How to paginate to previous page using AngularFire, Firestore and Firebase

Question

Sorry I have seen this question has been asked many times in different ways here such as:

• Howto paginate back to previous pages in a Angular(6) and firebase Fires

Answer 1

So I think I solved it by a comment inspired by the github issue mentioned above:

Supposing you have this array and you are sorting ascending

A,
B, 
C,
D,
E,
F, 

And you have a page limnit of 2 results

Then when you are in the third page you should have

E,
F

Now you need to go to previous page and what you need todo is:

1. Reverse the sorting order and our data should be [F,E,D,C,B,A]
2. startAfter the first document of the currently viewed page (in our case E)
3. Query the firestore to get the results (eg with your limit of 2). You should get with reverse order and starting after E and that is [D,C]
4. Reverse the above array so it will be [C,D]
5. Done