unable to insert json object inside another in node.js -

usersmodel.findone({"username":username}).exec(function (err, users) {           if (err) {                      logger.info("my error "+err);               return res.status(500).send(err);           }           else if(!users)               return res.status(404).send('no user found');           else {               users["account"]="xxxxxxxxx";               return res.status(200).send(users);           }         }); 

i want insert account key pair value inside users json.but return users json not contain account fields.where issue ?