how to append data to a field in mongoose model

const {name , id} = req.body
    Category.update(
        {_id: id}, 
        {$push: {items: {"name": name}}},{new: true, upsert: true }).exec();
        res.sendStatus(200)exports.addFriend = function (req, res, next)
{
var friend = {"firstName": req.body.fName, "lastName": req.body.lName};
Users.findOneAndUpdate({name: req.user.name}, {$push: {friends: friend}});
};//mongoose appending data

var objFriends = { fname:"fname",lname:"lname",surname:"surname" };
Friend.findOneAndUpdate(
   { _id: req.body.id }, 
   { $push: { friends: objFriends  } },
  function (error, success) {
        if (error) {
            console.log(error);
        } else {
            console.log(success);
        }
    });
)