Swift check if email already exist in collection firebase

Question

I want to check if the user uses the same email that already exists to sign up we'll tell the user "the email already exist" so let see the function that I created to do that but the status always return false please advise me also I'm new at Firebase thanks.

   func checkUserExist(phoneOrEmail:String)-> Bool {
        var isExistingUser = false
        // collection "tm_members"
        COLLECTION_USERS
            .whereField("email", isEqualTo: phoneOrEmail)
            .getDocuments() { (snapshot, error) in
                if let err = error {
                          print("Error getting documents: \(err)")
                      } else {
                      
                        for document in snapshot!.documents {
                            print("\(document.documentID) => \(document.data())")
                          let email:String = document.data()["email"] as! String
                       
                            if  !email.isEmpty {
                                isExistingUser = true
                            }
                            else{
                                isExistingUser = false
                            }
                        
                        
                        }
                      }
        }
        print("user status \(isExistingUser)")
        return isExistingUser
    }