how to solve circular structure in node.js

Question

i want to use cryptojs to encrypt cookies , i want to encrypt user id with his network card mac address.
cryptoJS needs to JSON.stringify this data then encrypts.
but i see this error in console:

TypeError: Converting circular structure to JSON

this is a snippet of my code:

var mac = 'e0:23:e3:46:57:52'
var userInfo = JSON.stringify({_id: user._id, mac: mac})
var cookie = crypto.AES.encrypt(userInfo, 'secret key')
res.cookie('userInfo', cookie, { expires: new Date(Date.now() + 900000000) })

Answer 1

user._id is ObjectId , just convert it to string:

var userInfo = JSON.stringify({ _id: user._id.toString(), mac: mac });

Also try to use util.inspect , it automatically replaces circular links with [Circular] .