Javascript and Firestore – Filter and Reducer at the same time

November 18, 2022

I have two arrays of object that I want to iterate over to produce a new filtered array. But also, I need to filter out some of the objects from the new array depending of a parameter. I’m trying this:

function loadAllUsersDontFollow() {
  
firestore()
        .collection("users")
        .where("id", "!=", user?.id)
        .get()
        .then((response) => {
          const data = following.filter((follow) => {
            return response.docs.reduce(function (res, item, index) {
              if (item.data().id !== follow.userId) {
                res.push(item);
              }
              return res;
            }, []);
          });
        });
}

the return of the function is being totally contrary to what I need, it is returning the users that I already follow, but I need the users that I still don’t follow. Help-me, please.

>Solution :

This would be easier if you created a hashed set of follower IDs…

const followerIds = new Set(following.map(({ userId }) => userId));

that you can then use to filter users…

const data = response.docs.filter((doc) => !followerIds.has(doc.data().id));

Your issue is that filter() expects a Boolean value returned from the callback. You were returning an array which, even if empty, is always truthy.