For Loop inner Object Assignment with Test.

forLoop.js

require('chai').should();
var expect = require('chai').expect;
var assert = require('chai').assert;

function xybkPushSame(dt){
  var xybk = [];
  var obj = {};
  for (let i=0; i<dt.length; i++){
    obj.title = dt[i].title;
    obj.id = dt[i].id;
    xybk.push(obj);
  }
  return xybk;
}

function xybkPushNotSame(dt){
  var xybk = [];
  for (let i=0; i<dt.length; i++){
    var obj = {};
    obj.title = dt[i].title;
    obj.id = dt[i].id;
    xybk.push(obj);
  }
  return xybk;
}

Array.prototype.foo = "foo!";
function forinVersion(dt){
  var xybk = [];
  for (var i in dt){
    if (dt[i].title){
      xybk.push(dt[i]);
    }
  }
  return xybk;
}

function mapVersion(dt){
  var cb = function(elem){
    if (elem.title) return elem;
  }
  var xybk = dt.map(cb);
  return xybk;
}

var dt = [{id: 13, title: 'first'}, {id: 14, title:'Second'}, {id: 15, title:'Third'}];
var dtSame = [{id: 15, title:'Third'}, {id: 15, title:'Third'}, {id: 15, title:'Third'}];

describe('XYBK', function(){
  describe('tran', function(){
    it('should return same right value within obj declared out of the loop', function(){
      var result = xybkPushSame(dt);
      assert.deepEqual(result, dtSame, 'with same last obj.');
    });
    it('should return right value with for loop', function(){
      var result = xybkPushNotSame(dt);
      result.should.deep.equal(dt);
    });

    it('should return right value with for in loop', function(){
      var result = forinVersion(dt);
      expect(result).to.deep.equal(dt);
    });

    it('should return right value with map', function(){
      var result = mapVersion(dt);
      expect(result).to.deep.equal(dt);
    });
  });
});