Changes cloud function

Changes.ts

import * as functions from 'firebase-functions';
import * as admin from 'firebase-admin';
import { Event } from 'firebase-functions';
import { DeltaSnapshot } from 'firebase-functions/lib/providers/database';

// intialize admin
admin.initializeApp(functions.config().firebase);

// export the function registration
export let writeTransactionChange = functions
    .database
    .ref('/transactions/{groupId}/{entityId}')
    .onWrite(event => writeEntityChange(event, "transaction"));

// functions which writes the Change to Firebase DB
async function writeEntityChange(event: Event<DeltaSnapshot>, entity: String)
{
    let entityId = event.params['entityId'];
    let groupId = event.params['groupId']; 
    let action = getAction(event); // can be insert / update / delete
    let changesRef = event.data.ref.root.child("/changes/" + groupId);
    let undocumentedEvent = event as any as UndocumentedEvent; // hack to get to uid
    let by = undocumentedEvent.auth.variable == null ? null : undocumentedEvent.auth.variable.uid;

    let change: Change = 
    {
        "action": action,
        "by": by,
        "entity": entity,
        "entityId": entityId,
        "serverTimestamp": admin.database.ServerValue.TIMESTAMP
    };

    await changesRef.push().set(change);

    return change;
}

function getAction(event: Event<DeltaSnapshot>)
{
    if (event.data.val() && !event.data.previous.val()) {
        return "insert";
    } else if (event.data.val() && event.data.previous.val()) {
        return "update";
    } else {
        return "delete";
    }
}

interface UndocumentedEvent {
  auth: {
    variable: {
      uid: string
    }
  }
}