Issue with packery functions while executing unit test's

I am using Angular 7 Here is my component code ngOnInit() {

 Packery.prototype.getShiftPositions = function (attrName) {
  attrName = attrName || 'id';
  return this.items.map(function (item) {
    return {
      attr: item.element.getAttribute(attrName),
      x: item.rect.x / this.pckry.width
    }
  });
};

Packery.prototype.initShiftLayout = function (positions, attr) {
  if (!positions) {
    // if no initial positions, run packery layout
    this.layout();
    return;
  }
  // parse string to JSON
  if (typeof positions === 'string') {
    try {
      positions = JSON.parse(positions);
    } catch (error) {
      console.error('JSON parse error: ' + error);
      this.layout();
      return;
    }
  }

  attr = attr || 'id'; // default to id attribute
  this._resetLayout();
  // set item order and horizontal position from saved positions
  this.items = positions.map(function (itemPosition) {
    const selector = '[' + attr + '="' + itemPosition.attr + '"]'
    const itemElem = this.element.querySelector(selector);
    const item = this.getItem(itemElem);
    item.rect.x = itemPosition.x * this.pckry.width;
    return item;
  }, this);
  this.shiftLayout();
};

this.pckry = new Packery('.grid', {
  itemSelector: '.grid-item',
  gutter: 10
});

// get saved dragged positions
// const initPositions = localStorage.getItem('dragPositions');
this.memberDashboardService.getUserPrefrences().subscribe(
  data => {
    this.pckry.initShiftLayout( data, 'data-item-id' );
   } ,
   error => {
    //console.log("Unable to retrieve User prefrences");
   }
);

this.pckry.getItemElements().forEach(itemElem => {
  const draggie = new Draggabilly(itemElem);
  this.pckry.bindDraggabillyEvents(draggie);
});

// save drag positions on event
this.pckry.on('dragItemPositioned', () => {
  const positions = this.pckry.getShiftPositions('data-item-id');
  this.memberDashboardService.updateUserPrefrences(positions).subscribe(
    data => {
      //console.log("User Prefrences Persisted");
    },
    error => {
      //console.log("Unable to store User prefrences");
    }
  );
});

}

when i run the unit test's using jasmine and karma I get the following error, can someone please help me with this

metafizzy / packery

Issue with packery functions while executing unit test's #534