Open up index.js (or create it) in the functions directory and add this code:

```js
const functions = require('firebase-functions');
const admin = require('firebase-admin');
const { Storage } = require('@google-cloud/storage');
const cron = require('node-cron');

admin.initializeApp();
const firestore = admin.firestore();
const storage = new Storage();

exports.scheduledFirestoreBackup = functions.pubsub.schedule('0 0 _ _ *').onRun((context) => {
const backupName = backup_${new Date().toISOString()}.json;
const bucketName = 'YOUR_FIREBASE_PROJECT_ID.appspot.com';
const bucket = storage.bucket(bucketName);

return firestore.collection('COLLECTION_NAME').get()
.then(snapshot => {
let data = [];
snapshot.forEach(doc => {
data.push(Object.assign({ id: doc.id }, doc.data()));
});
return bucket.file(backups/${backupName}).save(JSON.stringify(data));
})
.then(() => {
console.log(Backup ${backupName} completed successfully.);
return null;
})
.catch(error => {
console.error('Error backing up Firestore data:', error);
throw new Error('Backup failed');
});
});
```