TypeError Converting circular structure to JSON in nodejs

Question

Code :

 var formData = ({first_name:firstname,last_name:lastname,user_name:username, email:email,password:password});


  request.post({url:'http://localhost:8081/register', JSON: formData}, function(err, connection, body) {


exports.Register = function(req, res) {

    res.header("Access-Control-Allow-Origin", "*");

    console.log("Request data " +JSON.stringify(req));

This is a request package for node.js. But I am getting this error in return:

TypeError: Converting circular structure to JSON

Can someone help me with this?