Why is my js changing my Twig output array?

Question

PHP

I use Twig, and I connect a third-party script that breaks my array, I view the array through twig debag. Why does the script make some array keys empty? This piece of script changes the array:

var split = Split(['#leftPane', '#rightPane'], {
        sizes: sizes,
        onDragEnd: function () {
          localStorage.setItem('split-sizes', JSON.stringify(split.getSizes()));
        }