[{"username":"john123","email":"john@test.com","favorites":["5daef9a2761d4b1668214dbc"]},{"username":"jane75","email":"jane@test.com","favorites":[]},{"username":"johnwick","email":"johnwick@test.com","favorites":["5daef9a2761d4b1668214dbc"]}]

db.collection.aggregate([{$project:{username:1,email:1,favrites:1,totalFavritesCount:{$cond:{if:{$isArray:"$favorites"},then:{$size:"$favorites"},else:"NA"}}}}])