//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);
}
});
)