How to filter a List in Dart

Solution 1:

You are trying to remove the element from your List before it was even added.
Specifically, the map function will assign a List to your userList variable after it has mapped the snapshots.
From your code I can tell that you do not actually want to perform any mapping but only filtering.

In Dart, you can filter using the Iterable.where.
In your case that would look something like this:

final List<DocumentSnapshot> userList = snapshot.data.documents
      .where((DocumentSnapshot documentSnapshot) => documentSnapshot['userId'] != id).toList();

I am assuming that you only want documents that do not have a userId of id, otherwise, you would have to use an == operator instead.

You could also use List.removeWhere by assigning all the documents to your userList first and then calling removeWhere:

final List<DocumentSnapshot> userList = snapshot.data.documents;

userList.removeWhere((DocumentSnapshot documentSnapshot) => documentSnapshot['userId'] != id).toList();