Alright, let's get started with Firebase Cloud Functions. First things first, you need to set up your Firebase project. To do that, you'll want to install the Firebase CLI globally using npm. This way, you can run firebase commands from anywhere on your computer. Just pop open your terminal and run:

npm install -g firebase-tools

If you've already got the Firebase CLI installed, make sure it's up-to-date. You can check the version by running:

firebase --version

Next, log into Firebase using your Google Account:

firebase login

To kick off a new project, run firebase init functions. This will walk you through the setup. Just make sure you've already created a Firebase project in your Firebase console. Once your project is all set up and initialized, head into your functions directory and install Cloud Firestore:

cd functions && npm install @google-cloud/firestore

Now, let's dive into writing the Cloud Function. Open up the index.js file in your functions directory. You'll need to import both 'firebase-functions' and '@google-cloud/firestore'. Then, create a new Firestore instance:

const functions = require('firebase-functions');
const Firestore = require('@google-cloud/firestore');
const firestore = new Firestore({
  projectId: 'YOUR_PROJECT_ID',
  timestampsInSnapshots: true,
});

Don't forget to replace 'YOUR_PROJECT_ID' with your actual Firebase project’s ID. You can find this in the Project Overview section of the Firebase console.

Next up, let's build a function to clean up orphaned data. This function will listen for deletion events in a specific collection and then find and delete all related data in another collection.

exports.cleanUp = functions.firestore
  .document('/mainCollection/{docId}')
  .onDelete((snap, context) => {
    const deletedDocId = context.params.docId;

    return firestore
      .collection('associatedCollection')
      .where('mainDocId', '==', deletedDocId)
      .get()
      .then((snapshot) => {
        const deletionPromises = [];
        snapshot.forEach((doc) => {
          deletionPromises.push(doc.ref.delete());
        });
        return Promise.all(deletionPromises);
      })
      .catch((err) => {
        console.log(err);
        return false;
      });
  });

In this function, mainCollection is the collection where the deletion triggers the Cloud Function. associatedCollection is the collection with orphaned documents you want to delete. The mainDocId field in associatedCollection links to the deleted document in mainCollection.

Alright, now that we've written the function, it's time to deploy it. Run:

firebase deploy --only functions:cleanUp

This command deploys just the cleanUp function from your index.js file, leaving any other functions untouched.

Now, whenever a document is deleted in mainCollection, the Cloud Function will trigger and clean up all related documents in associatedCollection.

Remember, Cloud Functions are serverless, so they run in the cloud and you only pay for the compute time you use. You can create some pretty complex chains of Cloud Functions based on Firestore events, turning your Cloud Firestore into a real-time, reactive database.