javascript - Cannot changing JSON from a mongoDB query unless I copy it into new variable -

i've created mean app returns json, wanted create simple function change date field in json before displaying - can if make copy of data new variable. can explain why case? thought knew javascript pretty until this!! scope or mongodb thing?

my route:

 jobsimportctrl = require('../controllers/jobsimportctrl');     module.exports = function (app) {        app.get('/api/jobs', function (req, res) {          jobsctrl.getjobs(res);      });

my controller:

var jobspretty = require('../helpers/jobspretty'); // pretty job dates etc    module.exports = {    // jobs    getjobs: function(res)  {            job.find(function (err, jobs){            // if there error retrieving, send error. nothing after res.send(err) execute            if (err) {                res.send(err);            }             res.json(jobspretty.deadlinedates(jobs));          });    },

my helper function:

var moment = require('moment'); // momentjs library  moment().format();      // human readable deadline dates using momentjs    module.exports = {      deadlinedates: function(jobs) {          var prettyjobs = [];            jobs.foreach(function(job) {              var obj2 = json.parse(json.stringify(job));              obj2.deadline  = moment(job.deadline).format("dddd, mmmm yyyy");              prettyjobs.push(obj2);          });            return prettyjobs;      }  };