How to use the firebase-functions/lib/providers/firestore.document function in firebase-functions

const ALGOLIA_ID = functions.config().algolia.app_id;
const ALGOLIA_ADMIN_KEY = functions.config().algolia.api_key;

const ALGOLIA_INDEX_NAME = "notes";
const client = algoliasearch(ALGOLIA_ID, ALGOLIA_ADMIN_KEY);
// [END init_algolia]

// Initialize Firestore
const firestoreOptions = {
  projectId: process.env.GCLOUD_PROJECT
};
const db = new Firestore(firestoreOptions);

// [START update_index_function]
// Update the search index every time a blog post is written.
exports.onNoteCreated = firestore.document("notes/{noteId}").onCreate(event => {
  // Get the note document
  const note = event.data.data();

  // Add an "objectID" field which Algolia requires
  note.objectID = event.params.postId;

  // Write to the algolia index
  const index = client.initIndex(ALGOLIA_INDEX_NAME);
  return index.saveObject(note);
});
// [END update_index_function]

// [START get_firebase_user]
const admin = require("admin");

function getFirebaseUser(req, res, next) {