Use async/await and for..of in Cloud Firestore

1-standard.js

// In a Firestore standard example, we quickly create a 'xmas tree' of nested stuff 
// We use Promises directly: get().then(callback) and use snapshot.forEach() to iterate
let campaignsRef = db.collection('campaigns');
let activeCampaigns = campaignsRef.where('active', '==', true).select().get()
.then(snapshot => {
    snapshot.forEach(campaign => {
        console.log(campaign.id);
        let allTasks = campaignsRef.doc(campaign.id).collection('tasks').get().then(
            snapshot => {
                snapshot.forEach(task => {
                    console.log(task.id, task.data())
                });
            }
        )
    });
})
.catch(err => {
    console.log('Error getting documents', err);
});

2-async.js

// In the async/await example, we need to wrap our code inside a function
// and mark it as 'async'. This allows us to 'await' on a Promise.
async function process_tasks() {
    let campaignsRef = db.collection('campaigns')
    let activeRef = await campaignsRef.where('active', '==', true).select().get();
    for (campaign of activeRef.docs) {
        console.log(campaign.id);
        let tasksRef = await campaign.collection('tasks').get();
        for(task of tasksRef.docs) {
            console.log(task.id, task.data())
        } 
    }
}

// Call our async function in a try block to catch connection errors.
try {
    process_tasks();
}
catch(err) {
    console.log('Error getting documents', err)
}