebase-functions";
import * as firebase from "firebase-admin";
import {MD5} from "crypto-js";

export const createUserDoc = functions.auth.user().onCreate(event => {
    const firebaseUser = event.data;

    // Use gravatar as default if photoUrl isn't specified in user data
    const gravatarHash = MD5(firebaseUser.email).toString().toLowerCase();
    let photoURL = `https://www.gravatar.com/avatar/${gravatarHash}.jpg?s=1024&d=robohash`;
    let fileEnding = "jpg";
    if (firebaseUser.photoURL) {
        fileEnding = firebaseUser.photoURL.substr(firebaseUser.photoURL.lastIndexOf(".") + 1);
        photoURL = firebaseUser.photoURL;
    }
    
    const fileName = `users/${firebaseUser.uid}/profile.${fileEnding}`;
    const profilePhotoStorageOpts = {
        destination: fileName,
        metadata: {
            contentType: `image/${fileEnding}`
        }
    };

    const user = {
        name: firebaseUser.displayName || "No Name",
        email: firebaseUser.email,
        photoUrl: `gs://${firebase.storage().bucket().name}/${fileName}`
    };

    return Promise.all([
        firebase.storage().bucket().upload(photoURL, profilePhotoStorageOpts),
        firebase.firestore().collection("users").doc(firebaseUser.uid).set(user)
    ]);
});