Gists for the post - https://rizwaniqbal.com/posts/paginating-firestore-collections-with-snapshot-listeners/

sortByDesc.ts

// this method accepts the property to order by. In our example, this would be priority
// sortByDesc('priority')
sortByDesc(prop: string) {
    return this.returnedCardsArr.sort((a, b) => a[prop] < b[prop] ? 1 : a[prop] === b[prop] ? 0 : -1);
}

// also a slightly verbose way of keeping only unique IDs replacing the ones received from the new call - cause the data also might have changed.
for (const card of c) {
    if (!this.uniqueMap.has(card.id)) {
        this.uniqueMap.set(card.id, true);
        this.returnedCardsArr.push(card);
        continue;
    }
    this.returnedCardsArr = this.returnedCardsArr.filter((foundCard) => foundCard.id !== card.id);
    this.returnedCardsArr.push(card);
}