export Firestore collection into json file

firestore-export.js

/*
Requirements:  
1) To be able to access your Firestore collections, you must have serviceAccountKey.json ,
You can generate it 
- via Firebase Project Overview -> Settings -> Service accounts  
- Generate new private key, then download it as codementorship_serviceAccountKey.json file.  
Put this file in the same folder as this script. 

2) use this js file (firestore-export.js) as part of basic node.js project, 
and with a simple package.json that contains the following deps
  "firebase": "^3.4.0",
  "firebase-admin": "^6.0.0",
  
Run it like this:  
 node src/firestore-export.js collectionName
*/

const admin = require("firebase-admin");
const fs = require('fs');

const serviceAccount = require("./codementorship_serviceAccountKey.json");

let collectionName = process.argv[2];
let subCollection = process.argv[3];

// FIXME You should replace databaseURL with your own
admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: "https://codementorship.firebaseio.com"
});

let db = admin.firestore();
db.settings({ timestampsInSnapshots: true });

let data = {};
data[collectionName] = {};

let results = db.collection(collectionName)
    .get()
    .then(snapshot => {
        snapshot.forEach(doc => {
            data[collectionName][doc.id] = doc.data();
        })
        return data;
    })
    .catch(error => {
        console.log(error);
    })

results.then(dt => {
    getSubCollection(dt).then(() => {
        // Write collection to JSON file
        fs.writeFile(collectionName+".json", JSON.stringify(data), function(err) {
            if(err) {
                return console.log(err);
            }
            console.log("The file was saved!");
        });
    })
})

async function getSubCollection(dt){
    for (let [key, value] of Object.entries([dt[collectionName]][0])){
        if(subCollection !== undefined){
            data[collectionName][key]['subCollection'] = {};
            await addSubCollection(key, data[collectionName][key]['subCollection']);
        }
    }
}

function addSubCollection(key, subData){
    return new Promise(resolve => {
        db.collection(collectionName).doc(key).collection(subCollection).get()
            .then(snapshot => {
                snapshot.forEach(subDoc => {
                    subData[subDoc.id] =  subDoc.data();
                    resolve('Added data');
                })
            })
    })
}